Operational Standards Silver and the Coulomb

Experimentally, then, the two operational standards are intimately related. Silver of high purity (99.999-1-%) is obtainable at the present time. Also available are techniques and equipment for measuring current and time to high precision as well as conditions for generation of reagents at electrodes with high current efficiency. Thus either silver or the coulomb can be used as the operational standard with justification. 

In coulometric analysis the product of current in amperes and time in seconds that are required to complete a reaction or generate a reagent gives the number of coulombs. To bring about 1 equivalent of electrode reaction, 96,487 (96,487.0 1.6) C is required, and hence the number of equivalents E of reaction is 

Usually, after a reaction is about 98% complete, a current of about 0.1 of the normal value is used. In the apparatus developed for coulometry of high accuracy, the voltage of the regulated power supply seldom changes by more than 0.001% in a day, the current is accurately measurable through the use of a standard resistor (known to better than 1 ppm) of high precision, and adjustment of the iR drop is made equal to that of a Weston cell. The time can be measured to an accuracy of 1 ppm or better.  

In coulometry the stoichiometry of the electrode process should be known and should proceed with 100% current efficiency, and the product of reaction at any other electrode must not interfere with the reaction at the electrode of interest. If there are intermediate reactions, they too must proceed with the desired accuracy. In practice the electrolytic cell is designed to include isolation chambers. Losses of solute through diffusion, through ionic or electrical migration, and simply through bulk transfer must be minimal. Finally, the end point has to be determined by one of the many techniques used in titrations generally, whether coulometric or not. Both indeterminate and determinate end-point errors limit the overall accuracy achieved. Cooper and Quayle critically examined errors in coulometry, and Lewis reviewed coulometric techniques. 

Acids and bases have been standardized coulometrically with standard deviations of 0.003% halides by generation of silver ions to 0.005% potassium dichromate by generation of iron to 0.003% and conditions established under which 

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