128 TREATMENT OF CLASS III. add a small quantity of a prepared solution of magnesium and chloride of ammonium, and set aside for twelve hours. The formation of a whit precipitate of arseniate of ammonium and magne dence of the presence of arsenic. APPENDIX, + REAGENTS. [** Those reagents in the following list, which are marked with the double asterisk, are rarely employed; a single, small bottle of each of them in the labora tory, will be enough for many students. *Those marked with a single asterisk had also better be kept in one or two bottles of sufficient size, rather than be dis tributed to each student, because of their liability to spoil; several of them are used in considerable quantities.] Hcl. 1. Chlorhydric Acid (Concentrated).—The strong common acid prepared by chemical manufacturers, though usually far from pure, will answer for most of the purposes of this manual. It must, however, be continually borne in mind that the commercial acid is usually contaminated with sulphuric acid, and very often with traces of arsenic and iron. These impurities may be present in sufficient quantity to render the acid unfit for use when these very substances are to be tested for in the mixture to be analyzed. Hel The yellow color of the commercial acid, though often attributed to iron, is really due for the most part to the presence of a peculiar organic compound which is soluble in the strong acid. If in any experiment doubts arise concerning the character of a reagent, a quantity of it, somewhat larger than that which has been mixed with the substance under examination, should be tested by itself, and the reaction compared with that exhibited in the doubtful case. If the result of this trial is unsatisfactory, the experiment must be repeated with reagents which are known to be pure. 2. ** Chlorhydric Acid (Pure) may be prepared by distilling a mixture of fused chloride of sodium and sulphuric acid, and collecting the gas in water. (See Eliot and Storer's Manual of Inorganic Chemistry, Exp. 49.) HCH,0 3. Chlorhydric Acid (Dilute). — Mix 1 volume of the com mon concentrated acid, or - - where special purity is re the pure strong acid, with 4 volumes of water. 4. Nitric Acid (Concentrated).— Use the colorles cial acid of 1.38 or 1.40 specific gravity. Strong nitric ac able purity can usually be obtained from the dealers chemicals. An acid, which when diluted with five part gives no decided cloudiness with either nitrate of silver ( chlorhydric acid) or nitrate of barium (absence of sulphur good enough for most uses in qualitative analysis. 5. Nitric Acid (Dilute).— Mix 1 volume of the st with 5 volumes of water. 6. Aqua regia should be prepared only in small qua the moment of use, by mixing in a test-tube one volume nitric acid, with three or four times as much strong ch acid. 7. Sulphuric Acid (Concentrated). — The oil of v commerce will usually be found pure enough for the purpose manual. 8. ** Sulphuric Acid (Pure). — Sulphuric acid fre chlorhydric acid is necessary in testing for chlorine acco § 71. Such acid may be obtained from the dealers in fine icals, but acid purporting to be pure should invariably be by diluting a portion with a considerable quantity of water a ing a few drops of nitrate of silver. No turbidity should after the mixture has stood for some time. 9. Sulphuric Acid (Dilute) is prepared by gradually ing 1 part by measure of the concentrated acid to 4 parts of contained in a beaker or porcelain dish; the mixture must be stantly stirred with a glass rod. When the mixing is finishe sulphate of lead, which has liquid is left at rest until all the rated from the strong acid, has settled to the bottom; the liquid is then decanted into bottles. ** 10. Sulphuric Acid (Dilute) for the separation of per and cadmium (p. 25). Mix, as described in the preceding tion, 1 part of the strong acid with 5 parts of water. 11. Oxalic Acid. - Dissolve 1 part, by weight, of the com cial crystals, in 20 parts of water. 12. Acetic Acid. — The ordinary commercial acid. 13. Tartaric Acid should be kept in the state of powder, si solutions of it slowly decompose. For use, dissolve a small port of the powder in two or three times its volume of hot water, H2S H2S+ ag 14. Sulphuretted Hydrogen Gas (Sulphydric Acid), is prepared as needed, by acting upon fragments of sulphide of iron with dilute sulphuric acid in the apparatus described in §§ 89, 90 of this Appendix. The apparatus should always be placed either in the open air or in a strong draught beneath a chimney. 15. Sulphuretted Hydrogen Water. -Pass sulphuretted hydrogen gas into a bottle of water until the water can absorb no more. To determine when the absorption is complete, close the mouth of the bottle tightly with the thumb, and shake the liquid. If the water is saturated, a small portion of the gas will be set free by the agitation, and a slight outward pressure against the thumb will be felt. If the water is not fully saturated, the agitation will enable it to absorb the gas which lay in the upper part of the bottle, and a partial vacuum will be created, so that an inward pressure will be felt. Since sulphuretted hydrogen water soon decomposes when exposed to the air, it should always be kept in tightly closed bottles, and no very large quantity of it should be prepared at once. A good way of keeping the solution is to fill a number of small phials with the fresh liquid, cork them tightly, and invert them in water, so that their necks shall always be immersed and protected from the atmosphere. At the moment of using this reagent its quality should always be proved by smelling of it, or by adding a drop or two of the liquid to a drop of acetate of lead, which should be immediately blackened from the formation of sulphide of lead. HN+aq 16. Ammonia-Water.-Commercial aqua-ammoniæ may usu ally be obtained pure enough for the purposes of this manual. Dilate 1 volume of the strong liquor with 3 volumes of water. Ammoniawater should be free from carbonic acid; when diluted, as above, it ought not to yield any precipitate when tested with lime-water. HNHS 17. *Sulphydrate of Ammonium.-Pass sulphuretted hydrogen gas through ammonia-water, diluted as described in § 16, until a portion of the liquid yields no precipitate when tested with a drop of a solution of sulphate of magnesium (absence of free ammonia). Since sulphydrate of ammonium decomposes after a while, when exposed to the air, it is not advisable to prepare it in large quantities. In case any doubt arise as to the quality of the reagent, add some of it to a drop of acetate of lead. Unless a dense black precipitate of sulphide of lead is immediately thrown down, the sulphydrate is worthless. (H)CO, 18. Carbonate of Ammonium. — Dissolve 1 part, by weight, ༩ of the commercial salt, in 4 parts of water, and add to 1 part of strong ammonia-water. The solution of carbonate of ammonium should be kep made of glass which is free from lead. If kept in flint-gl the carbonate of ammonium takes up some lead so tha precipitating the carbonates of Class VI, this reagent is solution containing sulphydrate of ammonium, a dark appears in the liquid or obscures the precipitate to the an the operator. 19. Chloride of Ammonium.- Dissolve 1 part, by the crystallized commercial salt in 10 parts of water. 20. Oxalate of Ammonium. Dissolve 1 part, by the salt in 24 parts of water: or, dissolve 37 grms. of cr oxalic acid in 450 c. c. of water, neutralize exactly with a water and dilute to the bulk of 1 litre. 21. ** Molybdate of Ammonium.- Digest 1 part, b of molybdic acid for some hours in 4 or 5 parts of strong a water, and mix the clear solution with 12 or 15 parts of stro acid; or dissolve 1 part of molybdate of ammonium in 3 of of weak ammonia-water, and mix the liquid with 12 or 15 nitric acid, as before. 0 22. Caustic Soda. — Place 1 part, by weight, of the b mercial caustic soda in a large, stoppered bottle; pour upon parts of water, and shake the bottle at intervals until the w the soda has dissolved. Leave the bottle at rest until the li become clear, and finally transfer the solution, with a siphon small bottles in which it is to be kept for use. The soluti prepared, though pure enough for the uses prescribed in thi ual, is really far from pure. It would be unfit for use in a research, because it is usually contaminated with chloride, s and carbonate of sodium, and is liable to contain traces of nate, phosphate and silicate of sodium. Since some nitr sodium is added to it in the process of manufacture, the soda ble to be contaminated with this salt and the products of its position, including ammonia. This last impurity is liable to be off when the solution is boiled. Caustic potash, as prepared for surgeon's use, may be subst for caustic soda whenever it can be more readily obtained. potash should be dissolved in about 10 parts of water. Since solutions of the caustic alkalies act upon glass rather e especially when its outer surface or "fire-glaze" has once bee moved, it often happens, when the soda solution is kept in |