Analysis of mixtures of dialkyldithiophosphate, bis(dialkoxythiophosphinoyl) disulfide and elemental sulfur

Abstract

The method presented for the determination of zinc dialkyldithiophosphate, bis(dialkoxythiophosphinoyl) disulfide and elemental sulfur in the presence of each other is based on thiomercurimetric titrations. Zinc dialkyldithiophosphate is determined by titration with p-dimethylaminophenylmercury(II) acetate with Michler's thioketone as indicator. The sulfur and bis(dialkoxythiophosphinoyl) disulfide are determined after their reduction. Bis(dialkoxythiophosphinoyl) disulfide is reduced by using tributylphosphine and titration as for zinc dialkyldithiophosphate. The sulfur is treated with bis(2-methoxyethoxy) dihydride and titrated with o-hydroxymercuribenzoic acid in the presence of dithiofluorescein as indicator. The method is simple, rapid, reproducible and accurate and samples do not require separation or other preliminary stages prior to their analysis. The method is suitable for monitoring changes in thiophosphorus lubricating oil additives during engine runs.