574 AGRICULTURAL ANALYSIS 



The liquid is then poured off and the reduced platinum purified by 

 boiling with hydrochloric acid and water. It is then dried and 

 ignited to destroy any organic matter which may be present. It is 

 advisable to boil the finely divided platinum once with strong nitric 

 acid, and after this is poured off the solution of the platinum is 

 effected in a large porcelain dish over a water bath by adding 

 about four times its weight of hydrochloric acid, warming, and 

 adding nitric acid, little by little. After the platinum is in solu- 

 tion the evaporation is continued until a drop of the liquid, re- 

 moved by a glass rod, quickly solidifies. The crystalline mass 

 which is formed on cooling is taken up with water and filtered, 

 and then a sufficient amount of water added so that each 10 cubic 

 centimeters will contain one gram of platinum. The specific 

 gravity of this solution is 1.18 at ordinary temperatures. Special 

 care must be taken that the solution contains neither platinous 

 chlorid nor nitrogen compounds. If the first named compound 

 be present it should be converted into platinic chlorid by treat- 

 ment with fuming hydrochloric acid and a little nitric acid. The 

 last mentioned compound may be removed by evaporating suc- 

 cessively with hydrochloric acid and water. If the platinic chlorid 

 be made from waste platinum, the danger of contamination with 

 indium must be considered. In such a case the platinum should 

 be separated as ammonium platinochlorid, which can afterwards 

 be reduced as above indicated. A convenient test of the purity 

 of platinic chlorid solution is accomplished by the precipitation 

 of a known weight of chemically pure potassium salt, and deter- 

 mining the quantity of platino-potassium chlorid produced. 



(2) By Reduction in Nascent Hydrogen. The platinum resi- 

 dues, filtrates containing platinum, etc., are collected in a large 

 flask and evaporated in a large dish on a water bath, and re- 

 duced by means of zinc and hydrochloric acid to metallic plati- 

 num, the mass being warmed until all the zinc has been dis- 

 solved. The supernatant liquid standing over the spongy plati- 

 num is decanted and the spongy mass boiled twice with distilled 

 water. The spongy platinum is then brought on a filter and 

 washed till the filtrate shows no acid reaction. The filter and' 

 platinum sponge are next incinerated in a platinum dish and the 



