The blank for the end point required the following amounts of ferrocyanide solution: 0.60 c.c., 0.50 c.c., 0.55 c.c. In titrating the zinc in a sample of ore, the following results, inclusive of the amount of solution necessary to establish the end point, were obtained: Grams Sample c.c. K4Fe(CN)6 used What was the percentage of Zn in the ore? 20.38 20.35 Ans. 19.66% CHAPTER XIV ORGANIC PRECIPITANTS DETERMINATION OF NICKEL AS NICKEL DIACETYL-DIOXIME. THAN WATER SOLVENTS OTHER 225. The gravimetric determinations and separations which we have thus far considered are those where the precipitant has been inorganic in its nature and where the solution has been essentially aqueous. In the case of some of the elements, however, it so happens that it is not possible to effect a satisfactory separation by such means, and therefore recourse has been had to the employment of organic precipitants in some instances and to the use of solvents other than water in other instances. The more important organic precipitants which are employed at the present day are: diacetyl-dioxime (dimethyl glyoxime), cupferron, nitron, and nitroso-ẞ-naphthol; the more important solvents other than water are: ether saturated with hydrochloric acid, a mixture of absolute alcohol and ether in equal volumes, and amyl alcohol. The applications of these reagents follow herewith. 226. Diacetyl-dioxime (dimethyl glyoxime). This reagent was first synthesized by L. Tschugaeff in 19051 and two years later applied to the determination of nickel in steel by O. Brunck.2 Diacetyl-dioxime reacts quantitatively with nickel ion to form the very insoluble precipitate in which the content of nickel is 20.31%. This precipitate is one of the most satisfactory with which we have to deal since it can be easily filtered and washed on a Gooch crucible and readily dried to constant weight at 110°-120°. When pure it has a bright scarlet color with a characteristic golden luster. With respect to the interference of other metals it is to be noted that while palladium ion is the only other ion which of itself gives an insoluble precipitate with diacetyl-dioxime, cupric ion is coprecipitated with nickel diacetyl-dioxime if the alkalinity of the solution from which precipitation is being effected is greater than corresponds to p= 5.0, although cupric diacetyl-dioxime by itself is very soluble. For practically all the other metals with which nickel is found associated, such as cobalt, manganese, iron, chromium, aluminum, zinc, etc., the precipitation of nickel diacetyl-dioxime is independent of their presence. It might be mentioned that before the advent of diacetyl-dioxime it was almost impossible to obtain a satisfactory determination of nickel in the presence of cobalt, whereas now it is a very simple matter. There is a practical point that fits in here with respect to the use of diacetyl-dioxime, and this point has to do with the fact that diacetyl-dioxime is insoluble in water but is slightly soluble in 95% alcohol, namely, to the extent of about 1 gram per 100 c.c. alcohol, in which form it is used as a reagent. If very much. more diacetyl-dioxime solution is used than is necessary to precipitate the nickel quantitatively, the excess of diacetyl-dioxime will itself precipitate out and contaminate the precipitate of nickel diacetyl-dioxime. Therefore the diacetyl-dioxime should be added in slight excess only. This direction presupposes that the analyst knows beforehand the approximate nickel content of the solution from which precipitation is being effected; if this knowledge is not at hand the analyst must make progressive additions of diacetyl-dioxime solution until assured that same is in slight excess, but not more than in slight excess. 227. Exercise No. 24. Determination of Nickel in a Nickel Ore. Weigh out the necessary quantity of ore to furnish not more than 0.100 g. of nickel as the amount of nickel diacetyldioxime precipitate corresponding to this is the most that an 3 If a nickel ore is not at hand, a sample of German silver will answer equally well. ordinary size (20 c.c.) Gooch crucible can accommodate. Transfer to a 150 c.c. beaker and add about 10 c.c. of water; cover with a watch-glass and add 10 c.c. of 12 molar hydrochloric acid and 5 c.c. of 16 molar nitric acid. Heat until decomposition of the ore is complete, adding a little bromine if necessary in the case of sulphide ores. After solution has been effected, dilute with hot water and filter into a 750-c.c. beaker, washing the filter with hot water. If there is any residue, fuse same with potassium pyrosulphate in a quartz or porcelain crucible; dissolve the fusion by heating in 50 c.c. of water containing about 5 c.c. of 9 molar sulphuric acid; filter into main solution. To the combined filtrates, add, according to the amount of iron, aluminum, etc., present, about 50-100 c.c. 0.2 molar citric acid. Now add 15 molar ammonium hydroxide until the solution is faintly alkaline to litmus; if a precipitate forms, add more citric acid and again add ammonium hydroxide. Now add 3 molar hydrochloric acid until the solution is just acid to litmus paper; then add 3 c.c. 1 molar hydrochloric acid.5 Dilute the solution to about 500 c.c., add 10 grams ammonium chloride, and heat to 60°-70°. Add gradually and with constant stirring a 1% solution of diacetyldioxime in 95% alcohol (1 c.c. = 2.5 mg. nickel) until in slight excess, but no more than in slight excess. Digest at 60°-70° for an hour; then add 10 c.c. of 3 molar ammonium hydroxide (this should make the solution slightly alkaline), and set aside to digest for 12 hours, allowing the solution to cool of its own accord. Filter the precipitate through a weighed Gooch crucible which has been brought to constant weight in an electric oven at 110°120°; wash with hot water until the washings are free of chloride. During the filtration and washing keep the upper edge of the crucible dry for about 1/4 inch down from the top as otherwise the precipitate will creep over the top and down the outside of the crucible. Dry the crucible and its contents to constant weight 4 Tartaric acid or potassium sodium tartrate (Rochelle salt) may be used in place of citric acid for the purpose of forming complexes with the iron, aluminum, etc., to prevent their precipitation subsequently as hydroxides. The use of tartaric acid or Rochelle salt, however, is not so advisable, owing to the likelihood of acid ammonium tartrate precipitating out in the preliminary adjustment of acidity prior to the addition of the diacetyl-dioxime. 5 It is very important to establish the acidity in accordance with the foregoing directions; if the acidity is much greater, the nickel diacetyl-dioxime will not precipitate; if the acidity is much less, copper will be co-precipitated with the nickel. at 110°-120° in the electric oven. The precipitate is Ni(CH7O2N2)2 and contains 20.32% nickel. 228. Cupferron. The Ammonium Salt of Nitrosophenyl Hydroxylamine. Samples of this reagent on the market often contain a little zinc. It can readily be prepared according to the directions of Marvel & Kamm, J. A. C. S. 41, 276 (1919). This reagent precipitates copper, tin, iron, titanium, vanadium, zirconium and thorium from acid solutions and furnishes a method for separating these elements from cadmium, aluminum, chromium, manganese, cobalt, nickel, zinc, magnesium, calcium, barium and strontium. It is particularly valuable in separating iron, zirconium, and titanium from manganese and aluminum, the details for this procedure being given by J. Brown, J.A.C.S. 39, 2358 (1917). Cupferron also precipitates silver, mercury, lead, bismuth, antimony (Sb+++), molybdenum, uranium (U++++), and cerium, but these separations have not as yet been fully worked out. A comprehensive review of the analytical uses of this reagent will be found in the article by Lundell and Knowles, J. Ind. & Eng. Ch. 12, 344 (1920). 229. Nitron. Diphenyl-endo-anilo-hydro-triazole. This reagent, which is in reality a base, precipitates nitric acid as the fairly insoluble crystalline nitrate C20H16N4 HNO3.6 The following acids also form more or less insoluble precipitates with nitron and must be absent in the determination of nitric acid: perchloric, hydriodic, thiocyanic, chromic, chloric, nitrous, and M. Busch, Ber. 38, 861 (1905); A. Gutbier, Zeit. angew. Chem. 18, 494 (1905). |