Polarographic supporting electrolytes

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

The polarographic determination of metal ions such as Al3 + which are readily hydrolysed can present problems in aqueous solution, but these can often be overcome by the use of non-aqueous solvents. Typical non-aqueous solvents, with appropriate supporting electrolytes shown in parentheses, include acetic acid (CH3C02Na), acetonitrile (LiC104), dimethylformamide (tetrabutyl-ammonium perchlorate), methanol (KCN or KOH), and pyridine (tetraethyl-ammonium perchlorate), In these media a platinum micro-electrode is employed in place of the dropping mercury electrode. 

The polarographic method is applicable to the determination of inorganic anions such as bromate, iodate, dichromate, vanadate, etc. Hydrogen ions are involved in many of these reduction processes, and the supporting electrolyte must therefore be adequately buffered. 

In stripping voltammetry the stripping potential of a given ion is generally close to the polarographic half-wave potential of that ion in solutions with similar supporting electrolytes. Thus, typical stripping potentials in a 0.05M potassium chloride base solution are as follows Zn, — 1.00 V Cd, -0.07 V Pb, -0.45 V Bi, -0.10 V Cu(II), -0.05 V. 

The true residual current (faradaic) caused by reduction (or oxidation) of traces of impurities in the supporting electrolyte or in incidental components of the analyte. This current can be reasonably well compensated by the current offset "I comp. of the polarograph. 

The hydrazones precipitate or can be extracted with an inert solvent such as benzene or toluene after addition of a lower alcohol such as methanol together with supporting electrolyte, the hydrazones can be determined polarographically the dinitrophenylketimine structure predominates more or less in the values of E and td per mole. 

Molten carbamide is known to be a good solvent for the salts of many metals [1] and was used as a supporting electrolyte in polarographic investigations both at a dropping mercury electrode and at a stationary... 

The polarographic apparatus, reference electrodes, supporting electrolytes, solvent preparation, and preparation of the carbenium-ion solutions have been described in references 1-11 and 20. It is noteworthy that the reference electrode [9] described in 1978 maintained its EMF of -0.130 versus SCE until accidentally broken in 1983. 

Interesting results have been obtained from polarographic studies in various donor solvents. Measurements have been made of various metal perchlorates in solutions of donor solvents containing tetraalkylammonium perchlorate as supporting electrolyte against an aqueous saturated calomel electrode 113. In order to eliminate differences in liquid-liquid junction potentials bisbiphenylchromium (I) has been used as a reference ion 114 118). 

The existence of various oxidation states of technetium indicates the possibility of using polarography for its quantitative determination. Polarographic reduction of the pertechnetate ion at a dropping mercury electrode has been studied in different supporting electrolytes . 

In dilute supporting electrolyte, the maximum on the polarographic waves was observed, which is connected with accumulation of insoluble reduction product on the electrode surface. The apparent rate constant for cathodic reduction process of ammonia complexes of Zn(II) was obtained. 

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