725 
1908-9.] The Atomic Weight of Platinum. 
most accurate of our analytical operations. While it would be very 
interesting and most instructive, if it were feasible, to determine the ratio 
of platinum to oxygen or to silver directly, a few analyses, by means of a 
reliable method, are of more value than any number carried out according 
to methods which have been insufficiently studied to reveal their possibly 
large constant errors. 
The present investigation covers a study of the potassium and ammonium 
salts of chloroplatinic acid, and the corresponding salts of bromoplatinic 
acid. The method of analysis necessitated the preparation of the pure 
salt, the determination of its weight in a perfectly dry condition, its 
reduction in a stream of hydrogen, with the absorption in water of the 
hydrochloric acid formed, and subsequently the weighing of the platinum 
residue, and the determination of the potassium chloride left behind, and 
the hydrochloric acid formed, by precipitating and weighing the chlorine of 
each separately as silver chloride. 
Preparation of Pure Materials. 
Potassium Chloride . — Chemically pure potassium chloride was twice 
precipitated from an almost saturated solution with gaseous hydrogen 
chloride, prepared by boiling a solution of pure hydrochloric acid. After 
each of these precipitations, the salt was washed and dried in a centrifugal 
apparatus, as recommended by Richards.*' This treatment was usually 
resorted to for the purpose of removing the mother liquor from a mass of 
crystals. The value of the process of crystallisation as a means of purifying 
a substance is greatly enhanced by removing the mother liquor from the 
crystals as completely as possible. Consequently this point received a 
great deal of attention throughout this work. The potassium chloride 
from the second precipitation was once recrystallised from water. The 
above operations were carried out in platinum vessels. The product thus 
obtained was used in precipitating some platinum as potassium chloro- 
platinate. This platinum was very pure, having been through all but the 
last stage of its purification. The platinum salt was now reduced in pure 
hydrogen gas at a low temperature. The potassium chloride set free was 
dissolved in water, and twice precipitated in platinum vessels by means of 
gaseous hydrogen chloride. This product was employed in the preparation 
of the potassium chloroplatinate used in the analyses described below. 
Ammonium Chloride . — The method of preparing pure ammonium 
chloride was analogous to that used in the case of the potassium salt. After 
* Richards, Jour. Am. Chem. Soc., xxvii. 104 (1905). 
