EXAMINATION FOR THE ACID CONSTITUENT.

403

ence. One of the reactions thus employed consists in the decomposing influence which concentrated sulphuric acid exerts upon almost all saline combinations. The sulphuric radical unites with the basic constituent of the salt to form a sulphate, while the acid-radical in the substance either unites with the hydrogen to form a new acid which is liberated, or else splits up into characteristic products of decomposition. Whichever result takes place, its occurrence generally affords sufficient evidence of the nature of the acid constituent originally present.

In addition to the experiment with concentrated sulphuric acid, decisive information concerning the nature of the acidradical present in the substance under examination may often be obtained by gently warming the substance, or its solution, with dilute hydrochloric acid. All concentrated acids, particularly concentrated nitric and hydrochloric, when heated, themselves evolve pungent vapours, which mask the otherwise characteristic odour of the newly liberated acid.

In observing the action of sulphuric or hydrochloric acid upon a salt, it must be borne in mind that the nature of the basic radical present greatly influences the reaction. Thus the salts containing basic radicals of the first and second subdivisions united with weak acid-radicals, or with acid-radicals the hydrogen salts of which are gaseous at ordinary temperatures, are much more readily decomposed by strong acids than are the salts of other subdivisions.

To take an example of this difference in the behaviour of salts. NaCl or BaCl is instantly decomposed by the addition of H2SO4, the HCl escaping with effervescence,-while AgCl is not acted upon. KS or Ca, S is decomposed in a similarly rapid manner by HCl even when dilute, while Fe, S is but slowly acted upon, and Pb, S remains wholly intact. The varying solubility of the different compounds doubtless here influences the play of affinities. Carbonates and sulphites, the acids of which split, as soon as liberated, into water and carbonic and sulphurous anhydrides, are decomposed, perhaps without exception, even by weak acids, whatever may be the solubility of the individual salt operated on; but here it is obvious that the decidedly gaseous character of the chief product of the action must exert a powerful influence in determining the decomposition.

Preliminary examination for the detection of the acid-radical.

Many acid-radicals being by no means distinctly recognized by analysis in the wet way, it will be well for the student to follow

up the evidence he has elicited by the foregoing preliminary examination, by testing specially for various acid-radicals before undertaking the actual analysis.

4

SPECIAL TESTS.

Any suspicion of the presence of Chlorine may be followed by direct testing (with H, SO, and Mn,O,) of the original substance (see p. 255); of Bromine, by the starch test (see p. 259); Iodine, by the starch test (see p. 263); Fluorine, by the etching test (see p. 265); the Hypochlorous radical, by the lead test (see p. 268); the Chloric radical, by the action of concentrated H, SO, (see p. 269); Sulphur, by its odour, blue flame, and coloured salts; Selenium, by its horse-radish odour (see p. 286); Sulphocyanogen, by ebullition with HNO, (see p. 343); the Sulphurous radical, by the odour and action on H, S, of SO, the product of its decomposition (see p. 293); the Carbonic radical, by the action on BaHO solution of CO2, the product of its decomposition; Cyanogen, by passing the vaporized acid, which burns with a blue flame, into a test-tube containing a drop of KHO solution, dividing the solution of KCy thus obtained into two parts, and applying to one the Prussian-blue, and to the other the sulphocyanide test (see p. 342); the Acetic radical, by the acetic-ether test (see p. 350); the Nitric radical, by mixing with a fresh portion of the strong solution of the substance an equal bulk of concentrated H, SO, cooling the mixture, and then dropping in a crystal of FeSO,-on standing quietly for some seconds, a brown halo will appear.

We may remind the student that every result obtained in the above preliminary examination should be as carefully recorded as those observed in the examination for basic radicals. It is, indeed, more necessary to do this in the present case, since the detection and distinction of acid-radicals is more dependent on the preliminary analysis and the special testings consequent thereon, than is the recognition of the basic radicals, previously treated of.

ACTUAL ANALYSIS FOR THE DETECTION OF ONE ACID-RADICAL.

Attention to several points connected with this branch of the analysis of a salt is necessary to ensure success; and—

1. With regard to the solution of the substance under examination. If it dissolve in water, one great source of difficulty is evaded; but if not, it becomes a question in what acid it shall be dissolved. To the use of HCl is presented a great objectionthat it produces a dense precipitate with AgNO,, which is one of the principal reagents employed in the detection of acid-radicals. To the adoption, on the other hand, of HNO, as the solvent for salts insoluble in water, a scarcely diminished objection exists; for with many complex acids, if employed in sufficient strength to dissolve the salt, it oxidizes, and thus completely changes, the acid-radical. H, SO, too is rejected, because, if concentrated, it acts powerfully as an oxidizer, and even when dilute it produces precipitates with BaCl and CaCl, two reagents frequently employed in this course of analysis. Other acids, such as HF, HA, H2O, &c., are either too energetic, too weak, or too rare to be employed; or else they form insoluble salts with the reagents used; or, lastly, by their often complex constitution or ready decomposability they increase the difficulties of the analysis. Thus something may be urged against the use of each solvent acid; and the student must bear in mind the evils and advantages which attach to each, and use his judgment in all cases, by employing that solvent to which the fewest objections are attached. It is well that he should early learn that no strict rule can be laid down for his guidance in chemical analysis, and that this science demands of its practiser not only his whole manual skill, but also his best mental faculties. On the whole, the best acid solvent for general use is dilute HNO,; in this form the acid does does not oxidize very rapidly, and has the immense advantage over most others, of not forming any insoluble salts. The salts of organic acid-radicals should not be boiled, but only heated gently with this acid. In addition to dilute HNO,, HCl, con

centrated HNO,, and HÃ are advantageously employed in many special instances.

2. It will be seen presently that the method already adverted to in the detection of basic radicals may be employed for the recognition of the acid-radical also, in the analysis of salts insoluble in water. We allude to the mode mentioned (p. 395, § 2.) in which the decomposition is effected, and a soluble salt of the acid-radical thus obtained, by boiling the insoluble salt with a solution of NaHO or KHO. This method is frequently applicable where the attempt to make an acid solution would fail, and is especially used in the examination of cyanides, ferrocyanides, &c. The solid salt boiled with a concentrated solution of KHO, to which a little Na, CO, has been added in order to precipitate any metal of Subdivision II. present as a carbonate, gives the following result:

This mode of decomposition may be very advantageously employed in the case of salts containing a complex acid-radical liable to decomposition if treated with HNO,. It is better to boil such salts with Na,CO,, and not with the more powerful KHO. NaCO,, too, is an agent which almost invariably produces an insoluble precipitate: for nearly all carbonates are insoluble in neutral solutions; and in most other cases where an insoluble carbonate is not produced, an equally insoluble hydrate is the result, thus—

2Ba, C, H, O,+3Na, CO,=2Na, C, H, O,+3Ba, CO3.

6 5

an insoluble citrate.

6

a soluble citrate. precipitate.

By such means we obtain a soluble in the place of an insoluble salt of the acid-radical we wish to detect: nor is this the only object attained; we have an alkaline salt, i. e. a salt containing