Reversible calomel-mercury electrode

As stated earlier, the reference electrode in a cell used for electroanalysis is designed so that its potential is independent of the composition of the test solution. There are several general properties that reference electrodes should have in order to be useful in analysis (1) they should be reversible with an electrode potential which is independent of time and reproducible (2) they should have a small temperature coefficient (3) they should be ideally non-polarizable with negligible effects from the flow a small current through the system and (4) they should be easily constructed. The most commonly used reference electrodes are those based on on the mercury calomel system and the silver silver chloride system. The electrolyte most commonly used in these systems is KCl. Relevant parameters for commonly used reference electrodes are given in table 9.4. 

A simple calculation shows the effects of such a leak rate. Consider a 0.5 1L test solution volume having 4 pl./h of 4 M CE added (the KC1 concentration in a saturated calomel electrode). In a 24 h test the initial CE-free solution will develop a concentration of 7.7 X 10 5 M CE. Although low, this concentration will lead to pitting in many alloys (3), confounding interpretation of the results. A second effect of filling solution leakage into the bulk solution involves the cations that are released. In the case of the SCE, there is a small concentration of Hg2+ that makes its way into the bulk. Once in the bulk solution, the mercurous ions can be deposited electrochemically onto any surface at a potential below the reversible potential for Hg deposition. This deposition of metallic mercury can cause dramatic changes in the surface behavior. 

Fig. 7,8. Reversible mercury-calomel electrode with the parts consisting of a 250 ml wide-mouthed reagent bottle, a vented and drilled rubber stopper, a soft glass tube with platinum wire sealed into the end, mercury, a wire to the output of the power supply, saturated KCl, and a glass tube containing 5% polyacrylamide gel made up in 1 M KCl. The tube dips into the buffer reservoir of the electrophoresis apparatus. The anode and cathode should be interchanged after each run. This arrangement will prevent all contamination of the buffer with electrode products even in very long runs (J. C. Finder, Ph.D. Thesis, London University, 1974).

The standard potential for the reduction of No + to No(Hg) was measured by a modified radiopolarographic technique (31). Usually, the half-wave potential is determined by measuring the distribution of an element between the mercury and aqueous phases as a function of applied voltage. The half-life of No is too short to allow time for the recovery of No from the Hg phase for assay, therefore Meyer et al. measured the depletion of No in the aqueous phase as a function of a controlled potential. They assumed that equilibrium was reached in 3 min of electrolysis and that the electrode reaction was reversible. A sharp drop in No concentration in the aqueous phase occurred between -1.8 and -1.9 V v . the saturated calomel electrode or -1.6 V vs. the standard hydrogen electrode. Thus, their best estimates are summarized in the following equation. 

The source in this instrumental set-up is the reference electrode, the most popular examples of which are the silver/silver chloride electrode or the mercury/mercury chloride (saturated calomel) electrode. The important requirements of a reference electrode are reversibility, reproducibility and stability in time. Often these are incorporated into the ISE housing so that it contains the working and reference electrodes together and this combination electrode is attached to the voltmeter. 

In the case of standard electrode potential, it is appropriate to have a standard electrode whose reversible potential is made arbitrarily zero and against which the potentials of other electrodes can be measured. The hydrogen electrode is an accepted standard. It is composed of a rod of platinum covered with platinum black saturated with hydrogen gas at atmospheric pressure. Electrode potential based on this zero are said to refer to the hydrogen scale. However, in experimental work, it is often more suitable to use another standard electrode. Calomel is a common example. It consists of a pool of mercury covered with calomel (mercurous chloride) and immersed in a solution of potassium chloride. 

At any rate, the reaction is reversible and very fast. Recently, the toxicity for mercury has been mentioned often. Therefore, the calomel electrode using a mercury compound is going to be out of service. 

Mercury-mercurous chloride. This is probably the most widely used reference electrode. It is reversible to chloride ion and is usually made up in saturated aqueous potassium chloride solution, although Irnoldm " and O.lmoldm solutions are also common. In commercial electrodes, the solution is often retained with a porous plug or ceramic frit saturated aqueous KCl, being very dense, easily leaks out. A separate compartment will therefore be necessary for the reference electrode if chloride ions must be kept out of the working solution. Calomel electrodes can easily be prepared by shaking clean dry mercury with the powdered mercurous chloride which forms a skin around the mercury. The chloride ion solution is then carefully poured on top to complete the electrode. Home-made calomel electrodes can have a very low resistance and high performance. 

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