398 Lea — Platinochlorides and Platinum Subchloride. 



All these have objections — "With (1) it is not easy to obtain 

 a uniform conversion. (2) requires to be very closely watched 

 to catch the exact moment at which the change is complete. 

 (3) is liable to a vexatious reverse action by which platinous 

 salt is reconverted into platinic salt at the expense of the 

 cupric chloride present. Thomsen mentions this danger as 

 occurring in hot solutions. It probably depends however 

 more on concentration than on temperature. The larger the 

 proportion of cupric chloride present in any solution the 

 greater the tendency to reversal. In one case a half liter of 

 mother water containing platinous salt was set aside for spon- 

 taneous evaporation. In a few days large crystals of the red 

 salt began to form, in a few days more instead of these increas- 

 ing, there was not a crystal of the platinous salt left. 



These objections led me to look for something different. I 

 have found two methods, either of which gives good results. 



First Method. Potassium Acid Sulphite. 



Potassium platinic chloride is to be moderately heated with 

 solution of the acid sulphite ; convenient proportions are, 

 platinum salt 12 grams, acid sulphite 9 grams, water 160 cc . 

 The mixture can be placed over a hot water bath in a covered ves- 

 sel and left to itself. The reduction takes about 10 to 12 

 hours and is known to be complete when the solution has a 

 pure red color free from yellow. The cover is then removed 

 and the liquid evaporated to the crystallizing point. 



If, as may happen the red chloride and the other salts crys- 

 tallize out together, it is best to redissolve them by heat in a 

 small quantity of water saturated with potassium chloride. 

 The red salt then crystallizes out first. 



Second Method. Alkaline Hijpophosphites. 



By reason of its great reducing powers a very small propor- 

 tion of alkaline hypophosphite is capable of converting the 

 yellow platinum salt to the red ; theoretically one part of 

 hypophosphite should reduce nine or ten parts of platinum 

 salt. We can hasten the operation somewhat by using an 

 excess of hypophosphite, but then must work at a lower tem- 

 perature. Both methods will be given. 



In using an excess of hypophosphite it is convenient to take 

 10 grams of platinum salt, 2 grams or even more of potas- 

 sium hypophosphite, and 600 cc of water. These are placed in 

 a flask and very gently heated. The best temperature is 66° to 

 70° C. 



There is a very easy way of obtaining this temperature and 

 of keeping it perfectly constant for any length of time, by 



