68 On the Purification of Platina. 
and more completely, the solution ought to be slightly 
heated, i.e. to 50 or 60° of the centigrade thermometer. The 
iridium is then deposited in abundance, as T have shown in 
my memoir on the influence of this metal. in the colouring 
of the salts of platina*. In order that the separation may 
be as coinplete as possible, the solution must not be too 
much concentrated nor too alkaline. 
When the deposit no longer increases, we must filter and 
pour into the liquor, after it is cooled, as much muriatic 
acid as is necessary to make it again become very sensibly 
acid. We then precipitate with. sal ammonia; and when 
the precipitation is complete, we filter and wash the triple 
salt several times with small quantities of water. 
The liquors still contain platina and a little iridium, We 
precipitate these metals by a hydro-sulphuret, and treat the 
precipitate as above described. 
The triple salt which we obtain by this process, if it. is 
ure, ought to be of a clear golden yollow. The nitric acid 
which dissolves it by ebullition ought not to give it any 
more intenseness of colour than that which ought to be 
produced by the greatest volume of crystals which are form- 
ed again upon cooling. There is besides an easy method of 
ascertaining the smallest portion of iridium. It consists in 
reducing by heat 4 small portion of the salt which we wish 
to examine, dissolving the residue by the nitro-muriatic 
acid, (or we may dissolve directly the triple salt by this acid, 
which in this case decomposes the ammonia,) and again 
precipitating the metal by sal ammonia, taking care only to 
employ the necessary quantity. We separate “the triple salt 
by the filter, and evaporate the mother-waters. If the salt 
which they furnish is of a clear yellow, we may be assured 
that there 1s no iridium in that which we are examining. If, 
on the contrary, the salt is red, itis a proof of the metal be- 
‘Ing there; and we must treat in the same manuer the whole 
of the triple salt, if we wish to purify it entirely. 
The reduction by heat of a small quantity of triple salt 
requires some precaution. If we operate in a crucible, it 
frequently happens that the jets of vapour which rise, carry 
up with them great part of the salt: I prefer drying it first 
in a porcelain capsule, and finish the reduction in a stone re- 
tort. IT afterwards wash the metallic sponge, until it no 
Jonger contains any thing soluble: I even boil it with a 
title sulphuric acid, and after having well washed it, I re- 
* Annales de Chimie, tome xviii. p. 170. 
dissolve 
