INSTRUMENT FOR POTENTIOMETRIC ELECTROCHEMICAL MEASUREMENTS IN AN ELECTROLYTE SOLUTION

Abstract: There is provided a system for measuring a selected ion in a solution so as to avoid noise pickup from AC parasitic currents and measurement offset from DC parastitic currents, such as are frequently found in pH measurements on low conductivity grounded solutions like high purity water. The system in one form has an operational amplifier with an ion selective electrode connected to its inverting input, a counter electrode connected to its output, and circuit common connected to the non-inverting input, so that the potential of the ion selective electrode is continuously driven to circuit common. The reference electrode of the system is connected to a high impedance measuring circuit for measuring the voltage level of that electrode with reference to circuit common as a measure of the concen­ tration of the selected ion in the solution.

Description: This invention relates to apparatus for potentiometric electrochemical measurements employing ion selective probe systems such as glass pH electrode systems or other ion-selective electrode systems and ISFET pH devices. It is particularly useful in making pH measurements with these probe systems in solutions of low electrolytic conductivity, such as high purity water. A typical pH measuring system, as found in the prior art, is shown in U.S. Patent 4,189,367, issued to the present inventors on Feb. 19, 1980. That system utilizes an operational amplifier to drive the reference electrode through the low impedance output of the amplifier to maintain the pH electrode, connected to the amplifiers high input impedance summing junction, at signal common voltage. Such a measurement approach is entirely adequate for solutions with an electrolytic conductivity greater than 1 microsiemens. However, when such systems are used for pH measurements of earth grounded solutions of high purity water (less than 0.1 microsiemens), such as are found in electric power plants, it has been observed that they are subject to noise pickup and, also, to an offset or shift in the pH reading. It has further been discovered that the major source of error in high purity water measurements is caused by parasitic leakage currents flowing from the grounded solution through the reference electrode when said electrode is connected to a low impedance point in the electronic circuit. These leakage currents then flow through the low impedance path to the common or ground terminal of the power supply, through the power transformer, and through leakage on printed circuit cards in the system to produce, as a result, a leakage path between the circuit common and the AC power line. The instantaneous effects of the leakage currents which flow through this path are produced in direct proportion to the electrolytic resistance of the solution at the reference electrode/solution interface. The presence of parasitic AC currents in this path produces noise in the measurement and parasitic DC currents product a shift in output, both are directly observed as errors in the pH reading. Longer term integration of DC leakage currents and electrode rectified AC currents will result in a drift of electrode output with a commensurate drift in instrument accuracy. One arrangement for reducing leakage currents to ground is described by Creason and Delettrez in ISA Transactions, Volume 16, number 3, pages 67-71, "Problems and Solutions in pH Meaurement", wherein the reference electrode is tied directly to the input of a separate high-input-impedance amplifier (as is the glass pH electrode) and is not a convenient path to ground. In accordance with this invention there is provided an apparatus for measuring the concentration of a selected ion in a grounded electrolyte solution comprising an ion selective probe system including an ion selective device responsive to the concentration of a selected ion in the solution and a reference electrode; a potential measuring device connected to said reference electrode; and a control amplifier connected to said probe system, characterized in that the potential measuring device has a high impedance input connected to said reference electrode; and in that the output of the control amplifier is connected to said probe system through a counter electrode in such magnitude and sense to maintain the electrical characteristic of said ion selective device relative to the circuit common potential such that the potential of said reference electrode, as measured by said measuring device, provides an indication of the concentration of said selected ion in said solution. In the drawings, where like references refer to like elements: Fig. 1 is a circuit diagram of a known prior art potentiometric electrochemical measuring system for measuring pH. Fig. 2 is a circuit diagram of a pH measuring system which utilizes the invention with a glass electrode system. Fig. 3 is a circuit diagram of a pH measuring system which utilizes an ISFET type pH sensitive device. Fig. 1, which illustrates the prior art systems, utilizes the control amplifier, shown as an operational amplifier 10, to drive the reference electrode 12 from the amplifier's low impedance output on line 14 so as to maintain the glass pH electrode 16, connected by line 18 to the inverting input of the amplifier, substantially at circuit common. Circuit common is the potential at the non-inverting input of the amplifier, shown as line 20. By holding the glass electrode at circuit common, the potential on line 14 at the output of the amplifier 10 is indicative of the pH of the solution 22, shown as being in a grounded container 24. A microprocessor based measuring system 26 is shown as a preferred means for processing the output of the amplifier 10 to provide an indication of the pH of the solution on the display device 28: an analog implementation can also be used. With solutions having a conductivity below 1 microsiemens, for example, as is the case with high purity water, it has been found that spurious AC as well as DC currents may be present in the system and can therefore be picked up by the system. AC currents can cause noise and the DC currents can cause shifts in the readings on the display. An undesirable side effect, which is not immediately evident, is the possible deterioration of the reference cell which may result from those currents. In view of these problems, it is highly desirable to avoid such spurious currents in the reference electrode and to find a way to keep them from interfering with the measuring system. Preventing the spurious currents by enhancing power supply isolation is both difficult and costly. It has been found that the leakage currents, per se, are not the problem, but their flow into the reference electrode is the source of the effects noted. It is desirable, therefore, to connect the reference electrode to a low leakage, high impedance system in order to inhibit current flow. This has been accomplished by inserting an additional metal electrode, such as electrode 30, into the solution being measured in order to channel the spurious currents through that electrode rather than through the reference electrode. This additional electrode may be properly called a counter electrode. Thus, in Fig. 2 a metal wire is shown as counter electrode 30, and it is connected to the output of the amplifier 10, where the reference electrode was connected in the prior art systems. The reference electrode 12 is then connected to the measuring circuit, which consists of the microprocessor based measuring system 26 and the display unit 28. In order to further prevent the flow of spurious currents in the reference electrode, as was mentioned above, it is necessary to connect it to the measuring system through a high impedance circuit, such as a the buffer amplifier 32. A further advantage accrues from improved reference electrode stability because the reduction of spurious AC and DC currents reduces electrochemical changes in the reference electrode. In the circuit of Fig. 2, the counter electrode is driven by the output of the amplifier so that the pH electrode is maintained at signal common, as occurred in the prior art system of Fig. 1 with the reference electrode connected to the amplifier output. The voltage obtained at the metal counter electrode is unimportant, for it only serves to maintain the glass electrodes potential at signal common. The counter electrode may, by way of example, be a metallic electrode housing or an earth grounded pipe fitting in close proximity to the pH electrode. Because its potential is unimportant, parasitic currents in the counter electrode are of no consequence. Since the glass electrode is controlled to be at circuit common, it is only necessary to measure with a measuring circuit of high input impedance the potential of the reference electrode versus signal common in order to obtain a measure of the pH. With this circuit, it has been found that AC noise rejection has been improved, and there is virtual immunity to DC leakage currents. The principles of this improved circuit can also be applied to ISFET probe systems, as is shown in Fig. 3. In that figure a P-channel enhancement mode ISFET 36 has its source connected to the inverting input of the amplifier 10. The voltage source -V d controls the drain-source voltage at the preselected level. A source of potential is connected to the inverting input of the amplifier by way of a resistor R₁, shown as 38, which controls the drain current at V₁/R₁. As is also shown in Fig. 3, the reference electrode 12 is connected to the input of the measuring system and the output of the amplifier is connected to a counter electrode 30. The ISFET electrode system may, in one of its possible forms, be constructed as disclosed in U.S. Patent 4,505,799, issued on March 19, 1985 to Ronald D. Baxter, a coworker of ours.

Claims: Apparatus for measuring the concentration of a selected ion in a grounded electrolyte solution (22) comprising an ion selective probe system including an ion selective device (16, 36) responsive to the concentration of a selected ion in the solution and a reference electrode (12); a potential measuring device (32, 26) connected to said reference electrode; and a control amplifier (10) connected to said probe system, characterized in that the potential measuring device (32, 26) has a high impedance input connected to said reference electrode (12); and in that the output of the control amplifier (10) is connected to said probe system through a counter electrode (30)in such magnitude and sense to maintain the electrical characteristic of said ion selective device relative to the circuit common potential such that the potential of said reference electrode, as measured by said measuring device, provides an indication of the concentration of said selected ion in said solution. Apparatus according to claim 1, characterized in that the control amplifier (10) is an operational amplifier having its inverting input connected to the ion selective device (16, 36), its non-inverting input connected to circuit common, and its output connected to the counter electrode (30) so that said amplifier tends to drive the potential of the ion selective device to circuit common potential. Apparatus according to claim 2, characterized in that the ion selective device is a pH glass electrode. Apparatus according to claim 1, characterized in that said ion selective device (16, 36) is an ISFET, and in that the amplifier is connected to the probe system so that its output through the counter electrode (30) is effective to maintain a constant drain-to-source voltage and drain-to-source-current. Apparatus according to claim 1, characterized in that the counter electrode (3) is a metal electrode. Apparatus according to claim 4, characterized in that the ISFET is responsive to pH and has its source connected to a voltage supply by way of a resistor (38). Apparatus according to claim 6, characterized in that the counter electrode (30) is physically mounted in close proximity to said ISFET. Apparatus according to claim 1, characterized in that the electrolyte solution (22) is a grounded solution having low conductivity.

Inventors: CONNERY, JAMES G., SHAFFER, EARL W., JR.
Applicants: GENERAL SIGNAL CORPORATION
Application year: 1988
Main classification: G01N27
IPC codes: G01N27