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The excess reagent is determined residually by titration with standard silver nitrate solution (carried out at −0.23 v. vs. S. C. E. in an ammoniacal medium to prevent chloride interference), using the rotating platinum electrode as the indicating electrode. A blank is run to compensate for possible oxidation of the mercaptan to the disulfide. Microgram quantities of Cu, Ag, Pb, Hg, and Cd can be determined with precision and with an accuracy of within 0.5 p. p. m. over a range of 1.5 to 10 p. p. m. Additional applications were made on a number of inorganic and organic pharmaceuticals.