CHAPTER III. ANALYSIS OF MIXED SALTS. IN the recognition of the various basic and acid-radicals which may be present in a mixture of salts, the plans described in the following Tables are to be adopted, with, however, such modifications as the preliminary examinations may suggest. To the methods of preliminary examination already given (see pp. 391, 404) recourse must again be had; indeed, the indications which they afford are often of extreme value in the analysis of complicated mixtures. The methods for preparing the solution of the substance to be analysed will be the same as those already described. (See pp. 392, 408.) A few words may here be introduced concerning those combinations and mixtures of basic radicals with one another, known as alloys and amalgams. When once in solution, the manner of recognizing their constituents is identical with that adopted in the case of salts; but nitric acid, moderately concentrated, which is usually employed as the solvent for alloys, leaves many metals unacted upon and undissolved (e. g. platinum, gold, &c.), which require further treatment with hydrochloric or nitrohydrochloric acid. Nitric acid, moreover, partly separates tin and antimony in the form of insoluble compounds, which must then be treated according to the plan described on p. 394. Throughout the Tables of the present and preceding chapter, the more common radicals are indicated by a conspicuous type. For the detection, &c. of the acid-radicals contained in a mixture of salts, the Tables already given may be employed (see pp. 413, 414); from these Tables, the rarer organic radicals, the reactions of which have been detailed in Chapter VII., are for the most part omitted, since they seldom occur together in the course of analysis, and generally require the employment of very special methods of separation. For these methods, reference must be made either to Chapter VII. or to a comprehensive treatise on Organic Chemistry. The preliminary examination of the mixture of salts having been accomplished, and its results recorded, the student will then proceed to the actual analysis. 1 GENERAL TABLE FOR THE If HCl has been used for the solution of the substance, and no precipitate be assumed to be absent; but if the substance has been dissolved in water addition as long as any precipitate is produced: when the precipitate ceases, ride of Bi or Sb, or any HSiO, which may have been thrown down. Agitate The filtrate is first freed from HNO, by one or two separate Pt and the mass of the rarer allied metals, given in Table for Group II. If extremely acid, it suspected to contain As in the form of arsenic comof the latter is expelled*, when a rapid stream of H, S pitation of Pb, As, and Mo, it is well to warm the filtime, with H S. A blue colour on the first passage of precipitate may be only S, due to the oxidizing action ever, a mass of S might easily mask a small but imporsuffered to pass unexamined. Collect and wash the The precipitate may contain HS and Pb, S Sn2 S2 Hg S Sb S may be here precipit ated, since they are but CdS slightly soluble in As, S Pd S Mo St water or acids. Sn S Examine according to Examine according to Table If the filtrate is of a previously formed, or evaporated to dryness at moistened with water and separation of HSiO, as sent, the residue should not be interfered with. a current of air: the only dered insoluble in acids; bonaceous mass is left, well to burn the insoluble TABLE FOR THE ANALYSIS OF GROUP I. The precipitates produced by the addition of HCl, and remaining undissolved in excess of that reagent, may contain PbCl, AgCl, and Hg, Cl; the anhydride W2O3; and the acids HSbO, H, Sb. O, HB0O2, HBz, H, S, and H2 U. Boil the precipitate with several fresh portions of water, and wash it, when on the filter, with hot water. The solution may contain РЬСІ, HBOO, HBe and H, S. Add dilute H12SO; a white precipitate indicates the presence of Pb. The acids will be found in the analysis for acid-radical. The residue may contain AgCl and Hg, Cl; W203; HSbO3, H4 Sb. O, and H. Ū. EXAMINATION FOR BASIC RADICALS. either immediate or on cooling has been the result, the first group may or in HNO, or H, SO,, add a few drops of concentrated HCl, and repeat the add yet a few drops more of the concentrated acid, to redissolve any oxychlountil the solution is clear, and then filter. evaporations on a water-bath with HCl; it is then treated with NH, Cl, to and subsequently with H, CO, to precipitate Au, according to the details should then be partially neutralized with Na, CO, still keeping it acid; and if pounds, it must be saturated with SO, gas: it is then warmed until all excess gas is passed through it for at least half an hour. To ensure the perfect precitrate from the first HS precipitate, and to saturate it a second, and even a third HS should lead the student to look for Mo. A white or very pale yellow of unexpelled HNO3, of HCrO, or of ferric salts upon the H., S. Since, howtant quantity of Sb, S, or As, S, or other bodies, no such precipitate must be precipitate completely. fine blue colour, special search for Ru should be made in the Pt precipitate Mo may be looked for again in the following group. The filtrate should be now 100° C. with HNO,; this process should be repeated, and the residue finally just again evaporated to perfect dryness: absolute desiccation ensures the complete Si, O.; it would otherwise be mistaken for Al, H, O. If organic matter is prebe more highly heated, in order that the detection of Al, Cr, Fe, Mn, &c. may It is well to burn organic bodies off entirely, by igniting the residue strongly in danger is, that by exposing sesquioxides to so high a temperature they are rena compromise is therefore frequently effected by heating until merely a carwhich is boiled repeatedly with HCI, and the solution filtered. It would be residue white, to ascertain whether Si, O, is present. 3 To the HCl solution add some quantity of NH, Cl; then almost neutralize with NH, HO; place in a flask, and add (NH), S until no more precipitate is produced: warm gently, allow to rest for some time in the corked flask, and filter, washing with water slightly impregnated with (NH ̧), S. If the filtrate is brown, the presence of Ni may be inferred. First acidify with HA; warm, filter, and add the new precipitate to the one previously produced. The filtrate should be evaporated to expel excess of (NH), S, and any precipitate of S resulting from its decomposition ascertained to be only S by its entire volatility. Solution of (NH), CO is then added until the precipitation ceases; the liquid warmed (not boiled, lest the precipitate should redissolve in the NH, Cl) and filtered." The precipitate may contain BaCO3 Examine according to The filtrate, containing NH, Mg, K, and Na, is examined according to Table for GROUP V. Certain sulphates, as those of Ba, Sr, and Ca, may be precipitated here, the SO, being converted into H, SO,. If insoluble in acids, such precipitate must be examined by fusion. Mo is not perfectly precipitated unless H2S is passed into its alkaline solution, which is afterwards acidified. T 5 TABLE FOR THE The filtrate from Group I. may contain all other basic It must be freed from HNO,: this may be done by evaporating with slight To the HCl solution, which should be concentrated if it has been rendered too agitated, and allowed to rest for some hours. A yellow crystalline precipitate of KPtCl, indicates Pt. The largest part of the rare metals R, Ru, Ir, and also some PdCl, The filtrate should be mixed with a reasonable quantity of A precipitate con- would separate here in brown yellow sponge, the same form. indicates Au. Through the filtrate a current of second and even a third stream of thoroughly washed with hot water S; the undissolved portion must be The residue may contain Hg S, Pb S, Bi S, Cu,S, Cd,S, and Pd S: it must be washed until quite free from Cl, then boiled in concentrated ĤNO, until all red fumes cease. The liquor is diluted with HO, the residue allowed to settle, and dilute H2SO, added until precipitation is complete; it is then agitated and filtered. The residue and precipitate I. Boil in HC1; add one or two drops of HNO, and continue to boil. Drop in a bright fragment of copper foil. A silvery film on the foil indicates Hg. The filtrate may contain Bi, Cu, Cd, and Pd. Add some quantity of solution of KC1; evaporate to dryness on a water-bath, adding a little nitrohydrochloric acid towards the end of the operation. Redissolve in a solution of KCl, adding one drop of HC1; filter and wash with KCl. II. Ignite on a fragment of porcelain. A reddish To the filtrate add excess of NH ̧HO. A white yellow non-volatile crystalline residue of PbSO indicates Pb. A white | precipitate precipitate of KPdCl of BiH,O, indicates indicates Bi. Confirm by the oxychlo Pd. ride test (see p. 182). The filtrate may contain Cu and Cd. Acidify with HCl, and saturate with HS; filter and wash the precipitate with HO impregnated with HS. Boil the precipitate with dilute H2SO. and filter quickly. To the so- The pre lution add excess of Cu S. cipitate is ANALYSIS OF GROUP II. radicals; in the present Group we separate the following :— Pt, R, Ru, Ir, Os*, Au, and Mo. excess of HCl once or twice; in so doing, any Os would escape as osmic anhydride*. dilute, a reasonable quantity of NH Cl solution should be added, the liquid well concentrated solution of HCO1, and moderately heated for some hours. HS should now be passed for a considerable time, the liquid warmed, and a HS passed to ensure perfect precipitation. The precipitate must be collected, containing a little HS, then boiled for a few minutes with (NH),S containing filtered off. The filtrate may contain Sn, As, and Sb, combined with S, in the form of acid-radicals. Reprecipitate them as sulphides by acidulating with HCl; and, to prevent the HCl from decomposing a portion and dissolving it as chloride, pass a few bubbles of HS through the solution. Collect and wash the precipitate; boil it in HCl, and add, when boiling, a very little HNO,, drop by drop, until it is dissolved. Introduce the solution into an apparatus from which H is being evolved by the action of H,SO, upon Zn, both of these reagents having been ascertained to be free from As; pass the evolved gases through a wash-bottle containing solution of PbA, and thence into a test-tube containing a somewhat strong solution of AgNO,, continuing the passage as long as any precipitate is produced in the AgNO, solution. When the action has ceased, a black residue will be found upon the surface of the Zn in the generator t. * The separation of these four metals is most difficult: the process will be described in an Appendix to the present Table. This process was, we believe, originated by Dr. Hofmann, and is employed by him in the Royal College of Chemistry. |