PART III. CHAPTER I. ATOMIC WEIGHTS. Exp. 1. Determination of the atomic weights of copper and iron. Prepare and analyse the two oxides of copper, and the two chlorides of iron, and thus find the combining weights of copper and iron, i.e. the masses which combine with 8 parts by weight of oxygen, or 35.5 parts by weight of chlorine. Purify copper sulphate by solution in water, adding a little nitric acid, and heating to oxidise ferrous salts present, digesting with cupric oxide to precipitate ferric oxide, filtering, and crystallising. Recrystallise the crystals thus obtained, dissolve in water, heat to boiling, and add pure potash solution drop by drop until only a trace of copper sulphate remains undecomposed. Wash the pp. thoroughly by decantation with boiling water, dry at 100°, then heat over a Bunsen-lamp, and preserve the cupric oxide in a stoppered bottle. Dissolve 20 grams pure copper sulphate, 30 grams cream of tartar, and 40 grams dextrose, in a basin, in 240 c.c. water; heat, and add 30 grams caustic soda, and boil until the blue liquid is colourless; wash the red pp. of cuprous oxide thoroughly by decantation with boiling water, dry at 100o, and preserve in a stoppered bottle. Prepare ferrous chloride by dissolving very thin piano wire in pure hydrochloric acid, evaporating in contact with iron till the liquid begins to get thick, filtering, evaporating the filtrate rapidly to dryness in a basin, mixing the solid with about an equal quantity of pure ammonium chloride, filling a flask of about 150 c.c. with the mixture, and heating over a large lamp until the ammonium chloride is completely volatilised; on cooling the semi-fused mass, crystals of ferrous chloride are formed; place the crystals in a stoppered bottle. Prepare ferric chloride by strongly heating very thin iron wire in a rapid stream of dry chlorine; sublime the crystals, in a stream of chlorine, into a dry stoppered bottle. Estimate copper and oxygen in the oxides of copper by heating about 5 gram of each, placed in a porcelain boat in a tube of hard glass, in a stream of pure dry hydrogen (s. Exp. 2 in Chap. III. of Part II.). Weigh the residual copper in each case, and determine the oxygen by loss. Calculate the weight of copper combined with 8 parts by weight of oxygen in each oxide. Estimate iron in the chlorides of iron by dissolving about 5 gram of each in very dilute hydrochloric acid, and titrating with a carefully standardised permanganate solution: reduce the ferric chloride to ferrous, by means of pure zinc, before titration with permanganate. Also estimate the chlorine in the chlorides of iron by dissolving about 5 gram of each in dilute nitric acid, adding a slight excess of pure silver nitrate solution, and determining the mass of silver chloride precipitated by the ordinary gravimetric method. Calculate the weight of iron combined with 35-37 parts by weight of chlorine in each chloride. The results of your experiments shew that 31.6 and 63.2 parts by weight of copper combine with 8 parts by weight of oxygen to form cuprous and cupric oxide, respectively; and that 28 and 18.6 parts by weight of iron combine with 35.37 parts by weight of chlorine to form ferrous and ferric chloride respectively. The atomic weight of copper is then x31·6, and the atomic weight of iron is n18.6 and m28, where x, n, and m are whole numbers. Measurements of the specific heats of the two metals will serve to determine the values of x, n, and m. Prepare approximately pure iron as follows. Place 100 or 200 grams of thin iron wire in small pieces in a Hessian crucible and heat in the air until a portion of the iron is superficially oxidised; then cover the contents of the crucible with a plate of green glass, and heat to the highest temperature of a good wind-furnace. The oxygen of the ferric oxide combines with the small quantities of carbon in the iron, and the excess of ferric oxide dissolves in the molten glass. Prepare approximately pure copper by electrolysing a solution of pure copper sulphate acidulated with sulphuric acid, using platinum poles; wash the copper which separates on the negative electrode and hammer it in a clean steel mortar into a compact mass. Arrange a calorimeter as described in Chap. VII. Part II.; place about 20-30 grams of the iron you have prepared in a thin wide test tube; close the mouth of the tube by a plug of cotton wool through which passes a thermometer with its bulb in contact with the metal; immerse the bulb in steam until the temperature of the metal is constant; note the temperature of the metal and of the water in the calorimeter; quickly transfer the metal to the calorimeter, and determine the rise of temperature. Repeat a similar experiment with about 20-30 grams of the copper you have prepared. Calculate the specific heat of each metal, taking into account the water equivalent of the calorimeter stirrer and thermometer used. Use the results thus obtained to determine what multiples of the combining weights of copper and iron, respectively, represent the atomic weights of these elements. Exp. 2. Determination of the atomic weight of silver, that of chlorine being known; or of the atomic weight of chlorine, that of silver being known. This determination resolves itself into finding the ratio of the masses of silver and chlorine which interact to form silver chloride, when silver nitrate solution is added to a solution of ammonium chloride. Read STAS, Recherches sur les rapports réciproques des poids atomiques, p. 38 et seq; or the German translation Untersuchungen über die Gesetze der chemischen Proportionen &c., pp. 31-60. It is necessary to prepare pure silver, pure ammonium chloride, and pure sodium chloride; the measuring vessels used must be carefully corrected. Pure Silver. Dissolve ordinary coinage silver (containing copper) in dilute, boiling, nitric acid; evaporate to dryness, and heat till the mass melts; after cooling, dissolve in water containing ammonia, and allow to stand for 48 hours; then filter through a double filter, and dilute until the liquid does not contain more than 2 p.c. of silver. Measure the total volume of the liquid. Prepare a solution of ammonium sulphite by saturating pure ammonia solution with sulphurous oxide. The sulphurous oxide is prepared by heating pure sulphuric acid, diluted with toits volume of water, with pure copper, and leading the gas through water. Add excess of ammonia to a small measured quantity of the ammonium sulphite solution; heat to boiling, and run in the ammoniacal solution of silver nitrate containing copper nitrate so long as the blue colour of this solution is removed. Cuprous sulphite is formed and this reduces the silver nitrate with precipitation of silver. Having thus found the volume of the solution of silver and copper nitrates which is decolourised by a specified volume of the ammonium sulphite solution, add to the whole of the solution of silver nitrate containing copper nitrate that volume of the ammonium sulphite solution which will just suffice to decolourise it, place the liquid in a clean stoppered bottle and immerse it in water at 60°-70°. The whole of the silver in the solution is soon precipitated; wash the pp. by decantation with ammoniacal water until the washings cease to give a pp. with barium chloride solution; then allow the pp. to remain in contact with concentrated ammonia solution for a few days; finally wash thoroughly with cold water, and dry. 20 Ammonium chloride. One litre of a boiling concentrated solution of ammonium chloride is mixed with 100 c.c. of nitric acid, specific gravity-1-4; boiling is continued so long as chlorine is evolved, and the liquid is then allowed to cool; the ammonium chloride which separates is dissolved in boiling water, volume of nitric acid is added, and boiling is continued so long as chlorine is evolved. The liquid is then diluted with so much water that no solid separates out on cooling, run into a retort containing pure slaked lime, and distilled. The ammonia thus produced is washed by passing through a little water, and is then led into cold water. solution is nearly saturated with hydrochloric acid gas, made by warming pure hydrochloric acid. The liquid is concentrated; the ammonium chloride which separates is dried at 100°, in a long necked flask of hard glass, through which is passed a slow stream of pure dry ammonia gas. The dry salt is sublimed at the lowest possible temperature, the neck of the flask being kept full of ammonia gas. This Sodium chloride. Pure bicarbonate of soda is heated to dull redness in a platinum dish; the residue is dissolved in cold water; the liquid is evaporated in a platinum dish and crystallised; this process is repeated three or four times. The solid is dissolved in cold water, and saturated with pure hydrochloric acid gas; the sodium chloride which separates is dried, heated to dull redness in a platinum dish, and dissolved in water; the solution is allowed to stand for 24 hours, and is then poured off from the silica which has settled down, and evaporated in a platinum vessel; the residue is heated to dull redness and cooled in an exsiccator. A decinormal solution of the sodium chloride is prepared; 5.837 grams in 1000 c.c.: 1 c.c. precipitates 000766 grams of silver. Method of procedure. A wooden box about 70 centims. long, 22 centims. wide, and 45 centims. deep, is arranged with doors and openings as shewn in Figs. 52 and 53. The inside Fig. 52. Fig. 53. |