Method and apparatus for testing the integrity of an electrode in a potentiometric measuring electrode system

A testing method and apparatus for testing the condition of electrodes and their conductors in ion sensitive measuring or controlling systems, wherein the measuring circuit is disruptive and replaced by an impedance measuring circuit containing only one electrode, so that also failures of, for example, reference electrodes, soiling and deposits, and electrolyte loss, can be determined. The arrangement has sufficient accuracy to detect faults, defects and failures as they slowly come into existence.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to a method and apparatus for testing the integrity 
of an electrode in an electrode system for measuring an ion concentration 
or a redox potential in a solution, wherein in the circuit containing the 
electrode, a two directional electrical quantity is injected to determine 
the impedance of the electrode. 
2. Discussion of the Prior Art 
A conventional method and apparatus to carry out such testing is disclosed 
for example in U.S. Pat. No. 4,189,367 wherein the electrode system 
contains a high resistance ion selective membrane and a reference 
electrode. The impedance of the ion selective membrane is considerably 
greater than that of the reference electrode. For example, this may be by 
a factor 10 times greater than the combined impedance of the reference 
electrode and the solution. In practice, the impedance of the reference 
electrode is often about 10.sup.-3 to 10.sup.-5 times smaller than that of 
the high resistance ion selective electrode, which may be, for example, of 
glass. 
In another example, U.S. Pat. No. 3,661,748 discloses a method and 
apparatus for testing the integrity of electrodes, wherein apart from 
electrodes for measuring concentrations of a gas, such as O.sub.2 or 
C0.sub.2 (which electrodes need a drving voltage and are not considered in 
this invention), a testing circuit is used for a series connection of a 
potentiometric electrode, such as a pH electrode, and a reference 
electrode. In this circuit, an AC voltage is injected in the series 
connection and phase-sensitive measuring is done of the AC current passing 
through the circuit. The AC voltage is applied to the system via an 
auxiliary electrode in contact with the liquid monitored by the ion 
sensitive electrode. 
In both prior art disclosure, disadvantageously, the measuring circuit is 
maintained intact during the testing period. The result is that no 
individual testing of the ion sensitive electrode and the reference 
electrode is possible and that test values of each may be relatively 
inaccurate, whereas several types of failures may escape discovery, as may 
be apparent from the below discussion. 
Apart from the high resistance membrane electrodes, such as glass 
electrodes, low impedance potentiometric electrodes exist, such as redox 
electrodes, which are selectively sensitive to for example Na, K or other 
ions. 
When testing electrode systems, many types of electrode failures, faults 
and defects may occur. For example, apart from a short circuit originating 
from a crack in a glass electrode, for example, misreadings may be due to 
many other causes. Examples of other causes are: 
(A) The connection with an electrode may be interrupted. In that case, its 
reading is highly constant, but may be in the region of normal readings. 
(B) The electrode may be soiled or coated by deposits, so that its 
impedance increases and its sensitivity decreases, thus causing 
misreadings. 
(C) The reference electrode may fail, due to loss of electrolyte, in which 
case its impedance will increase and its output may float, so that the 
value indicated by the electrode system will be false, but remain within 
the region of possible readings. 
(D) The reference electrode is poisoned. For example, in case of a 
reference electrode of the type, metal-metal halide electrolyte, this may 
lead to a very strong increase of impedance, and even isolation of the 
electrode from the electrolyte, which will cause misreadings. 
(E) The membrane of the reference electrode between its electrolyte and the 
liquid being monitored may become clogged, thus, leading to a high 
impedance of the reference electrode, which can even be put out of action. 
In practice, failure to the reference electrode is a greater danger than 
the failure of, for example, the glass electrode. Furthermore, it is 
desired to be able to indicate not only the break down of a high 
impedance,but also other types of failures. 
Thus, it can be appreciated that the prior art is replete with 
disadvantages and deficiencies. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to overcome the aforementioned 
and other disadvantages and deficiencies of the prior art. 
Another object is to provide a highly reliable method and apparatus for 
testing and monitoring electrodes in an electrode system. 
A further object is to provide a monitoring method and apparatus, which are 
able to detect failure of each of the different electrodes of an electrode 
system for measuring ion concentration or a redox potential. 
A still further object is to provide a method and apparatus which allow 
detection of impedances which are outside of a normal range, either being 
too high or too low. 
The foregoing and other objects are attained by the invention which 
provides, in its broadest aspects, a method as indicated above, wherein 
the measuring circuit is replaced by test circuits, each containing only 
one of the electrodes of the system, and an auxiliary electrode, which 
test circuits each contain at least one impedance adapted to the impedance 
value of the one electrode of the system. Because the measuring circuit is 
not used during the test periods, the injected two dimensional electrical 
quantity cannot disturb the measurement, so that higher accuracy is 
obtained. 
In several electrode systems for measuring ion concentration in a solution, 
a potential equalization electrode is present. such an electrode serves to 
clamp the electrical potential of the liquid to be measured at a 
predetermined voltage level, for example, to prevent the measuring 
electronics or instruments from receiving voltages outside their working 
range. Such an electrode may be of an inoxydizable metal, such as 
stainless steel. For normal measurements, it cannot replace the so-called 
reference electrode, which preferably contains a metal, a halide of that 
metal, and a halide electrolyte. For the purpose of the invention, the 
potential equalization electrode is, however, quite suitable, because any 
DC voltage component does not play any part whey carry out the method of 
the invention. Accordingly, a preferred embodiment provides that the 
auxiliary electrode be the potential equalization electrode. 
In the known system of U.S. Pat. No. 4,189,367, the electrical quantity to 
be injected in the system is a current. This may result in difficulties in 
case of circuit interruption. 
Accordingly, it is preferred to provide that the two dimensional electrical 
quantity be a square wave or a block voltage. A block voltage has the 
advantage that a very stable level is present for sampling. When sampling 
in the second half of the duration of one of the voltage levels, the 
sample value will be practically stable. 
With the known method of the above U.S. Pat. No. 4,189,367, the temperature 
of the liquid is measured and the value of the electric current to be 
supplied to the electrode system is determined on the basis of the 
measured temperature. A temperature compensation is applied in the system 
because the impedance of the glass membrane is strongly dependent on the 
temperature and in fact doubles for temperatures increases of about 
10.degree. C. 
When practicing the invention, an analoguous temperature compensation can 
be applied by varying the applied voltage or the circuit for measuring the 
current flowing as a result of this voltage. A further possibility is to 
measure the impedance of a monitored electrode and to compare the 
impedance with a value calculated on the basis of the liquid temperature. 
The invention also encompasses an apparatus for testing the integrity of an 
electrode in an electrode system for measuring ion concentration or a 
redox potential in a solution, comprising terminals connected to a 
measuring electrode, a reference electrode and an auxiliary electrode, 
means for generating an electrical test quantity for feeding it to at 
least one of the terminals, switching means for forming a test circuit 
connected to the terminal of the auxiliary electrode and the terminal of a 
selected electrode of the measuring electrode system, first switching 
means for connecting the means for generating the electrical test quantity 
to one of the terminals, second switching means for connecting a balance 
impedance to the terminal of the one electrode of the measuring electrode 
system or that of the auxiliary electrode, and third switching means for 
connecting the terminal of the selected electrode to an output device. 
Advantageously, the apparatus is suitable for successively testing the 
electrodes of an ion sensitive electrode system. A further advantage of 
such an apparatus is, that it is possible to switch in a measuring circuit 
adapted for the monitored impedance. In a practical case, a glass membrane 
is compared with a resistance which is considerably greater, for example, 
5 times greater, than a resistance to be used for comparing with a 
reference electrode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention is further described with reference to the single FIGURE of 
the drawing, wherein a vessel 1 contains liquid 2, of which ion 
concentration, for example, the H ion concentration pH or the sodium ion 
concentration pNa is to be measured. This is done with an ion sensitive 
electrode system having a reference electrode 3 and an ion sensitive high 
impedance glass electrode 4. A potential equalization electrode 5 is also 
immersed in liquid 2. Electrodes 3,4 and 5 are connected to terminals 
6,7,8 respectively, which serve as inputs of the measuring and testing 
systems of the invention. 
MEASUREMENT MODE 
During normal ion concentration measurements, switch 9 is open and switch 
10 is closed. Consequently, a comparing resistor 11, having a value of, 
for example, 10M ohms, is out of circuit and a filter, formed by resistor 
12 and capacitor 13, is active to remove disturbances picked up in the 
lead between electrode 4 and terminal 8. The smoothed voltage of terminal 
8 is fed to the positive input 14 of an operational amplifier 15. Input 16 
of amplifier 15 is connected to output 17. The operational amplifier 15 
serves as an impedance match and its output voltage corresponds directly 
to the voltage of input 14. 
In a corresponding manner, terminal 6 is connected to positive input 18 of 
operational amplifier 19, switch 20 being open, so that a comparing 
resistor 21, of, for example 2M ohms, is out of the circuit, whereas a 
disturbance is diverted by filter comprising resistor 22 and capacitor 23 
being activated and switch 24 being closed. 
The output 25 of operational amplifier 19 is connected to the positive 
inputof operational amplifier 26, the negative input of which is connected 
to the output 17 of amplifier 15. The output of amplifier 26 is used as a 
measuring terminal for measurement of ion concentration. 
Terminal 7 of potential equalization electrode 5 is connected to earth by a 
relatively small resistor 27, of, for example, 10 k ohms. Switch 28 is 
open. 
TEST MODE 
When testing either of electrode 3 or 4, switch 28 is closed and a low 
impedance symmetrical block voltage generator 29 delivers a block voltage 
of, for example, 1 volt at 40 Hz to electrode 5 via terminal 7. 
(TESTING ELECTRODE 4) 
When testing electrode 4, switch 9 is closed, so that in the 
circuit,comprising earth, electrode 5, liquid 2, electrode 4, resistor 11, 
earth, the AC voltage of input 14 of amplifier 15 mainly depends on the 
impedance value of electrode 4 and liquid 2. An extremely high impedance 
points to rupture of connection or no liquid 2 between electrodes 4,5. An 
impedance which is higher than may be expected, but not as high as results 
from a rupture, points to soiling or coating of at least one of the 
electrodes 4,5 or an extemely low conductivity of the liquid 2. A low 
impedance points to a break down of electrode 4. 
The output of amplifier 15 is fed to a capacitor 31, for example, of 2 F, 
through a switch 30, which is closed when testing electrode 4. Capacitor 
31 is connected to earth via resistor 32 of, for example 10 k ohms. 
A sampler switch 33 is adapted to close during a short period of the 
duration of one of the levels of the block voltage so that samples of this 
level are fed to a sample holding capacitor 34, connected to the positive 
input of an output operational amplifier 35. The voltage on the output of 
amplifier 35 is a measure for the impedance of electrode 4 and the liquid 
2, independent of any failure of electrode 3. Further, this impedance can 
be measured even in the region of very high impedance pointing to rupture 
of leads. 
Furthermore, filter circuit comprising resistor 12 and capacitor 13 is 
disrupted because of the opening of switch 10, so that it does not 
influence the measurement. 
When testing electrode 4, switch 20 is open in order to prevent a shunt 
circuit via relatively small resistor 21 to earth. Switch 24 may be 
closed, whereas switch 36 is open, to prevent interference from electrode 
3. 
(TESTING ELECTRODE 3) 
For testing reference electrode 3, switch 20 is closed and switch 24 is 
opened. Consequently, the filter circuit comprising resistor 22 and 
capacitor 23, is put out of action and resistor 21 is connected in series 
with the impedance of electrodes 5 and 3, and liquid 2 connecting them. At 
the same time, resistor 11 is switched off by opening switch 9 and the 
filter circuit comprising resistor 12 and capacitor 13 is activated by 
closing switch 10. Switch 28, for supplying the block voltage, is still 
closed, but the connection to blocking capacitor 31 is through switch 36, 
whereas switch 30 is opened. In the circuit comprising earth, electrode 5, 
liquid 2, reference electrode 3, resistor 21, earth, the impedance of 
reference electrode 3 is compared to that of resistor 21. 
A high impedance value may be an indication of a lead rupture, soiling or 
coating of the outer membrane, loss of electrolyte or poisoning. A low 
impedance may point to some type of short circuit. 
When testing electrodes 3, or 4, the output of amplifier 26 has no 
relevancy to any quantity to be measured and should be neglected. 
Auxilliary electrode 5 is of a type which rarely breaks down. However, it 
is possible to become covered with an insulating deposit. In that case, an 
increase of the impedance measurement of electrode 3 will show up, which 
itself probably will have some additional resistance due to the deposit. 
Because of the normally high resistance value of electrode 4, such a 
failure would be less clear in case only the impedance of that electrode 
is being measured. Moreover, a synchronous impedance increase of 
electrodes 3, and 4 could be an indication that something is wrong. 
With the invention, monitoring of the temperature may be combined with 
adaption of the amplitude of the block voltage from generator 29, adaption 
of resistor 11, or simply, varying the region in which the impedance value 
has to be. Also, it is possible to use the invention with other 
electrodes, such as metal electrodes for redox measurements. Such 
electrodes have a low impedance value, but, in cases of soiling, coating, 
poisoning or rupture of connection, may show high impedances, thus always 
indicating any type of failure. 
Although unnecessary, further switching means may be provided to switch off 
any electrode not incuded in any test circuit at the moment of testing. 
In order to synchronize block voltage generator 29 with the sampling switch 
33, a block voltage generator may provide a block voltage having ten times 
the frequency of generator 29. The generator 37 feeds a counter 38, which 
sends the first five out of each ten pulses toward generator 29 and the 
ninth and tenth pulses to switch 33. 
The foregoing description is illustrative of the principles of the 
invention. Numerous modifications and extensions thereof would be apparent 
to the worker skilled in the art. All such modifications and extensions 
are to be considered to be within the spirit and scope of the invention.