~Nitre.~--This is potassium nitrate. ~Platinum Chloride~, 2HCl.PtCl_{4}. (In the crystallised form it has 6H_{2}O).--It may be made as follows:--Take 5 grams of clean platinum scrap and dissolve in a flask at a gentle heat in 50 c.c. of hydrochloric acid with the occasional addition of some nitric acid; evaporate to a paste; and then dissolve in 100 c.c. of water. It is used for separating and determining potassium. ~Phenolphthalein~ is an organic compound used as an indicator; more especially in determining the weaker acids, it cannot be used in the presence of ammonia. Dissolve half a gram in 100 c.c. of dilute alcohol. ~Potassium Bicarbonate~, KHCO_{3}.--It may be purchased pure; on ignition it leaves the carbonate, K_{2}CO_{3}, which may be used as a standard. ~Potassium Cyanide~, KCN.--It is used in the dry assay as a reducing agent. The commercial salt is very impure. Purchase that sold as potassic cyanide (gold) which contains about 95 per cent. of KCN. It is used for copper assaying and occasionally in separation. Make a 10 per cent. solution when wanted. ~Potassium Bichromate~, K_{2}Cr_{2}O_{7}. It may be purchased nearly pure. It is used as an oxidising agent, for determining iron; and as a test solution. For this last purpose a 10 per cent. solution is prepared. ~Potassium Chlorate~ (KClO_{3}) can be purchased pure. It is used with hydrochloric acid as a substitute for aqua regia. ~Potassium Ferrocyanide~ (K_{4}Fe(CN)_{6}.3H_{2}O), or "yellow prussiate of potash," is used as a test; as an indicator; and for the determination of zinc. Make a 5 per cent. solution. ~Potassium Ferricyanide~ (K_{6}Fe_{2}(CN)_{12}), or "red prussiate of potash," is used for testing; and as an indicator. Make a 5 per cent. solution when wanted, as it decomposes on keeping. ~Potassium Hydrate~, KHO. Purchase that purified with alcohol. It is an alkali, and is used for absorbing carbonic acid, &c. ~Potassium Iodide~, KI. It may be purchased nearly pure. It is used as a test and for dissolving iodine. It should be used in a 10 per cent. solution freshly made. The solution decomposes on exposure to light, with separation of iodine. ~Potassium Nitrate~ (KNO_{3}) can be purchased pure. It is used in the dry way as an oxidizing agent. It is very fusible. It decomposes at a low temperature into potassium nitrite (KNO_{2}) and free oxygen; and at a higher temperature leaves potash (K_{2}O). It oxidizes sulphur and carbon with explosive violence. This action may be moderated by mixing the nitre with carbonate of soda, common salt, or some other inert body. ~Potassium Nitrite~, KNO_{2}.--The commercial article is not pure, but is sufficiently so for the purpose required. A saturated solution is used in the separation of cobalt; the solution is made when wanted. ~Potassium Permanganate~, KMnO_{4}.--This salt can be purchased sufficiently pure. It is much used as an oxidizing agent.