1520
ANALYTICAL CHEMISTRY
the sample contain tungsten, columbium, tantalum, titanium, or the material is not decomposed by any other acid combination. molybdenum, add approximately 3 grams of tartaric acid as soon Because of the insolubility of potassium perchlorate, all perchloric as decom osition of the sample is complete and evaporate t o dryacid must then be expelled either by evaporation to dryness or water bath A4ddto the dry salts about 5 grams of by fuming with sulfuric acid. If any chromium(V1) is intfiness on tartaric acid and 75 ml: of water and heat until solution is comcated, it should first be volatilized as chroniyl chloride from t h r perchloric acid solution, because expulsion of perchloric acid I>y plete. Carbides, Ceramals, Ceramets, and Tun sten, Tantalum, and evaporation would lead to acid-insoluble chromic oxide, while Columbium Alloys. Some material can %e decomposed with fuming with sulfuric acid aould cause formation of inso1ui)le nitric or hydrochloric acids or with varying proportions of aqua rhromium sulfat,e. regia. Many carbides, however-for instance, cemented carbides of tantalum, tungsten, columbium, titanium, and zirPRECIPITATION OF POT.ASSIUM COB.ALTINITRITE cymium-cannot be decomposed with the usual acid combinations. I n such cases weigh 0.5000 t o 1.0000 gram of the material T o the tartaric acid solution obtained hy one or a combinatioii into a platinum dish. Add 20 ml. of hydrofluoric acid and from of the procedures outlined in the preceding paragraphs, add slowly time to time, in small portions, about 5 ml. of nitric acid. Somea saturated solution of potassium hydroxide until the solution is times the reaction is vigorous; the dish should then be covered definitely alkaline to litmus paper or any other suitable indicator. with a platinum or gold cover. Sometimes the reaction is slow, -4dd an excess of potassium hydroxide solution if any precipitate and gentle heating is required to effect complete solution of the forms at the neutral point or if insoluble tungstic or rare earths sample. Some carbides dissolve faster or more completely in acids are present (if no tartaric acid was employed in the original nitric acid to which small quantities of hydrofluoric acid are decomposition or if the sample is high in these elements). added. Others can first be ignited in platinum dishes to their T o the alkaline solution having a volume of less than 100 nil. oxides, which then can be dissolved in nitric and hydrofluoric carefully add glacial acetic acid until the solution is just acid, then acids. As soon as solution of the sample is complete, evaporate add 4 ml. in excess. The solution should IIP clear a t this point. the solution to dryness on the water bath. Add 20 ml. of 40% If for any reason a cloudiness or small prwipitate forms i t should tartaric acid solut,ion and heat until all salts have dissolved, dishe disregarded, because reprecipitation nil1 eliminate the conregarding any undissolved graphitic carbon or tungsten trioxide. taminating impurity while precipitation of the cobalt is complete. Transfer to a 600-ml. beaker and dilute with water to about i 5 To the warm acetic acid solution atid, with vigorouc: stirring, nil. 200 ml. of hot potassium nitrite solution (100 grams of potassium Cobalt in Its Ores and Concentrates. Treat 1 to 2 grams of nitrite dissolved in hot water and R frw drops of acetic acid anti the sample with nitric and hydrochloric acids. Omit the hydrodiluted to 200 ml,). If the samplr is high in cobalt the potassium When action t,hloric acid when analyzing silver-bearing ores. cobaltinitrite precipitates instantly. Smaller amounts (
