WEIGHING THE PRECIPITATE 569- 



rid of platinum and potash as such, although he acknowledges 

 that in the gooch this weighing can be made with great accu- 

 racy. 49 He prefers to estimate the platinum in the metallic 

 state and uses for this purpose a platinum crucible the inside of 

 which, half way up from the bottom, is coated with a layer of 

 platinum sponge, which is conveniently prepared by the decom- 

 position of a few decigrams of the platinum double salt by in- 

 clining the crucible and rotating it during the progress of the 

 reduction, which should require about a quarter of an hour. The 

 platinum sponge produced in this way greatly favors the decompo- 

 sition of the double salt for analytical purposes. The decompo- 

 sition of the salt takes place quickly and quietly and at conven- 

 iently low temperatures. 



When the decomposition is ended the crucible is strongly 

 heated so as to hold the platinum sponge together sufficiently to 

 prevent its being removed in the subsequent washing of the cru- 

 cible by decantation. By the ignition at a high temperature 

 necessary to secure this, the greater part of the potassium chlorid 

 is volatilized. After cooling, a few drops of concentrated hydro- 

 chloric acid are placed in the crucible and if the slightest yellow 

 color be shown the acid is evaporated and the ignition repeated, 

 with the addition of a little oxalic acid. In most cases the slight 

 yellow color produced comes from a trace of iron and will, there- 

 fore, appear again after the second ignition. The crucible is sub- 

 sequently washed by repeated decantations, finally with boiling 

 water, and after drying is ignited and weighed. 



The advantage of this process is that without further trouble 

 the reduced metal is completely freed of any salts of the alkaline 

 earths, etc., which have been carried down with it and also from 

 any of the uncombined sodium chlorid which may not have 

 been washed out by the alcohol. In fact, the results obtained 

 in this way are nearly always lower than those obtained through 

 the direct weighing of the double salt, and the wash water which 

 is first poured off contains, as a rule, traces of the alkaline earths 

 and almost without exception some sodium chlorid. Correction 

 49 Zeitschrift fur analytische Chemie, 1893, 32 : 184. 



