Analysis of Subdivision IV. SECTION II.-Salts of metals which form Sulphides soluble in Sulphide of Ammonium. The salt may be one of TIN, ANTIMONY, ARSENIC, PLATINUM, or GOLD. If, as is probable, it is a sulphide, wash thoroughly; add hydrochloric acid; boil, and add, while boiling, a few drops of nitric acid, drop by drop, until the precipitate is dissolved. Pour the solution down the funneltube of a hydrogen apparatus fitted with an "arsenic-tube" (see p. 213). Apply the heat of a spirit-lamp, urged by a blowpipe, to the arsenic-tube. The formation of a mirror after the lapse of a few minutes indicates the presence of Arsenic or Antimony. If no mirror is obtained when the action in the hydrogen apparatus has ceased, brush the black powder which is found, carefully from the surface of the zine; it may contain Tin, Antimony, Platinum, or Gold. Dissolve the Wash it by decantation, boil it with hydrochloric acid, and filter. mirror out of the tube by a drop of hot nitric acid; evaporate in a porcelain dish to dryness at 100° C.; redissolve in a drop of water, and add nitrate of silver. A yellow or Any residue indicates the acid remaining, then add A brown red precipitate precipitate, or yellow If no precipitate occurs, the solution should be neutralized with ammonia; indicates Arsenic ; no change spangles, in indicates Antimony. dicates the presence of Gold. a yellow precipitate will indicate the presence of Platinum. Perfect solution indicates the presence of Tin or Antimony. Boil the hydrochloric solution, add a drop or two of nitric acid, or a crystal of chlorate of potassium; cool, and add excess of solution of sesquicarbonate of ammonium : the formation of a precipitate insoluble in excess indicates the presence of Tin. precipitate by dissolving the tochloride of mercury a precipitate Tin. The solution may contain Antimony. Pass hydrosulphuric acid gas into it, and then acidify with hydrochloric acid: a reddish-orange precipitate indicates the presence of Antimony. (see page 192) (see page 196) (see page 204) (see page 207) * Signifies that a precipitation occurs, but not Signifies that the precipitate which is formed consists of of the salt indicated in the horizontal column. the salt indicated in the horizontal column next below. CHAPTER VII. DETECTION OF THE ACID-RADICALS IN THEIR COMPOUNDS. THE reactions of the basic radicals having been fully described in the preceding Chapter, the student may now acquaint himself with the tests by the application of which the presence or absence of the acid-radicals is determined. The slightest consideration will show that, to a great extent, the same means which serve for the detection of the basic constituents of salts will also be available for the recognition of their acid-radical; for it is obvious that, if a soluble chloride produces a white, curdy, insoluble precipitate in solutions of silver salts, a solution of a silver salt will prove an equally certain test for the presence of a soluble chloride. Still, owing to the numerous broad distinctions, physical and chemical, which exist between the two classes of radicals themselves, other means of no less decisive nature are at hand in abundance, by the employment of which, the class and also the individual characteristics of the acid constituent present in any given salt can be determined. A few remarks upon each of these methods of testing may not be out of place as an introduction to the present Chapter. And, firstly, of the detection of the acid-radical by the formation of certain saline compounds of well-defined chemical or physical properties. As just now stated, a very large number of salts (easily recognizable by such characteristics as the following,-solubility in certain menstrua and insolubility in others, or insolubility in all, or again, the possession of some remarkable features of form or colour) are employed in common, as the products by the formation of which the presence either of the basic or of the acidradical contained in them can be safely predicated. Thus chloroplatinic acid (HPtCl,), usually employed as the test for the presence of potassium, may itself be recognized by the employment, as a reagent, of a soluble salt of potassium. Sulphuric acid, often employed to detect the presence of barium, may itself be recognized infallibly by a soluble barium salt. The presence of mercury in a solution may be evidenced by the action of a soluble chromate, which, in its turn, may be recognized by a mercury salt. Lead may be detected by hydrosulphuric acid, and hydrosulphuric acid by lead. By these instances it may therefore be seen, that the same reaction differently applied may be used to indicate both a basic and an acid-radical. Nor, be it observed, is it necessary that the hydrogen salt of the acid-radical be employed; in almost every instance (as in the third of the examples just given), any salt will be as efficient as the acid itself or salt of hydrogen, provided only it be soluble: thus, in the cases cited above, chloroplatinate of sodium is as good a precipitant as chloroplatinate of hydrogen, i. e. chloroplatinic acid; the sulphides and sulphates of potassium, sodium, and ammonium act equally well with the acids themselves, i. e. the corresponding salts of hydrogen, the hydrosulphuric and sulphuric acids. And indeed it frequently happens that the desired reaction does not take place when the acid is employed, but only upon the addition of some other than the hydrogen salt; the reason of this may be stated to consist in this,— that the hydrogen compound or acid liberated at the time of the formation of the new compound, either keeps that compound completely in solution, or prevents its entire precipitation. Thus, if hydrosulphuric acid (HS) be added to ferrous chloride (FeCl), no sulphide of iron (Fe, S) falls, since this ferrous sulphide is soluble in hydrochloric acid, the necessary complementary product of the reaction. But if, on the other hand, a soluble sulphide, such as the sulphide of potassium, sodium, ammonium, barium, strontium, or calcium, be added to ferrous chloride, an immediate precipitation of ferrous sulphide occurs; for it is no longer the chloride of hydrogen, i. e. hydrochloric acid, which forms the secondary product of the reaction, but the chloride of a far more powerfully basic radical, a chloride therefore which exerts no decomposing or solvent action on the ferrous sulphide: in this case the reaction is as follows: K, S+2FeCl = Fe2 S+2KC1. Occasionally, as the student will have already observed in the reactions employed to detect the basic radicals, the recognition of substances does not depend upon the formation of a salt of extreme insolubility, or of remarkable physical features of form or colour, but upon the production of a body which is readily volatilizable, and possessed of an easily recognized odour. The detection of the basic radical ammonium depends upon a decomposition of this kind, which the following equation will recall: NH,C1+KHO=NH_HO+KC1. In this case the hydrate of ammonium is resolved immediately into the gas ammonia (NH3) and water (H, O); and ammonia is remarkable for a peculiar |