Thallium amalgam/thallous-chloride electrode

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

The solution to the right of the glass electrode is usually a buffer solution of KH2P04 and Na2HP04, with 0.1 mol dm"3 of NaCl. The reference electrode is usually a calomel electrode, silver/silver chloride electrode, or a thallium amalgam/thallous chloride electrode. The emf of this cell depends on a(H + ) in the solution X in the same way as that of the cell with the Pt H2 electrode, and thus the same procedure is followed. 

Propylene Carbonate (PC) and Water. Data from both spectroscopic and thermodynamic studies for other solvent systems are sparse and some of it is of doubtful quality. For propylene carbonate, Salomon (40) has obtained emf data using lithium metal and thallium amalgam-thallous chloride or bromide electrodes. 

Other reference electrodes for use in polar aptotic solvents. Emphasis has been given to the use of the silver-silver ion reference electrode because it is almost universally applicable, and because standardization on the use of one reference electrode system simplifies the comparison of data between different workers. However, a number of other reference electrodes have been used (see Table 5.4), particularly those that have resulted from the vast amount of batteiy research. These include the Li/Li(solv)+ and other alkali metal electrodes that function reversibly in Me2SO, propylene carbonate, and hexa-methylphosphoramide. The thallium-thallous halide electrodes of the second kind also function reversibly in Me2SO and propylene carbonate. The cadmium amalgam-cadmium chloride reference electrode also functions reversibly in dimethylformamide and may be a useful substitute for the silver-silver ion reference electrode, which may be unstable in dimethyformamide.54... 

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