JP4860588B2 - Ion concentration measuring device - Google Patents

Description

  The present invention relates to an ion concentration measuring apparatus for measuring an ion concentration by measuring a potential difference generated between an ion selective electrode immersed in a sample solution and a reference electrode.

  Conventionally, an ion electrode method has been used for measuring an ion concentration in an aqueous solution. In the ion electrode method, for example, as shown in Non-Patent Document 1, measurement is performed by combining an ion electrode that responds to specific ions and a reference electrode that outputs a reference potential, and measuring the potential difference between the two electrodes. Obtain the concentration of the target ion.

  As the ion selective electrode, a glass thin film type, a solid film type, a liquid film type, etc. are used. However, these ion selective electrodes change in sensitivity and responsiveness with use time and the number of times of use. No response to. For this reason, an ion measuring device that combines an ion-selective electrode and a reference electrode can be calibrated periodically (for example, every month) to maintain the required accuracy, and those that can no longer be calibrated are replaced as deteriorated lifetimes. The function is maintained by replacing the sensor portion used for a certain period of time or by replacing the sensor portion.

  However, even if a deteriorated product or a product with a limited service life is eliminated by periodic preventive maintenance measures based on the time of use or the number of times of use or sensitivity check during calibration, measurement may suddenly become impossible during use. Therefore, it has been difficult to determine whether or not the deterioration is caused only by the calibration result. In other words, the ion sensor is calibrated periodically, but if the sensor malfunctions during this time, the measured value is used in an inaccurate state, causing a malfunction of the plant. It was.

  Here, it is known that the internal impedance (resistance) of the ion-selective electrode and the comparative electrode changes when a deterioration phenomenon such as contamination of the sensitive film surface, deterioration of the sensitive film, or change in the concentration of the internal liquid proceeds. Thus, several attempts have been proposed to use this phenomenon for detection of deterioration.

  For example, in Patent Document 1, a power source means for supplying a low-frequency alternating current between an ion-selective electrode and a comparison electrode is provided, and an alternating current is generated in a direct-current potential generated between both electrodes according to the ion concentration in the sample solution. Has disclosed an apparatus for measuring the internal impedance by superimposing the alternating current potential generated by the above and measuring the alternating current potential.

  Further, in Patent Document 2, a third electrode is immersed in the sample solution in addition to the ion selective electrode and the comparison electrode, and an alternating current is passed between the third electrode and the comparison electrode in the inspection mode, An apparatus for inspecting a reference electrode is disclosed.

  Further, in Patent Document 3, an AC voltage is applied to one input terminal of an operational amplifier that measures a potential difference between an ion selective electrode and a comparison electrode, and an alternating current signal output from the operational amplifier is used as a comparison electrode and a sample solution. Measures internal impedance by means of applying to a series circuit composed of ion-selective electrodes and AC current flowing through the series circuit via a DC cut capacitor, converting the AC current into AC voltage, rectifying and smoothing An apparatus is disclosed.

JP 58-92854 A JP 58-99745 A Japanese Patent Laid-Open No. 8-21115 G. J. et al. Moody and J.M. D. R. Co-authored by Thomas “Ion-selective electrode” Kyoritsu Shuppan (1977)

  As described above, various ion concentration measuring apparatuses capable of coping with sensor deterioration have been proposed, but each has the following drawbacks.

  In the apparatus of Patent Document 1, a switch is connected to the high impedance input side of an amplifier that measures a potential generated between an ion selective electrode and a comparison electrode, and problems are likely to occur in terms of insulation. In addition, the technique disclosed in the patent cannot perform ion concentration measurement and deterioration determination at the same time.

  In the apparatus of Patent Document 2, only the deterioration of the reference electrode can be detected. In addition, the cost is increased because the third electrode is required.

  In the apparatus of Patent Document 3, ion concentration measurement and internal impedance measurement can be performed simultaneously. However, the cable-to-line capacitance and inductive noise from the sensor's surrounding environment (mainly hum noise from the commercial power supply) are not taken into account, so it is difficult to measure the internal impedance when the cable is long, and A large impact.

  The present invention relates to an ion concentration measuring apparatus for measuring an ion concentration by measuring a potential difference between an ion selective electrode and a reference electrode immersed in a sample solution by using a buffer amplifier having a high input impedance. And an application means for applying an alternating current signal to the comparison electrode, and an ion component connected to the ion-selective electrode and connected to the common input point with the buffer amplifier. AC signal measuring means for measuring using a voltage conversion amplifier, and isolating the power source and grounding point of the buffer amplifier from the power source and grounding point of the current-voltage conversion amplifier, and the buffer amplifier and current − While isolating the output of the voltage conversion amplifier and measuring the potential difference, And detecting the deterioration of the ion-selective electrode.

  Further, it is preferable to sample in synchronization with the frequency of the AC signal to which the measurement value of the AC component is applied.

  According to the present invention, the DC voltage detection amplifier and the AC signal detection amplifier power supply and ground are separated from each other and are connected to the ion-selective electrode in a shared manner, whereby the DC input resistance of the current-voltage conversion amplifier is obtained. The electrode impedance can be measured without affecting the ion concentration measurement.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows an example of an electrode impedance circuit block diagram, FIG. 2 shows an equivalent circuit around the sensor, FIG. 3 shows the configuration and equivalent circuit of the ion selective electrode and the comparison electrode, and FIG. 4 shows an equivalent circuit of the AC impedance.

  The ion sensor unit 1 includes an ion selective electrode 10 that generates a voltage (Eg) proportional to an ion concentration, and a comparison electrode 12 that generates a reference voltage (Er). The internal resistance (Rg) of the ion selective electrode 10 is a non-defective product and is several hundred kΩ to several MΩ, but the internal resistance (Rr) of the comparison electrode 12 is about 10 kΩ and is much lower than the impedance of the ion selective electrode. . Therefore, there is no problem even if the impedance between the two electrodes is the impedance of the ion selective electrode. The ion selective electrode 10 has an internal capacitance (Cg) in parallel with its internal resistance (Rg), but this internal capacitance is as small as several tens of pF and is almost negligible.

  The comparison electrode 12 is connected to the comparison electrode terminal (RT), and the ion selective electrode 10 is connected to the ion selective electrode terminal (GT). The comparative electrode terminal (RT) and the ion selective electrode (GT) are connected to the measuring device by a shield wire, and there is a capacitance between cable wires (Cs) between the shield and the core wire, and this capacitance is, for example, 10 m. The cable is about 1500 pF.

  The impedance measurement block 2 is connected to the comparison electrode terminal (RT) and the ion selective electrode terminal (GT). The impedance measuring block 2 is provided with an oscillator 20, and the output of the oscillator 20 is supplied to a driver 22. The driver 22 sets the output of the oscillator 20 to a rectangular wave having a predetermined size. The output of the driver 22 is connected to a reference electrode terminal (RT) via a DC blocking capacitor C. For this reason, a rectangular wave having a predetermined frequency is applied to the comparison electrode 12 via the comparison electrode terminal (RT).

  A current-voltage conversion amplifier 24 is connected to the ion selective electrode terminal (GT), where the current signal is converted into a voltage signal. The output of the current-voltage conversion amplifier 24 is supplied to the AC-DC conversion circuit 26. The AC-DC conversion circuit 26 is also supplied with an output from the oscillator 20, where the output of the current-voltage conversion amplifier 24 is detected to convert it into a voltage proportional to the current.

  As described above, in the impedance measurement block 2, a rectangular wave having a predetermined frequency is applied to the comparison electrode 12, and the magnitude of the AC component output through the ion selective electrode 10 is output. Therefore, the impedance of the ion sensor unit 1 can be measured with this size.

  On the other hand, the potential difference (DC potential) measurement output according to the concentration is performed in the ion measurement block 3. That is, the ion selective electrode terminal (GT) is supplied to the low-pass filtering (LPF) circuit 32 of the ion measurement block 3. The LPF circuit 32 attenuates AC signals and noise. The output of the LPF circuit 32 is input to a buffer amplifier 34 having a high input resistance. The buffer amplifier 34 impedance-converts the DC potential of the ion selective electrode 10 and outputs it as a stable voltage.

  The output of the current-voltage conversion amplifier 24 of the impedance measurement block 2 is supplied to the control unit 4 via the isolator 52, and the output of the buffer amplifier 34 of the ion measurement block 3 is supplied to the control unit 4 via the isolator 54. The control unit 4 includes a multiplexer (MPX) 42, an analog-digital converter (ADC) 44, and an arithmetic processing unit (MPU) 46, and impedance measurement values and ions supplied via the isolators 52 and 54. A DC voltage signal for the concentration measurement value is supplied to the MPX 42. The multiplexer 42 switches analog signals for impedance measurement values and ion concentration measurement values and supplies them to the ADC 44, and the ADC 44 converts the supplied analog signals into digital data. The impedance measurement value and the ion concentration measurement value as digital data that is the output of the ADC 44 are processed by the MPU 46, and the impedance measurement value and the ion concentration measurement value are calculated here. That is, what is supplied to the MPU 46 is digital data indicating a DC voltage value. The MPU 46 calculates the ion concentration using a calibration curve based on standard data and the like, and evaluates the impedance measurement value by comparing with a threshold value determined based on the impedance value of a normal sensor unit. To do.

  FIG. 2 shows an equivalent circuit for the ion sensor unit 1, the impedance measurement block 2, and the ion measurement block 3 described in FIG.

  A buffer amplifier 34 is connected to the ion selective electrode terminal (GT) via the LPF 32. That is, one end of the buffer amplifier 34 is connected to the first ground GND1, and the output from the LPF 32 is input to the other end. Accordingly, the buffer amplifier 34 outputs a DC voltage that has passed through the LPF 32 and is a difference between the DC component and the ground GND1. In FIG. 2, the LPF 32 is indicated by a resistor R and a capacitor C.

  Note that the ground GND1 is isolated (electrically separated) from the ground GND2, and the ground GND1 rises and falls according to the AC signal, but a difference in DC component appears in the output of the buffer amplifier 34.

  The ion selective electrode terminal (GT) is input to one end of the current-voltage conversion amplifier 24, and the other end is connected to the ground GND2 that is isolated from the ground GND1. The output of the current-voltage conversion amplifier 24 is negatively fed back to the current-voltage conversion amplifier 24 via the feedback resistor Rf. The AC component at the ion selective electrode terminal (GT) is amplified and output at the output of the current-voltage conversion amplifier 24.

  3 shows an equivalent circuit for the ion sensor unit 1 and the ion measurement block 3, and FIG. 4 shows an equivalent circuit for the ion sensor unit 1 and the impedance measurement block 2. Thus, the ion sensor unit 1 and the ion measurement block 3 can be written as separate circuits. In addition, the power supply of each circuit of the impedance measurement block 2 and the ion measurement block 3 is also a separate power supply.

  As shown in FIG. 3, the output DC voltage of the ion sensor unit 1 is output from the buffer amplifier 34. This output basically corresponds to Eg-Er.

  As shown in FIG. 4, an AC signal is applied to the reference electrode terminal (RT), and the impedance between the electrodes is determined by the magnitude of the AC signal flowing through the ion selective electrode terminal (GT).

  Here, since the internal impedance of the ion selective electrode 10 and / or the comparison electrode 12 is high, in the ion measurement block 3 for measuring the voltage difference between the two electrodes, the internal resistance of the ion sensor unit 1 composed of both electrodes is reduced. Therefore, it is necessary to have a high DC input resistance that does not affect the operation. On the other hand, in the impedance measurement block 2, it is necessary to reduce the input resistance in order to measure the electrode current.

  In order to perform the ion concentration measurement in the ion sensor unit 1 and the impedance measurement of the ion sensor unit 1 at the same time, a voltage follower type buffer amplifier 34 with a high input resistance and a current-voltage conversion amplifier 24 with a low input resistance are provided. If these are connected to the sensor in common, the input resistance is lowered, and as a result, ion measurement cannot be performed.

  In the present embodiment, the ground of each circuit is isolated, and only the electrode of the ion measuring unit is shared and connected, so that the DC input resistance of the current-voltage conversion amplifier 24 becomes so high that it can be ignored, thereby affecting the ion measurement. Electrode impedance can be measured.

  In this embodiment, the ion selective electrode 10 generates and outputs a voltage corresponding to a specific ion concentration, and can be applied to a glass electrode, a solid film type, a liquid film type ion electrode, or the like. .

  Moreover, the comparative electrode 12 can use a well-known thing, for example, what has a potassium chloride solution as an internal liquid, has a silver-silver chloride electrode as an internal electrode, and used porous bodies, such as ceramics, as a liquid junction part. Can be used.

  As the buffer amplifier having a high input impedance, a known one can be used as long as the input current is 1 nA or less, such as an FET (Field Effect Transistor) input OP-AMP.

  For the oscillator 20 serving as an AC signal source, a known circuit such as a multivibrator oscillation circuit can be used. Here, the frequency of the AC signal is preferably several tens Hz to several hundreds Hz. This is because the influence on the ion concentration measurement system circuit side is unavoidable at 10 Hz or less, and the influence of the capacitance Cs between the cables cannot be excluded at 1000 Hz or more. Furthermore, it is more preferable not to become an integral multiple of the commercial power supply frequency (50 Hz / 60 Hz). This is because if it is an integral multiple of the commercial power supply frequency, it is likely to be affected by inductive noise synchronized with the commercial power supply frequency.

  The current-voltage conversion amplifier 24 preferably has a feature that the internal noise voltage and the input current are small.

  As a means for separating the grounding point of the buffer amplifier 34 and the current-voltage conversion amplifier 24, a photocoupler, an isolation amplifier, or the like can be used. The analog isolation amplifier is particularly preferable because the circuit scale can be reduced and high-density mounting can be supported.

  As a method of detecting the deterioration of the ion selective electrode from the measured value of the AC signal component in the MPU 46 of the control unit 4, a method of comparing with a preset value or a method of detecting a sudden change in the change rate can be used. .

  Here, if there is a capacitance (Cs) between the cable lines between RT and GT, the high frequency component of the drive signal is bypassed, and as shown in FIG. 5, overshoot and undershoot are generated at the rising and falling portions of the signal. Produce. For this reason, if it converts into direct current | flow using general peak rectification, the impedance inside an electrode cannot be measured correctly.

  As a method of sampling without being affected by this influence, it is possible to use synchronous detection using a sample / hold amplifier.

  That is, as shown in FIG. 7, a ¼ frequency divider 62 is provided between the oscillator 20 and the driver 22. As a result, as shown in FIG. 8, with respect to the output from the oscillator 20, a signal divided by 1/2 and a signal divided by 1/4 are obtained. Then, the signal divided by 1/4 is output from the driver 22. On the other hand, the signal divided by 1/2 from the output of the oscillator 20 is input to the NOR gate 64, from which a sampling clock is obtained. This sampling clock is a signal that rises at 3/4 passage of the H level portion and L level portion of the signal divided by 1/4. Therefore, this sampling clock is supplied to the AC-DC conversion circuit 26, and at the rising edge of the sampling clock, the AC signal can be detected at the H level and the L level at a timing that excludes the sampling overshoot and undershoot portions, In the AC-DC conversion circuit 26, the amplitude of the AC signal can be correctly detected from the obtained difference.

  FIG. 9 shows the gradient potential difference (mV), which is the difference in the output voltage of the buffer amplifier 34 according to the change of the ion concentration 1 → 10 mg / l, for eight samples of the ion selective electrode 10 (manufactured by Toa DKK). It is a graph of the output voltage of the current-voltage conversion amplifier 24 corresponding to the amplitude of the alternating current output from the ion selective electrode terminal (GT). Here, the electrode whose sample electrode number is younger is used for a longer time. From this, it can be seen that the gradient potential difference, which is a general lifetime determination factor, is reduced by use. It can also be seen that the longer the electrode used, the higher the internal impedance and the smaller the current amplifier output voltage.

  FIG. 10 shows the difference in measurement voltage between synchronous detection and peak detection when the sensor cable is 10 m long. As described above, according to the synchronous detection, the voltage is almost 0 in the sample electrode numbers 1 to 4, but the peak detection has an output of about 100 mA. From this, it can be understood that the degree of deterioration cannot be accurately detected in the peak detection due to the influence of the cable line capacitance Cs.

  Thus, in this embodiment, the synchronous detection system which samples the amplitude of the part in which overshoot and undershoot fall is introduced, and the influence of the line capacitance is eliminated. As a result, measurement is possible even when the cable is long.

  As described above, according to the present embodiment, the degree of deterioration of the ion sensor unit can be checked in real time simultaneously with the ion concentration. Therefore, it is possible to take a quick response to a sudden sensor abnormality. In addition, sensors that have been regularly replaced for safety reasons can also be determined to extend the usage period, which can reduce running costs. Since it is applicable even when the cable is long, it is suitable for a device that requires high reliability, such as an industrial management device or an inspection device for continuous operation that processes the signal from the ion sensor to control the ion concentration.

It is a block diagram of an electrode impedance circuit. It is an equivalent circuit diagram around the sensor. It is an equivalent circuit of an ion selective electrode. It is an equivalent circuit diagram of AC impedance. It is a figure which shows the influence of the capacity | capacitance (Cs) between the lines of a cable. It is a figure which shows the timing which is not influenced by the capacity | capacitance between cables. It is a figure which shows the circuit of the production | generation of a sampling clock. It is a figure which shows the production | generation of a sampling clock. It is a figure which shows the difference of the gradient value of an ion selective electrode, and current amplifier output. It is a figure which shows the difference between the time of synchronous detection, and peak detection.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Ion sensor part, 2 Impedance measurement block, 3 Ion measurement block, 4 Control part, 10 Ion selective electrode, 12 Comparison electrode, 20 Oscillator, 22 Driver, 24 Current-voltage conversion amplifier, 26 AC-DC conversion circuit, 32 LPF circuit, 34 buffer amplifier, 42 multiplexer, 52, 54 isolator, 62 frequency divider, 64 NOR gate.

Claims (2)

In an ion concentration measurement apparatus that measures an ion concentration by measuring a potential difference between an ion selective electrode and a reference electrode immersed in a sample solution using a buffer amplifier with a high input impedance,
Applying means for applying an AC signal for measuring electrode impedance to the reference electrode;
AC signal measuring means connected to the ion selective electrode, connected to a common input point with the buffer amplifier, and measuring an AC component of a current flowing between the electrodes using a current-voltage conversion amplifier with a low input impedance; ,
With
While isolating the power source and grounding point of the buffer amplifier and the power source and grounding point of the current-voltage conversion amplifier, isolating the output of the buffer amplifier and the current-voltage conversion amplifier while measuring the potential difference An ion concentration measuring device for detecting deterioration of an ion selective electrode from a measured value of the alternating current component.   2. The ion concentration measuring apparatus according to claim 1, wherein sampling is performed in synchronization with a frequency of an AC signal to which the measurement value of the AC component is applied.

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