Rational Application of the Standard Gravimetric Method for Determination of Platinum in Pt-Ru Alloy

As a participator of the international interlaboratory testing of a jewellery sample,a platinum-ruthenium alloy was received and required to analyze according to the assigned methods. Gravimetric method (ISO 11210-1995) was used by this laboratory for the determination of platinum. It was known that the content of ruthenium in the alloy is around 5%. To eliminate the interference of ruthenium,modification of the conventional gravimetric method was performed in the following way: ① the sponge platinum obtained through the combustion (at 950 ℃) of the precipitate (NH4)2PtCl6 was dissolved again with aqua regia and the black insoluble substance,which was proved to be ruthenium,was filtered onto a close-texture fillter paper,transferred to a tarred crucible,combusted at 950 ℃ for 1.5 h and weighed to constant weight. ② platinum in the filtrate was re-precipitated as (NH4)2PtCl6 and treated in the same way as in the first precipitation. Content of platinum was obtained by calculation according to the formula given in the paper,in which a substraction of the ruthenium content was included. The results obtained by this modified gravimetric method were in consistency with the results obtained by other methods.