Reference half-cell electrode

Details Reference half-cell electrode Reference half-cell electrode

1993-04-21

1994-11-01

Niebling, John

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204416, 204433, G01N 2726

Patent

active

053605292

DESCRIPTION:

BRIEF SUMMARY
This invention relates to reference half-cell electrodes and in particular to electrodes intended for use in the measurement of halide ions.
In electroanalytical measurements, a reference electrode is used in conjunction with a sensing electrode. When both electrodes are immersed in a solution of an ionic salt, a potential difference is established between the electrodes, whose magnitude is a function of the concentration of the dissolved ionic species.
Examples of commercial reference electrodes, which are commonly used for such purposes include saturated aqueous calomel and silver/silver chloride electrodes. These comprise an internal reference element which is immersed in a highly concentrated salt solution. This serves the purpose of maintaining a constant ionic strength environment for the reference element.
Many electrodes suffer from inherent disadvantages that result from leakage of the electrolyte solution. Firstly, the electrode solutions require periodic replenishment which results in regular maintainance being necessary. This can be inconvenient, costly and time consuming, especially in an industrial environment. Secondly, leakage may cause the electrode to be unsuitable for particular industrial or medical applications where contamination poses a problem.
Attempts to overcome these problems have focussed on the replacement of the electrolyte solution with a gelled electrolyte equivalent. These types of reference electrode have a much lower rate of seepage of electrolyte from within the gel and consequently only require minimal maintenance until the electrolyte is exhausted.
British Patent Specification 1 353 209 describes a single junction electrode comprising a silver wire coated with silver chloride surrounded by a polyolefin tube filled with a gel containing potassium chloride. In use the open end of the polyolefin tube filled with the gel is placed in the solution to be measured. However it is stated that the electrode will need renewal and this is done by cutting the lower end of the polyolefin tube thus exposing a fresh surface of the gel. Example 3 indicates that such renewal needs to be carried out about every 24 hours. Such variable behaviour we call long term variability.
A commonly used polymer gelling agent is polyacrylamide.
If it is desired to make sure that absolutely none of the electrolyte in the electrode contaminates the liquid being tested it is known to add a further electrolyte bridge comprising a different salt (which is tolerated by the test liquid). Such a double junction electrode is described in German Specification 3415089 A1. Unlike the electrode described in the British specification however the gelled electrolyte is not in direct contact with the liquid being tested because there is an additional ion exchanger unit therebetween.
Reference electrodes, in addition to displaying long term variability due to junction contamination also show short term variability possibly caused by changes of the silver wire half-cell potential or temperature effects. Variability can also occur when the reference electrode is immersed in solutions of varying ionic strength, such as environmental samples. This is caused by junction potentials developing at the electrolyte/sample interface. Electrodes subject to both long and short term variations require frequent calibration and this is clearly undesirable.
According to the present invention there is provided a half-cell reference electrode comprising an electrode member in electrochemical contact with at least one gelled electrolyte solution which itself is contactable with the liquid to be tested wherein the gelling agent comprises a polymer containing acrylamide units substituted with a group or groups which lower the polymer's tendency to hydrolyse compared to polyacrylamide while providing a polymer that can form a gelled solution in 2.6M sodium/potassium nitrate solution (2.21M potassium nitrate and 0.39M sodium nitrate) comprising at least 75% (w/w) sodium/potassium nitrate solution.
It is preferred that the polymer can form a

REFERENCES:
patent: 3912614 (1975-10-01), Spracklen et al.
patent: 4959138 (1990-09-01), Brinkmann et al.

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