High-precision gravimetric coulometry using the silver-perchloric acid coulometer: Titration of arsenious oxide with electrogenerated iodine.

High-precision gravimetric coulometry with a silver-perchloric acid coulometer is evaluated as an alternative to the conventional titrimetric method. The loss of weight (caused by electrolytic dissolution) of a highly pure silver anode in series with the cathode of a conventional constant-current titration system is measured and related to the number of equivalents of substance titrated. The precision of the method is determined by titrations of the Standard Reference Material 83C arsenious oxide (99.99% pure) with electrogenerated iodine, using biamperometric end-point detection. Depending on the size of the sample, an ultimate precision of 25 ppm is obtained. The assay for 0.5-g samples of the SRM material is 99.993(9) +/- 0.002(5)% purity.