2. Other assays.1 a. The ore, etc., is dissolved in aqua regia and evaporated with the addition of sulphuric acid. Aqueous sulphurous acid is added to this in order to reduce the arsenic acid. The sulphurous acid is then removed by boiling, and the copper, arsenic, etc., are precipitated with sulphuretted hydrogen. The liquid is then filtered, the sulphuretted hydrogen removed by evaporation, and the ferrous oxide oxidized by potassium chlorate. Sodium carbonate boiling hot, is then added until a permanent precipitate is formed. A drop of hydrochloric acid is now added, until the precipitate just disappears, and then a large quantity of sodium acetate added to the hot solution. This is boiled for 10 to 15 minutes, and the precipitate of iron and alumina quickly filtered off, washed, and again dissolved in hydrochloric acid, etc., in order to separate such traces of nickel as it may contain. Chlorine water is added to the nickeliferous filtrate, followed by potassium hydrate (some zinc also will be precipitated), and if necessary, more chlorine water. This is boiled for a quarter of an hour, or until the precipitate of nickel and cobalt sesquioxides has assumed a black color. It is then filtered, the precipitate dried, and the oxides reduced in a current of hydrogen gas (p. 121). For the separation of nickel and cobalt, the weighed metals are dissolved in hydrochloric acid. The solution is slightly supersaturated with sodium carbonate. A concentrated solution of potassium nitrite is added, and then some acetic acid until a weak acid reaction is established. The liquid is then allowed to stand for 24 hours, when the yellow cobalt precipate is filtered off and washed with a concentrated Fresenius' Verfahren für Erze, Leche und Speisen in Fresenius' Ztschr. xii. 70. solution of potassium chloride or potassium sulphate. The precipitate is now dissolved in hydrochloric acid, the solution precipitated with sodium hydrate and chlorine water, etc., and the cobalt sesquioxide reduced in a current of hydrogen gas. The solution carrying the nickel is precipitated with potassium hydrate and chlorine water, etc. In order to separate the iron, the oxidized iron is partly precipitated with sodium carbonate (p. 195). Some acetic acid is added, and the liquid is heated to 30 to 40° C. (86 to 104° F.) in order to dissolve the precipitated iron. It is then boiled, the precipitated iron is again dissolved two or three times, and the precipitation repeated to separate the nickel from it. Finally, by adding sodium sulphide the ferric oxide is converted into iron sulphide, and this is treated with diluted hydrochloric acid, whereby a trace of nickel sulphide may still be found in the residue. b. Nickeliferous solution from the assay with sulphocyanide for determining copper in nickel coins.—In order to decompose the sulpho-cyanide, the solution is evaporated with the addition of 10 cubic centimeters (0.61 cubic inch) of nitric acid, which will cause the fluid to become first red, and then colorless. The nickel is precipitated by pouring the nickeliferous liquid into a boiling solution of 100 cubic centimeters (6.1 cubic inches) of sodium hydrate, 10 per cent. strength, which is boiling in a platinum evaporating dish. It is allowed to boil, then diluted with water, and again heated to the boiling point. It is allowed to settle, and then decanted through a filter. The precipitate is again boiled three times, each time with 200 cubic centimeters (12.2 cubic inches) of water, and then filtered. The precipitate is dried, heated to redness, and rubbed fine, again washed with boiling water, dried, ́ignited, and the nickel determined from the nickel protoxide' (now free from alkali) with 78.38 per cent. nickel. 1 Fresenius' Ztschr. 1878, p. 58. Inaccurate results will follow if less, or less concentrated, sodium hydrate is used, or if the washing is continued too long, as under these circumstances some nickel is dissolved as hydrated oxide. B. Volumetric assay with sodium sulphide.—The copper which may be present is removed by means of sulphuretted hydrogen, and its percentage determined by the potassium cyanide method (p. 124). The filtrate from the precipitation with sulphuretted hydrogen is evaporated with nitric acid, and the iron precipitated with ammonia. The precipitate is dissolved twice or three times, and again precipitated with ammonia (the methods with ammonium sesquicarbonate (p. 19) or with sodium acetate (p. 195) give a more accurate separation); a standard solution of sodium sulphide, 50 cubic centimeters (3.05 cubic inches) of which precipitate 0 25 gramme (3.85 grains) of nickel, is then added to the vigorously boiling solution, until all the nickel (with cobalt) has been, separated. The addition of sodium sulphide is made in quantities of not more than cubic centimeter (0.015 cubic inch) at a time, the solution being maintained at the boiling point, until a filtered drop becomes brown when brought in contact with lead solution upon a porcelain plate. The solution of lead is prepared by dissolving equal quantities of lead acetate and potassium tartrate in caustic potassa. The normal solution is prepared by dissolving 0.25 gramme (3.85 grains) of pure nickel (or cobalt), or an equivalent quantity of pure oxide or salt, in 5 cubic centimeters (0.3 cubic inch) of concentrated nitric acid. This is diluted with water, supersaturated. with ammonia, tested with a saturated solution of sodium sulphide, which is thereupon suitably diluted. 1 Journal für prakt. Chemie, lxxxviii. 486 (Küntzel); xcii. 450 (Winkler); Fresenius' Ztschr. vi. 66 (Braun). - Separation of cobalt. The metallic sulphides of nickel and cobalt, which have been precipitated with sodium sulphide, are filtered off, and the precipitate is washed with sulphuretted hydrogen water. is then dissolved in aqua regia and evaporated with hydrochloric acid to expel the nitric acid. It is now strongly diluted with water in a suitable flask, and nearly neutralized. Elutriated barium carbonate is added, chlorine gas introduced, and the precipitated black cobalt sesquioxide dissolved in hydrochloric acid. The barium is precipitated with sulphuric acid, ammonia in excess is added, and the cobalt titrated with solution of sodium sulphide. The barium is then precipitated from the nickeliferous filtrate, ammonia is added, and the nickel similarly titrated with solution of sodium sulphide. When zinc and manganese are present, the ores are fused with potassium cyanide, arsenious acid, soda, and black flux, to a button containing all the nickel and cobalt, as well as a part of the copper and iron, while zinc and manganese are slagged off. The button is dissolved in aqua regia, the copper and arsenic are precipitated, hot, with sulphuretted hydrogen, and the nickel and cobalt determined as above. C. Colorimetric assay.'-A colorimetric method for ́determining nickel has been proposed by Winkler, but it is of little practical value. VII. COBALT. 42. ORES. Smaltine, CoAs2, with 28.19 Co; cobalt glance (cobaltine), CoASS, with 35.5 Co; cobalt pyrites (linnaeite), (NiS.COS.FeS) (Ni2S3.Co2S. Fe2S), with 14.6 Ni and 11 to 40.7 Co; glaucodot, (Fe Co) AsS, with 24.77 Co; earthy cobalt, (CoO.CuO)2MnO2+4H2O; cobalt bloom, COAS2O+8H2O, with 37.5 CoO 29.5 Co. = 1 Journ. f. prakt. Chem., xcvii. 414. 43. ASSAYS OF COBALT. The object of the assays is 1. The determination of cobalt by the dry (p. 183) or the wet method; in the latter case by gravimetric (p: 195) and volumetric analysis (p. 197), of which the details have already been given in the foregoing chapter on nickel. 2. The determination of the blue coloring power (density), and the beauty of the colors (smalt colors) which are formed in fusing ores and products containing cobaltous oxide with different quantities of potassium silicates (smalt assay). 44. SMALT ASSAY. Cobaltous oxide, either contained as such in the ores (earthy cobalt, cobalt bloom), or produced by roasting sulphurized and arsenized ores, gives a blue color to fused potassium silicate (smalt glass, very likely CoO.3SiO2+ K2O.3SiO2), the color, under otherwise equal conditions, being the more intense, the richer the ore in cobaltous oxide. The presence of foreign metallic oxides, which also dissolve in potassium silicate, exerts an injurious effect upon the beauty of the blue tint. Nickel protoxide, the most injurious foreign substance, produces an objectionable reddish or violet tint; ferrous oxide, if present in small quantities, gives a greenish shade, but ferric oxide, bismuth oxide, lead oxide, and manganous oxide effect the cobalt color to a very small extent only. Manganic oxide gives a violet tint; cupric oxide, a green; cuprous oxide, a red tint; while the coloring power of manganous oxide is counteracted by that of ferrous oxide, if both are present. In roasting arsenized and sulphurized cobalt ores, the various metals become oxidized in succession, cobalt the |