JP5752564B2 - Method for analyzing water-soluble selenium - Google Patents

Description

  The present invention relates to a method for analyzing water-soluble selenium. More specifically, the present invention relates to a method for analyzing water-soluble selenium that is suitable for analysis of wastewater that may contain sulfur compounds, such as desulfurization wastewater discharged from a coal-fired power plant.

  Selenium (Se) is an element widely used in industry. For example, in addition to being used for glass colorants and decolorizers, it is used for photosensitive drums and rectifiers of copiers by utilizing the semiconductivity and photoconductivity of metal selenium. Accordingly, wastewater containing water-soluble selenium may be discharged in the process of manufacturing industrial products containing selenium, or water-soluble selenium may be eluted into the environment due to use or disposal of industrial products containing selenium.

In the electricity business, water-soluble selenium (tetravalent selenium: SeO ₃ ²⁻ , hexavalent selenium: SeO ₄ ²⁻ ) is contained in the flue gas desulfurization effluent of coal-fired power plants due to the small amount of selenium contained in coal. May be included. Moreover, also in a coal ash disposal site, water-soluble selenium may be contained in coal ash disposal site elution water due to a small amount of selenium contained in coal.

  Here, selenium is known as an element that causes various disorders due to excessive intake. For example, it is known that symptoms such as neuropathy, gastrointestinal dysfunction, muscle weakness, and hypotension occur and ultimately cause circulatory collapse and pulmonary edema. Therefore, if selenium is dissolved in the waste water, there is a risk of adverse effects on the human body. In Japan, the selenium water quality standard (0.01 mg-Se / L) was established in 1993. In 6 years, a part of the Enforcement Ordinance for Water Pollution Control Law was revised and the selenium drainage standard (0.1 mg-Se / L) was established.

  In order to comply with the above effluent standard value, it is necessary to continuously monitor and manage the selenium concentration in the effluent.

  Under such circumstances, the present applicant has proposed a method for analyzing tetravalent selenium dissolved in an analysis sample in Patent Document 1. Specifically, at least one of hydrochloric acid and sulfuric acid is added to an analytical sample in which tetravalent selenium is dissolved to prepare an acidic analytical sample, and sodium tetrahydroborate is added to the acidic analytical sample. By reacting tetravalent selenium with sodium tetrahydroborate, hydrogen selenide gas is generated, and this hydrogen selenide gas is detected with a hydrogen selenide gas detector, and the sample is analyzed. A method for quantitative analysis of dissolved tetravalent selenium concentration is proposed. In addition, when this analytical method is applied to desulfurized wastewater, the analytical sample is pretreated by an official method according to the JIS method (see JIS-K0102.67.3., Non-patent Document 1). It has also been proposed that the influence of analysis interfering components (organic substances, etc.) contained in the analysis sample can be eliminated, and that the analysis sample can be acidified by this pretreatment, so that the acidification treatment can be omitted.

  In addition, the above pretreatment method according to the official method is a method in which sulfuric acid and nitric acid are added to an analytical sample and heated to generate sulfuric acid white smoke. The time required was a long time of 4 to 5 hours. Therefore, the applicant of the present invention, in Patent Document 2, for the analysis method proposed in Patent Document 1, the first pretreatment in which potassium permanganate is added and heated to the sample for analysis, and hydrochloric acid is added and heated. Then, the second pretreatment for reducing hexavalent selenium to tetravalent selenium is performed in order, and the acidification treatment is omitted, so that the influence of analysis interfering components (organic substances, etc.) contained in the sample for analysis is determined by the official method. In addition, a method for analyzing the total concentration of water-soluble selenium including not only tetravalent selenium concentration but also hexavalent selenium concentration with high accuracy is proposed.

JP 2009-8668 A JP2011-2424

Japan Industrial Standards Committee, Industrial Wastewater Test Method, JIS K 0102.67.3, (2008).

  However, the waste water may contain not only organic substances but also sulfur compounds. It has been reported in References 1 and 2 that a part of the sulfur compound reacts with sodium tetrahydroborate to be reduced to form hydrogen sulfide gas (Reference 1: W. kijowski and PA Steudler, “Determination of total reduced sulfur in natural waters ", Limnol. Oceanogr., 27 (5), 975-978, (1982), Reference 2: JFTysona and CDPalmerb," Simultaneous detection of selenium by atomic fluorescence and sulfur by molecular emission by flow -injection hydride generation with on-line reduction for the determination of selenate, sulfate and sulfite ", Anal. Chim. Act., 652 (1-2), 251-258, (2009)).

  Here, the hydrogen sulfide gas can be a component that interferes with the measurement of the hydrogen selenide gas by the hydrogen selenide gas detector. In addition, as a result of intensive studies by the inventors of the present application, the desulfurization effluent may contain a kind of sulfur compound that generates hydrogen sulfide by reaction with sodium tetrahydroborate and interferes with the measurement of hydrogen selenide gas. From this, it has become clear that accurate analytical values may not be obtained.

  Therefore, the present invention provides a method for analyzing the concentration of water-soluble selenium in sample water (analytical sample) by measuring hydrogen selenide gas using a hydrogen selenide gas detector. An object of the present invention is to provide a method for obtaining an accurate analytical value by avoiding the generation of hydrogen sulfide gas due to a possible sulfur compound.

  In order to solve such a problem, the inventors of the present application have conducted intensive research. As a result, in the first pretreatment (organic substance decomposition treatment) in the method described in Patent Document 2, by using potassium permanganate and sulfuric acid in combination, New ability to avoid generation of hydrogen sulfide by enhancing the oxidizing power of potassium permanganate and decomposing sulfur compounds in sample water (sulfur compounds that can react with sodium tetrahydroborate to produce hydrogen sulfide gas) As a result, knowledge has been obtained and various studies have been made to complete the present invention.

  That is, the water-soluble selenium analysis method of the present invention includes a first pretreatment in which potassium permanganate is added and heated as an organic substance decomposing agent to sample water, and a reducing agent that reduces hexavalent selenium to tetravalent selenium. After the second pretreatment in which hydrochloric acid is added and heated in order, tetravalent selenium contained in the sample water is reacted with sodium tetrahydroborate to generate hydrogen selenide gas, and this hydrogen selenide gas is converted into selenium. In the method of analyzing the concentration of water-soluble selenium in the sample water based on the signal value detected by the hydrogen selenide gas detector, the sulfuric acid is further added during the first pretreatment. Thus, sulfur compounds that can react with sodium tetrahydroborate together with organic substances contained in the sample water to generate hydrogen sulfide gas are decomposed.

  Here, in the water-soluble selenium analysis method of the present invention, it is preferable to adjust the sulfuric acid concentration of the sample water to 1.3 to 3 mol / L by adding sulfuric acid.

  According to the present invention, in a method for analyzing water-soluble selenium concentration in a sample for analysis (sample water) by measuring hydrogen selenide gas using a hydrogen selenide gas detector, In addition to avoiding the influence of organic substances that can be generated, generation of hydrogen sulfide gas caused by sulfur compounds can be avoided, and accurate analytical values can be obtained.

It is process schematic of the water-soluble selenium analysis of this invention. It is a figure which shows the water-soluble selenium analysis system used in the Example. It is a figure which shows the experimental result which examined the density | concentration of sulfuric acid and the addition amount in a 1st pretreatment.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings.

  The water-soluble selenium analysis method of the present invention includes a first pretreatment in which potassium permanganate is added as an organic substance decomposing agent and heated to sample water, and hydrochloric acid is used as a reducing agent to reduce hexavalent selenium to tetravalent selenium. After sequentially adding and heating the second pretreatment, tetravalent selenium contained in the sample water is reacted with sodium tetrahydroborate to generate hydrogen selenide gas, and this hydrogen selenide gas is converted to hydrogen selenide. In the method of analyzing the water-soluble selenium concentration of the sample water based on the signal value detected by the hydrogen selenide gas detector, led to the gas detector, sulfuric acid is further added during the first pretreatment, A sulfur compound capable of generating hydrogen sulfide gas is decomposed by reacting with organic tetrahydroborate together with organic substances contained in the sample water. In other words, in the present invention, the first pretreatment using both potassium permanganate and sulfuric acid (S1) → the second pretreatment for reducing hexavalent selenium contained in the first pretreated sample water to tetravalent selenium (S2). → Hydrogen selenide gas generation / measurement (S3) is performed, and the water-soluble selenium concentration of the sample water is analyzed (FIG. 1).

  When the sample water contains an organic substance, this can hinder the generation of hydrogen selenide gas and cause a decrease in analysis accuracy. In addition, the sample water may contain a type of sulfur compound that generates hydrogen sulfide by reaction with sodium tetrahydroborate. When such a sulfur compound is contained in the sample water, detection of hydrogen selenide gas is hindered by hydrogen sulfide generated by the reaction with sodium tetrahydroborate, which may cause a decrease in analysis accuracy. In the present invention, in the first pretreatment, by adding potassium permanganate and sulfuric acid to the sample water, the oxidizing power of potassium permanganate is enhanced, and not only organic substances contained in the sample water but also sulfur compounds are added. It decomposes to prevent generation of hydrogen sulfide, and avoids a decrease in analysis accuracy caused by organic substances and sulfur compounds. The decomposition of the sulfur compound will be described in more detail. Sulfur compounds such as dithionic acid and hydroxylamine sulfonic acid compound (NS compound) are oxidized and decomposed by potassium permanganate whose oxidizing power is enhanced by sulfuric acid, and finally sulfuric acid Become an ion. Sulfate ions are chemically stable, and hydrogen sulfide gas is not generated even when sodium tetrahydroborate is added. Therefore, a sulfur compound capable of generating hydrogen sulfide gas by reaction with sodium tetrahydroborate by potassium permanganate whose oxidizing power is enhanced by sulfuric acid, that is, a stable sulfur compound that does not react with sodium tetrahydroborate (sulfuric acid It is possible to eliminate the interference of the measurement of hydrogen selenide gas by a sulfur compound other than (such as peroxodisulfuric acid), and so on, and to perform accurate analysis.

The amount of potassium permanganate added in the first pretreatment is an amount capable of decomposing organic substances contained in the sample water, and the excess is added in an amount that does not inhibit the reduction in the second pretreatment. That is, if the amount of potassium permanganate added is too large, the excess reacts with hydrochloric acid as the reducing agent in the second pretreatment as follows, and the amount of hydrochloric acid decreases. As a result, the reduction reaction of hexavalent selenium to tetravalent selenium can be inhibited.
2KMnO ₄ + 16HCl → 2MnCl ₂ + 2KCl + 5Cl ₂ + 8H ₂ O (1)

  About the specific addition amount of potassium permanganate, it is preferable to set it as 3-10 times amount of COD amount of sample water, and it is more preferable to set it as 5-10 times amount. The case where 5 mL of sample water having a COD concentration of 16 mg-O / L is collected will be described as an example. 1 mL of an aqueous potassium permanganate solution having a concentration of 1 g / L to 3 g / L may be added.

  Note that the sample water often has a variation in COD concentration within a certain range depending on the source. For example, in the case of desulfurization effluent, the COD concentration is approximately 10 to 30 mg-O / L. Therefore, the amount of potassium permanganate added can be set to an appropriate value depending on the source of the sample water. Of course, the COD concentration of the sample water may be measured, and the amount of potassium permanganate added may be set for each analysis based on this measured value.

  It is preferable that the amount of sulfuric acid added in the first pretreatment is such that the sulfuric acid concentration of the sample water after adding sulfuric acid and potassium permanganate (aqueous solution) is adjusted to 1.3 to 3 mol / L. More preferably, the amount is adjusted to 2.3 mol / L. If the sulfuric acid concentration is too low, the effect of increasing the oxidizing power of potassium permanganate by sulfuric acid is reduced, so that the organic compound cannot be sufficiently decomposed and the analysis accuracy can be lowered. If the sulfuric acid concentration is too high, the amount of sulfuric acid added becomes too large, increasing the total amount of sample water, leading to an increase in reaction time. To give a specific example, when 5 mL of desulfurization waste water containing a sulfur compound is used as sample water, 1 to 2 mL of 6 to 18 mol / L sulfuric acid may be added to obtain the above concentration, and 1 to 9 to 12 mol / L sulfuric acid. It is preferable to add ˜2 mL to the above concentration, and it is more preferable to add 2 mL of 9 mol / L sulfuric acid to the above concentration.

  Here, in the first pretreatment, the sample water, potassium permanganate and sulfuric acid are added in the order of sample water → sulfuric acid → potassium permanganate or in the order of potassium permanganate → sample water → sulfuric acid. It is preferable to control. Alternatively, it is preferable to add potassium permanganate after sufficiently stirring and mixing the sample water and sulfuric acid, or adding sulfuric acid after sufficiently stirring and mixing the sample water and potassium permanganate. Thereby, it is possible to suppress boiling of the sample due to the neutralization reaction of sulfuric acid, and it is possible to prevent a decrease in the resolution of the organic matter and sulfur compound due to the solid formation caused by direct contact between potassium permanganate and sulfuric acid.

  The heating temperature of the sample water to which potassium permanganate and sulfuric acid have been added in the first pretreatment is preferably 100 ° C. or higher. If the temperature is lower than this, the organic matter and the sulfur compound cannot be decomposed, and the analysis accuracy can be lowered. If the heating temperature is too high, the sample water will boil vigorously, so the heating temperature is preferably 100 to 120 ° C.

  The heating time in the first pretreatment is preferably 15 minutes or more at the above temperature. When shorter than this, an organic substance and a sulfur compound cannot be decomposed | disassembled and analysis accuracy may fall. It should be noted that even if the decomposition processing time is too long, no particularly advantageous effect is seen, but rather the analysis time is lengthened. Therefore, the decomposition treatment time may be approximately 15 to 30 minutes at the above temperature, preferably 15 minutes to 20 minutes, and considering the certainty of the decomposition treatment, it may be approximately 20 minutes. More preferred.

  The sample water is subjected to the first pretreatment and then the second pretreatment. The second pretreatment includes hexavalent selenium contained in the first pretreated sample water (tetravalent selenium originally contained in the sample water oxidized into hexavalent selenium in the first pretreatment, and sample water). (6 selenium originally contained in selenium) is reduced to tetravalent selenium. Although sodium tetrahydroborate can react with tetravalent selenium to produce hydrogen selenide gas, it does not react with hexavalent selenium. Therefore, by adding hydrochloric acid as a reducing agent, hexavalent selenium is reduced to tetravalent selenium, all water-soluble selenium contained in the first pretreated sample water is converted to tetravalent selenium, and the total amount of water-soluble selenium is reduced. It can be subjected to a reaction with sodium tetrahydroborate.

  The amount of hydrochloric acid added in the second pretreatment is preferably such that the hydrochloric acid concentration of the first pretreated sample water is 4 mol / L to 6.7 mol / L, and is 6.7 mol / L. Is more preferable. If the hydrochloric acid concentration is too low, the total amount of hexavalent selenium may not be reduced to tetravalent selenium. If the concentration of hydrochloric acid is excessively increased, the amount of hydrochloric acid added is excessively increased, and the total amount of the first pretreated sample water is increased, resulting in a prolonged analysis time. As a specific example, when 5 mL of desulfurization waste water is used as sample water, it is preferable to add about 5 mL of concentrated hydrochloric acid (12 mol / L).

  The heating temperature of the first pretreated sample water after addition of hydrochloric acid in the second pretreatment is preferably 100 ° C. or higher. If the temperature is lower than this, hexavalent selenium that is not reduced to tetravalent selenium remains, and the analysis accuracy may be lowered. If the heating temperature is too high, the sample water will boil vigorously, so the heating temperature is preferably 100 to 120 ° C.

  The heating time in the second pretreatment is preferably 10 minutes or more at the above temperature. If it is shorter than this, hexavalent selenium that is not reduced to tetravalent selenium remains, and the analysis accuracy may be lowered. Note that even if the time for reducing hexavalent selenium to tetravalent selenium is too long, no particularly advantageous effect is seen, but rather the analysis time is prolonged. Therefore, the time for reducing hexavalent selenium to tetravalent selenium may be about 10 to 15 minutes at the above temperature.

  As described above, by performing the first pretreatment and the second pretreatment on the sample water, the organic matter and sulfur compound of the sample water are decomposed, and the factor that lowers the analysis accuracy can be removed. Furthermore, the total amount of water-soluble selenium contained in the sample water can be converted to tetravalent selenium, and the total amount of water-soluble selenium can be subjected to a reaction with sodium tetrahydroborate, which enables accurate analysis of the water-soluble selenium concentration. It becomes possible.

  Quantitative analysis of water-soluble selenium is performed using sample water obtained by sequentially performing the first pretreatment and the second pretreatment. The quantitative analysis of water-soluble selenium can be carried out by the same method as described in JP2009-8668A and JP2011-2424A, for example. Hereinafter, a method for quantitatively analyzing water-soluble selenium using sample water obtained by sequentially performing the first pretreatment and the second pretreatment will be described. In the following description, the sample water obtained by sequentially performing the first pretreatment and the second pretreatment is referred to as pretreated sample water.

  To the pretreated sample water, sodium tetrahydroborate is added to react tetravalent selenium and sodium tetrahydroborate to generate hydrogen selenide gas, and the hydrogen selenide gas is measured with a hydrogen selenide gas detector. To do.

  As the hydrogen selenide gas detector, various types of detectors capable of detecting hydrogen selenide gas can be used. In particular, a hydrogen selenide gas detector having a diaphragm galvanic type or concentration cell type sensor unit. Is preferred. This type of detector has the advantage that since the sensor itself constitutes a battery, there is very little residual current when there is no gas to be detected, so that zero point stability is high and running cost is low. doing. An example of such a detector is 1GWA / V70 manufactured by Bionics, but is not limited thereto. For example, a new Cosmos Electric PS-7 (dedicated hydrogen selenide sensor unit CDS-7) having a constant potential electrolysis method may be used.

  Sodium tetrahydroborate is added to the pretreated sample water in the form of an aqueous solution. The addition rate is such that the generation of hydrogen from the pretreated sample water can be suppressed. As a result, peak splitting of the detection signal value in the hydrogen selenide gas detector due to rapid hydrogen generation due to the reaction between hydrochloric acid and sodium tetrahydroborate contained in the pretreated sample water can be avoided. The reliability of the detection signal value in the hydrogen fluoride gas detector is increased to enable highly accurate analysis. The rate at which the generation of hydrogen from the pretreated sample water can be suppressed depends on the sodium tetrahydroborate concentration of the solution containing sodium tetrahydroborate and the hydrochloric acid concentration of the pretreated sample water. When the hydrochloric acid concentration of the treated sample water is 2.5 mol / L to 3.5 mol / L and the sodium tetrahydroborate concentration of the solution containing sodium tetrahydroborate is 3 g / L, sodium tetrahydroborate is added. The solution containing 10 mL may be added over 20 seconds, and it is preferable to add over 40 to 60 seconds. In terms of addition rate, it is less than 30 mL / min, preferably 10-15 mL / min.

  The solution containing sodium tetrahydroborate may be an alkaline solution containing only sodium tetrahydroborate, but may also contain, for example, ethylenediaminetetraacetic acid and sodium hydroxide. While sodium tetrahydroborate is an inexpensive and easily available material widely used as a reducing agent, it has the property of being easily decomposed. Therefore, it is known that ethylenediaminetetraacetic acid can be dissolved in a 0.1 mol / L sodium hydroxide solution and stably stored at room temperature for 30 days (ADIdowu, PK Dasgupta, Z. Genfa, and K Toda: “A Gas-Phase Chemiluminescence -Based Analyzer for Waterborne Arsenic”, Anal. Chem., 78, 7088-7097 (2006)). That is, it is a reagent having both ease of use and stability, and is very suitable as a reagent used for on-site analysis. In addition, when a solution containing sodium tetrahydroborate is stored and used in a certain place for a long time, it is stored in a refrigerator that can be cooled to 0 ° C. to 25 ° C., preferably 0 ° C. to 10 ° C. Is preferred. As a result, a solution containing sodium tetrahydroborate can be stored without being affected by the analysis environment, and stable sodium tetrahydroborate that is not decomposed and deteriorated can be added over a long period of time.

  For the addition of the sodium tetrahydroborate solution, for example, a separator such as a separatory funnel or burette can be used which can control the addition rate by adjusting the opening of the cock (valve), or when the valve is open. Only those that can be added at a constant rate can also be used. Further, by using a highly controllable device such as a peristaltic pump, more accurate quantitative analysis can be performed.

About the addition amount of sodium tetrahydroborate, the quantity more than the quantity which can reduce | restor the total amount of the tetravalent selenium considered to exist in pre-processed sample water to hydrogen selenide is selected suitably. In order to reduce 1 mol of H ₂ SeO ₃ (SeO ₃ ²⁻ ) to generate hydrogen selenide, _3/4 mol of NaBH ₄ (BH ₄ ⁻ ) is required. Therefore, a minimum of 0.38 mg NaBH ₄ is required for 1 mg of tetravalent selenium. Here, in order to reliably reduce the total amount of tetravalent selenium present in the pretreated sample water to hydrogen selenide, the total amount of tetravalent selenium considered to be contained in the pretreated sample water is reduced. It is preferable to add an excess amount of sodium tetrahydroborate. That is, assuming that the tetravalent selenium concentration in the pretreated sample water is 1 mg / L or less, 0.38 mg to 6.7 g of sodium tetrahydroborate is added to 1 L of the pretreated sample water. It is preferably 3.8 mg to 3.4 g, more preferably 38 mg to 2.0 g. Even if an amount of sodium tetrahydroborate exceeding 6.7 g is added, it does not affect the measurement of hydrogen selenide, but it is useless because it does not participate in the hydrogen selenide generation reaction.

  Further, it is preferable to continue stirring the pretreated sample water from the start of the addition of sodium tetrahydroborate to the pretreated sample water until the measurement by the hydrogen selenide gas detector is completed. If the agitation is not sufficient, sodium tetrahydroborate added to the pretreated sample water will not sufficiently diffuse and will react unevenly with the pretreated sample water, resulting in selenization with a hydrogen selenide gas detector. There is a possibility that the signal strength of hydrogen may decrease.

  The hydrogen selenide gas is measured at any time by a hydrogen selenide gas detector, and a measured value is output. Here, it is preferable to supply a carrier gas to the generated hydrogen selenide gas, and guide the hydrogen selenide gas to the hydrogen selenide gas detector along with the carrier gas. Furthermore, it is preferable to suck the hydrogen selenide gas alone or together with the carrier gas using the gas suction function of the hydrogen selenide gas detector. As the carrier gas, not only an inert gas such as nitrogen or argon but also air can be used.

  Based on the measurement value (signal value) measured by the hydrogen selenide gas detector, the water-soluble selenium concentration of the sample water can be analyzed by the calibration curve method.

  The calibration curve used for the calibration curve method was created based on the correlation between the water-soluble selenium concentration obtained in advance from a plurality of standard samples with known water-soluble selenium concentrations and the signal value detected by the hydrogen selenide gas detector. Is. More specifically, for each of a plurality of standard samples with known water-soluble selenium concentrations, analysis is performed under the same conditions as when actually analyzing the sample water, and the water-soluble selenium concentration and hydrogen selenide gas detector are used. A calibration curve is obtained by fitting a correlation with the detected signal value by a known method such as the least square method, and the sample water is obtained from the signal value obtained when analyzing the sample water. The water-soluble selenium concentration can be determined.

  The above-described embodiment is an example of a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the gist of the present invention.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

(1) Experimental apparatus A flow-type water-soluble selenium analysis system shown in FIG. 2 was used. This water-soluble selenium analysis system is roughly divided into a sample water collection unit, a first processing unit for decomposing analysis interference components contained in the sample water sent from the sample water collection unit, and a first processing unit. Included in the second treatment part that performs the second treatment for reducing hexavalent selenium contained in the first treated sample water to be reduced to tetravalent selenium, and the second treated sample water fed from the second treatment part A reduction vaporization unit for reducing and vaporizing tetravalent selenium to hydrogen selenide gas, and a signal value obtained by detecting hydrogen selenide gas fed from the reduction vaporization unit through a gas introduction pipe with a hydrogen selenide gas detector. And an analysis unit for analyzing the water-soluble selenium concentration of the sample water. Hereafter, each structure and operating conditions of the water-soluble selenium analysis system shown in FIG. 2 in a present Example are demonstrated in detail. In addition, when the flow system shown in FIG. 2 is operated under the following operating conditions, it has been confirmed that the water-soluble selenium concentration can be analyzed with high accuracy for desulfurization wastewater containing organic substances but not containing sulfur compounds.

<Sample water collection unit>
The sample water collecting unit has two adjustment tanks 11 and 12 and a standard sample storage tank 13, and the sample water or the standard sample is sample water from any of the tanks by opening and closing the valve 11a, the valve 12a, and the valve 13a. The sample was fed to the first treatment tank 21 via the sample water feeding line 16 provided with the liquid feeding device 15. In addition, by providing two adjustment tanks, for example, desulfurization waste water before selenium removal treatment can be collected in one tank, desulfurization waste water after selenium removal treatment can be collected in the other tank, and these can be continuously measured. This is advantageous because it makes it easier to examine the status of selenium removal treatment. However, providing two adjustment tanks is not an essential condition, and there may be one adjustment tank or three or more adjustment tanks.

  5 mL of sample water was sampled from either of the adjustment tanks 11 and 12 and used for analysis. In the selenium analysis system shown in FIG. 2, a water-soluble selenium concentration of approximately 0.02 to 0.3 mg-Se / L can be analyzed with a sample water sampling amount of 5 mL.

<First processing part>
The 1st processing part shall have the following (1a)-(1f). The first processing unit corresponds to the first preprocessing (S1) in the present invention.
(1a) First treatment tank 21 for storing sample water
(1b) Organic matter decomposing agent addition means 22 for adding potassium permanganate in an amount capable of decomposing organic matter as an organic matter decomposing agent in the first treatment tank 21 and not inhibiting the reduction in the second treatment section.
(1c) The first stirring device 24 that stirs and mixes the sample water in the first treatment tank 21 and the organic substance decomposing agent to form a first mixed solution.
(1d) The 1st liquid feeding line 26 provided with the 1st liquid feeding apparatus 25 which liquid-feeds a 1st liquid mixture toward a 2nd process part.
(1e) The 1st heating apparatus 27 which heats the 1st liquid mixture which is passing the 1st liquid feeding line 26 to the temperature at which organic substance is decomposed | disassembled
(1f) Cleaning agent supply means 28 for supplying hydrochloric acid as a cleaning agent to the first liquid feeding line 26 after the first mixed liquid has passed.

  The first treatment tank 21 accommodated the sample water sent from the sample water collecting unit. The capacity of the first treatment tank 21 was 30 mL.

  The organic substance decomposing agent adding means 22 is for adding potassium permanganate as an organic substance decomposing agent for decomposing an organic substance which is an analysis interfering component in the first processing tank 21. Specifically, the organic substance decomposing agent adding means 22 stores 2 g / L of potassium permanganate aqueous solution in the organic substance decomposing agent storage tank 22a, and 1 mL of this is added to the first treatment tank 21 by a syringe pump. Controlled to be. In this embodiment, the potassium permanganate aqueous solution was added to the first treatment tank 21 before the sample water was stored in the first treatment tank 21.

  The first stirring device 24 is for stirring and mixing the sample water and the organic substance decomposing agent to form a first mixed solution. In this embodiment, the first stirring device 24 is a carrier gas supply unit that performs stirring and mixing by supplying a carrier gas (air) into the first treatment tank 21 and bubbling. The carrier gas (air) supply rate was 500 mL / min.

  The first liquid mixture is directed from the first processing tank 21 to the second processing tank 31 by the first liquid feeding device 25 via the first liquid feeding line 26 connecting the first processing tank 21 and the second processing tank 31. In the process, the sample is heated by the first heating device 27 to decompose the analysis disturbing component. As a result, the analysis interference component can be decomposed by the flow method.

  In the present embodiment, the first heating device 27 is a cylindrical heater, and a part of the capillary is spirally wound around the heater as the first liquid feeding line 26 to provide the first liquid feeding device 25 (peristaltic). The flow rate of the first mixed liquid was controlled by a pump). Thereby, the decomposition processing time of the analysis interfering component of the first liquid mixture is controlled by the flow rate of the first liquid mixture set by the first liquid feeding device 25 and the length of the capillary wound around the heater. In this example, a PTFE capillary with an inner diameter of 1 mm and a length of 6 m was used as the capillary, the flow rate was set to 0.5 mL / min, the heating temperature of the heater was 100 ° C., and the first mixed solution was 20 ° C. at 100 ° C. Heated for a minute. In addition, about the desulfurization waste_water | drain which does not contain a sulfur compound including an organic substance, even when heating time is 10 minutes, a water-soluble selenium density | concentration can be analyzed accurately.

In addition, manganese dioxide etc. resulting from potassium permanganate contained in the first liquid mixture is deposited on the first liquid feed line 26 after the first liquid mixture has passed, and the first liquid feed line 26 is blocked. Alternatively, the diameter of the first liquid feeding line 26 is narrowed, and the smooth flow of the first liquid mixture can be hindered during the next analysis. Therefore, in the present embodiment, the first liquid feeding line 26 after the first mixed liquid has passed is provided with a cleaning agent supplying means 28 for supplying hydrochloric acid as a cleaning liquid. Specifically, the cleaning agent supply means 28 stores hydrochloric acid in the cleaning agent storage tank 28a, and 2.5 mL of concentrated hydrochloric acid (12 mol / L) passes through the first treatment tank 21 using a syringe pump. To be added into the first liquid feeding line 26. By supplying hydrochloric acid, manganese dioxide, which is a main precipitate that can block the first liquid feeding line 26, is removed by the chemical reaction shown below.
MnO ₂ + 4HCl → MnCl ₂ + H ₂ O + Cl ₂ (2)

<Second processing section>
The 2nd processing part shall have the following (2a)-(2e). The second processing unit corresponds to the second preprocessing (S2) in the present invention.
(2a) The second treatment tank 31 connected to the first treatment tank 21 by the first liquid feeding line 26 and in which the first treated sample water is fed and stored.
(2b) Reducing agent adding means 32 for adding hydrochloric acid as a reducing agent for reducing hexavalent selenium to tetravalent selenium in the second treatment tank 31.
(2c) The second stirring device 33 that stirs and mixes the first treated sample water in the second treatment tank 31 and the reducing agent to form a second mixed solution.
(2d) The second liquid feeding line 36 including the second liquid feeding device 35 for feeding the second mixed liquid toward the reduction vaporization unit.
(2e) The second heating device 37 that heats the second liquid mixture passing through the second liquid feeding line 36 to a temperature at which the reduction reaction of hexavalent selenium to tetravalent selenium proceeds.

  The second treatment tank 31 accommodated the first treated sample water sent from the first treatment unit, that is, the sample water in which the organic matter contained in the sample water was decomposed. The capacity of the second treatment tank 31 was 30 mL.

  The reducing agent adding means 32 is originally contained in the second treatment tank 31 in the sample water that is hexavalent selenium contained in the first treated sample water (oxidized in the first treatment unit to become hexavalent selenium. In addition, hydrochloric acid is added as a reducing agent for reducing tetravalent selenium and hexavalent selenium originally contained in the sample water) to tetravalent selenium. In the present embodiment, the reducing agent adding means 32 was controlled so that 5 mL of hydrochloric acid (12 mol / L) was added from the cleaning agent storage tank 28 a into the second treatment tank 31 using a syringe pump. In the present embodiment, the location where the hydrochloric acid is fed from the cleaning agent storage tank 28a can be switched to one of the first treatment tank 21 and the second treatment tank 31 by the valve 32a.

  In the present embodiment, the second stirring device 33 is a carrier gas supply means for supplying a carrier gas (air) to the first treated sample water and the reducing agent in the second treatment tank 31 to perform bubbling. As a result, the first treated sample water and the reducing agent were stirred to obtain a second mixed solution. Moreover, since chlorine gas is generated when the first treated sample water is brought into contact with hydrochloric acid, the chlorine gas is excluded by bubbling. Specifically, bubbling with 500 mL / min of air was performed for 2 to 3 minutes. Thereby, in the hydrogen selenide gas detector 52, the interference of the analysis by generation | occurrence | production of the negative peak resulting from chlorine gas was excluded.

  The second liquid mixture is fed from the second processing tank 31 toward the reaction tank 41 by the second liquid feeding device 35 via the second liquid feeding line 36 connecting the second processing tank 31 and the reaction tank 41. In this process, the hexavalent selenium is heated by the second heating device 37 and reduced to tetravalent selenium. As a result, reduction treatment of hexavalent selenium to tetravalent selenium by the flow method was made possible.

  In the present embodiment, the second heating device 37 is a cylindrical heater, and a part of the capillary is spirally wound around the heater as the second liquid feeding line 36 to provide the second liquid feeding device 35 (peristaltic). The flow rate of the second mixed liquid was controlled by a pump). Thereby, the reduction process which reduces the hexavalent selenium contained in the second mixed liquid to tetravalent selenium by the flow rate of the second mixed liquid set by the second liquid feeding device 35 and the length of the capillary wound around the heater. The time was controlled. In this example, a PTFE capillary having an inner diameter of 1 mm and a length of 6 m was used as the capillary, the flow rate was set to 1.5 mL / min, the heating temperature of the heater was 100 ° C., and the second mixed solution was 10 ° C. at 100 ° C. Heated for a minute.

<Reduction vaporization department>
The reduction vaporization part shall have the following (3a)-(3f). The reduction vaporization section corresponds to hydrogen selenide gas generation (first stage of S3) in the present invention.
(3a) A sealed reaction tank 41 connected to the second treatment tank 31 by the second liquid feeding line 36 and in which the second treated sample water is fed and stored.
(3b) Water supply means 42 for supplying an amount of water capable of suppressing generation of hydrogen chloride gas from the second treated sample water into the reaction tank 41
(3c) Carrier gas supply means 43 for replacing the gas flow path from the reaction tank 41, the gas introduction pipe 51, and the gas introduction pipe 51 to the sensor part of the hydrogen selenide gas detector 52 with a carrier gas.
(3d) A third stirring device 44 for stirring the second treated sample water supplied with the water in the reaction tank 41
(3e) After the reaction tank 41, the gas introduction pipe 51, and the gas flow path are replaced with the carrier gas, the water in the reaction tank 41 stirred by the third stirring device 44 is supplied with the second treatment. Hydrogen selenide gas generator addition means 45 for adding a solution containing sodium tetrahydroborate to the sample water at a rate less than the rate at which the peak of the signal value detected by the hydrogen selenide gas detector 52 splits.
(3f) Refrigerator 45a that stores a solution containing sodium tetrahydroborate after cooling to 0 ° C to 25 ° C.

  In the reaction tank 41, the second treated sample water sent from the second treatment section, that is, the organic matter and sulfur compound contained in the sample water are decomposed, and the hexavalent selenium is reduced to tetravalent selenium. Sample water was stored. The capacity of the reaction tank 41 was 50 mL. The reaction tank 41 has a sealed structure.

  The water supply means 42 supplies the reaction tank 41 with an amount of water that can suppress generation of hydrogen chloride gas from the second treated sample water. As a result, the second treated sample water is diluted with water (hereinafter sometimes referred to as a diluted second treated sample water), and generation of hydrogen chloride gas from the second treated sample water is suppressed. In the hydrogen selenide gas detector, the interference of analysis due to the occurrence of an abnormal peak was eliminated. In this embodiment, before the second treated sample water is sent and stored in the reaction tank 41, 5 mL of distilled water is added from the water supply means 42 to the reaction tank 41, and the diluted second process is performed. The hydrochloric acid concentration of the used sample water was 3 mol / L.

  The carrier gas supply means 43 replaces the inside of the reaction tank 41, the gas introduction pipe 51, and the gas flow path from the gas introduction pipe 51 to the sensor part of the hydrogen selenide gas detector 52 with a carrier gas. . Thereby, the baseline of the signal of the hydrogen selenide gas detector 52 is stabilized, and the analysis is performed with high accuracy based on the stable signal value. In this embodiment, hydrogen selenide gas is detected from the reaction tank 41, the gas introduction pipe 51, and the gas introduction pipe 51 by supplying a carrier gas (air) to the diluted second treated sample water and bubbling. The gas flow path leading to the sensor part of the vessel 52 is replaced with a carrier gas. The carrier gas flow rate was 500 mL / min and the bubbling time was 7 minutes. When bubbling was performed, the valve 49a was opened and the valve 49b was closed.

  The third stirring device 44 is a stirring blade and replaces the gas flow path from the reaction tank 41, the gas introduction pipe 51, and the gas introduction pipe 51 to the sensor part of the hydrogen selenide gas detector 52 with a carrier gas. After that, the diluted second treated sample water was stirred so that the reaction with sodium tetrahydroborate smoothly proceeded.

  The hydrogen selenide gas generating agent addition means 45 replaces the gas flow path from the gas introduction pipe 51 to the sensor part of the hydrogen selenide gas detector 52 in the reaction tank 41, the gas introduction pipe 51, and the carrier gas. Then, a solution containing sodium tetrahydroborate is added to the diluted second treated sample water stirred by the third stirring device 44 to generate hydrogen selenide gas from tetravalent selenium. . In this embodiment, the hydrogen selenide gas generating agent adding means 45 uses a peristaltic pump from the hydrogen selenide gas generating agent storage tank 45a to transfer a solution containing sodium tetrahydroborate into the reaction tank 41 at 15 mL / min. So that 10 mL was added. The hydrogen selenide gas generator storage tank 45a was a refrigerator, and a solution containing sodium tetrahydroborate was cooled to 0 ° C. to 10 ° C. and stored.

  The solution containing sodium tetrahydroborate had a sodium tetrahydroborate concentration of 3 g / L, an EDTA concentration of 1.0 mmol / L, and an NaOH concentration of 0.1 mol / L.

<Analysis Department>
The analysis unit has the following (4a) to (4c). An analysis part corresponds to the hydrogen selenide gas measurement (after S3) in this invention.
(4a) Entrained gas supply means for supplying a carrier gas as an entrained gas to the head space in order to send the hydrogen selenide gas in the head space 41a in the reaction tank 41 to the hydrogen selenide gas detector 52 through the gas introduction pipe 51 53
(4b) A third heating device 54 that heats the entire gas introduction pipe 51 to a temperature at which condensation does not occur on the inner wall of the gas introduction pipe.
(4c) Analysis means 55 for analyzing the water-soluble selenium concentration of the sample water based on the signal value detected by the hydrogen selenide gas detector.
(4d) A fourth heating device 55 that heats the gas flow path to a temperature at which no condensation occurs on the inner wall of the gas flow path and to a temperature at which the electrolyte of the sensor unit does not evaporate and dry.

  The hydrogen selenide gas detector 52 was 1 GWA / V70 manufactured by Bionics.

  The gas introduction pipe 51 was a pipe inner mold of 4 mm and a length of 2 m.

  The accompanying gas supply means 53 is for causing the hydrogen selenide gas staying in the head space 41 a of the reaction tank 41 to accompany the carrier gas (air) and sending it into the hydrogen selenide gas detector 52 through the gas introduction pipe 51. Is. In this example, air was supplied to the head space 41a of the reaction tank 41 at a flow rate of 250 mL / min as the accompanying gas. At this time, the valve 53a was opened and the valve 53b was closed.

  The third heating device 54 was a cord heater that covered the entire outer peripheral surface of the gas introduction pipe 51 and heated to a temperature (90 ° C.) at which no condensation occurred on the inner wall of the gas introduction pipe 51. This prevents a decrease in analysis accuracy caused by the hydrogen selenide gas dissolved in the dew condensation water generated on the inner wall of the gas introduction pipe 51.

  The fourth heating device 55 is a device that heats the gas flow path leading to the sensor portion of the hydrogen selenide gas detector 52. The hydrogen selenide gas fed from the gas introduction pipe 51 is introduced into the sensor portion of the hydrogen selenide gas detector 52 through a through hole (gas flow path) provided in the base portion 52a of the hydrogen selenide gas detector 52. Is done. When the analysis environment is lower than 15 ° C., dew condensation occurs on the wall surface of the through hole, and the hydrogen selenide gas dissolves in the dew condensation water, so that the analysis accuracy can be lowered. Therefore, in the present embodiment, in order to enable accurate analysis without depending on the analysis environment, the fourth heating device 55 for heating the pedestal 52a is provided so that no condensation occurs on the inner wall of the gas flow path. The temperature was heated to a temperature (65 ° C.) at which the electrolyte solution in the sensor part did not evaporate to dryness.

  In the water-soluble selenium analysis system shown in FIG. 2, reference numeral 61 denotes a waste liquid tank for discharging the liquid in the reaction tank 41 as a high concentration waste liquid. Reference numeral 62 denotes a waste liquid line for collecting and discharging the washing water sent to the tank and the line in which the sample water is sent in the first processing unit, the second processing unit, and the reduction vaporization unit as a low concentration waste liquid. Is.

(1) Example 1
A method for eliminating the influence of sulfur compounds that could be contained in wastewater was investigated.

  In this embodiment, as shown in FIG. 2, sulfuric acid addition means 23 capable of adding a predetermined amount of sulfuric acid having a predetermined concentration into the first treatment tank 21 is provided. Specifically, the sulfuric acid addition means 23 stores a predetermined concentration of sulfuric acid in the sulfuric acid storage tank 23a, and a predetermined amount of sulfuric acid is added to the first treatment tank 21 by a syringe pump. I tried to control it.

The sulfuric acid addition conditions were as follows. The sulfuric acid was added after the sample water was fed into the first treatment tank 21 and stored.
(1) No addition of sulfuric acid (2) 3 mol / L, 1 mL
(3) 3 mol / L, 2 mL
(4) 6 mol / L, 1 mL
(5) 6 mol / L, 2 mL
(6) 9 mol / L, 1 mL
(7) 9 mol / L, 2 mL
(8) 12 mol / L, 1 mL
(9) 12 mol / L, 2 mL
(10) 18 mol / L, 1 mL
(11) 18 mol / L, 2 mL

The results are shown in FIG. When sulfuric acid was not added, when 1 mL or 2 mL of 3 mol / L sulfuric acid was added, or when 1 mL of 6 mol / L sulfuric acid was added, interference (abnormal peak) due to hydrogen sulfide was confirmed. On the other hand, when 2 mL of 6 mol / L sulfuric acid was added, no abnormal peak was observed when the sulfuric acid concentration was 9 mol / L or more.

  From this result, it was clarified that when the sulfuric acid concentration of the first mixed solution is 1.3 to 3.0 mol / L, the sulfur compound can be decomposed and an abnormal peak due to the sulfur compound can be avoided.

  Next, eight kinds of sulfur compounds were added as standard to the blank sample, selenium standard sample, and desulfurization drainage sample so as to be 100 mg / L, and interference with the hydrogen selenide gas detector 52 due to hydrogen sulfide generation was examined. The results are shown in Table 2. In Table 2, HAOS is hydroxylamine-O-sulfonic acid. HABS is hydroxylamine-N, O-bissulfonic acid. Further, “no pre-processing” means that the first processing by the first processing unit is omitted. The pretreatment of 10 minutes means heating at 100 ° C. for 10 minutes in the first treatment part. Pretreatment 20 minutes means heating at 100 ° C. for 20 minutes in the first treatment part.

  In the case of 20 minutes of pretreatment, no abnormal peak was observed in any sample.

  In the case of 10 minutes of pretreatment, an abnormal peak was observed in the sample to which dithionate ion was added.

  Without pretreatment, abnormal peaks were observed in 6 samples to which sulfurous acid, thiosulfuric acid, dithionic acid, HAOS, imidodisulfonic acid and HABS ions were added.

  Next, the sulfur compound concentration in the waste water was measured in five types of desulfurization waste water. The results are shown in Table 3.

  All five samples contained dithionate ions of 100 mg / L or more, and four HABS ions contained 0.9 to 58 mg / L.

  When all the five samples were heated at 100 ° C. for 20 minutes in the first treatment section, no abnormal peak was observed. However, an abnormal peak was observed after heating for 10 minutes.

Next, 5 mL of sample water (referred to as a standard sample) obtained by adding 100 mg / L dithionic acid to a selenium standard sample (0.1 mg / L), a desulfurization drainage sample (Se: 0.042 mg / L, dithionate ion: 160 mg) / L) About 5 mL of sample water (drainage sample) to which 100 mg / L dithionate ion was added, the heating time in the first treatment part was changed in various ways to examine the presence or absence of abnormal peaks. The results are shown in Table 4.

  When the heating time was 10 minutes or 12 minutes, an abnormal peak was detected in any sample, whereas when it was 15 minutes or longer, no abnormal peak was detected in any sample.

  As a result of analyzing the selenium concentration in the case of a heating time of 15 minutes, it was 0.115 mg / L for the standard sample and 0.04 mg / L for the drainage sample, which almost coincided with the selenium concentration of the sample.

  From the above results, it was found that the heating time in the first processing section was 15 to 30 minutes, preferably 20 minutes.

(2) Example 2
The amount of potassium permanganate added in the first treatment part was examined.

  The sample water was desulfurization effluent having a water-soluble selenium concentration of 0.055 mg / L and a COD concentration of 16 mg-O / L. Moreover, the density | concentration (g / L) of potassium permanganate aqueous solution was 1.0, 2.0, 4.0, and 8.0. 2 mL of 9 mol / L (1 + 1) sulfuric acid was added. In other cases, water-soluble selenium analysis was performed as the above basic condition. Moreover, the residual liquid in the reaction tank 41 of the reduction vaporization part after completion | finish of analysis was collect | recovered, and water-soluble selenium density | concentration was measured by JISK0102. The results are shown in Table 5.

  In the case of potassium permanganate concentrations of 1.0 g / L and 2.0 g / L, no difference was observed in the measured selenium value. However, when the potassium permanganate concentration is 4.0 g / L or 8.0 g / L, the measured selenium value decreases as the potassium permanganate concentration increases, and the selenium in the residual liquid tends to increase. Was recognized.

  The cause of this was thought to be that excess potassium permanganate reacted with hydrochloric acid, which was a reducing agent, in the second treatment part, resulting in insufficient reduction of hexavalent selenium.

  Here, since concentrated hydrochloric acid (12M) is used for the reduction treatment of hexavalent selenium, it is difficult to increase the hydrochloric acid concentration. In addition, an increase in the amount of hydrochloric acid added leads to an increase in the hexavalent selenium reduction treatment time, causing an increase in the analysis time of the entire system.

  Therefore, in order to solve this problem, the amount of sample water collected should be reduced, or the amount of potassium permanganate added should be adjusted so that the potassium permanganate concentration is suitable for the COD concentration of the waste water. It was considered.

Here, the oxidation reaction by KMnO ₄ is represented by the following formula.
MnO ₄ ⁻ + 8H ⁺ + 5e ⁻ → Mn ²⁺ + 4H ₂ O (a)
3Mn ²⁺ + 2MnO ₄ ⁻ + 2H ₂ O → 5MnO ₂ + 4H ⁺ (b)
When KMnO ₄ is excessively present relative to the oxidizable substance, the formulas (a) and (b) proceed sequentially, and MnO ₂ is generated.
Next, the oxidation reaction with oxygen is represented by the following equation.
O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (c)
From the formulas (a) and (c), it can be seen that 1 mol of MnO ₄ ⁻ oxidizes the oxidizable substance 2.5 times as much as one oxygen atom in the formula (c). That is, 158 g (1 mol) of KMnO ₄ oxidizes an oxidizable substance equivalent to 40 g-O.
Here, the amount of oxygen necessary for the oxidizable substance of the wastewater is 0.08 mg-O when oxidizing 5 mL of wastewater having a COD concentration of 16 mg-O / L.
The amount of oxygen that can be supplied by KMnO ₄ is 1.0 mg-O, for example, when 1 mL of a ₄ g / L KMnO ₄ aqueous solution is added. That is, when 1 mL of ₄ g / L KMnO ₄ aqueous solution is added, it becomes 12.5 times (1.0 / 0.08) the amount of oxygen atoms necessary for the organic substance decomposition treatment.
When the same calculation is carried out, it becomes 3.1 times at 1 g / L, 6.3 times at 2 g / L, and 25 times at 8 g / L (all added amounts are 1 mL).

  From the above calculation results, if the amount of COD in the wastewater is 3 to 10 times, preferably 5 to 10 times, the organic matter in the sample water can be decomposed without inhibiting the reduction reaction in the second treatment section. It was considered.

Claims (2)

A first pretreatment in which potassium permanganate is added and heated as an organic substance decomposing agent to the sample water; and a second pretreatment in which hydrochloric acid is added and heated as a reducing agent to reduce hexavalent selenium to tetravalent selenium; In order, the tetravalent selenium contained in the sample water is reacted with sodium tetrahydroborate to generate hydrogen selenide gas, the hydrogen selenide gas is led to a hydrogen selenide gas detector, and the selenium In the method of analyzing the water-soluble selenium concentration of the sample water based on the signal value detected by the hydrogen fluoride gas detector,
In the first pretreatment, sulfuric acid is further added to decompose a sulfur compound capable of generating hydrogen sulfide gas by reacting with sodium tetrahydroborate together with organic substances contained in the sample water. Selenium analysis method.   The water-soluble selenium analysis method according to claim 1, wherein the sulfuric acid concentration of the sample water is adjusted to 1.3 to 3 mol / L by adding the sulfuric acid.

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