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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 (H, S) 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, 8+2FeCl=Fe, 8+2KCI.

precip. 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, Cl-KHO=NH, HO+KCl. In this case the hydrate of ammonium is resolved immediately into the gas ammonia (NH,) and water (H, 0); and ammonia is remarkable for a peculiar

and pungent odour. Upon a similar formation of an odorous and volatile body depends, in great measure, the recognition of the presence of the acetic acid-radical. To the substance supposed to be or contain an acetate, sulphuric acid and alcohol (C, H, HO) are added, and the mixture heated. The former reagent will produce, with an acetate, acetic acid (i.e. acetate of hydrogen) (HC,H,O, or HĀ); and this, acting in the nascent state on the alcohol or hydrate of ethyle (C, H, HO, or EHO), yields a fragrant body, the acetate of ethyle, thus

HĀ+EHO=H, 0+EĀ.

acetate of

ethyle. The acetate of ethyle, or acetic ether, is volatilized at the temperature employed, and, possessing a most peculiar and agreeable fruity odour, is easily identified.

We will now consider briefly the second method of detecting acid-radicals, by the actual elimination of the acid-radical itself, and the subsequent identification of its peculiar properties. And here it must be borne in mind, that a great distinction must necessarily exist between the behaviour of the acid- and basic radicals, simply on account of their inherent nature. The basic radicals with which the student has been made acquainted, and which occur in great abundance, are (with a few exceptions) elements, and, moreover, they are all (ammonium, mercury, and hydrogen only excepted) solid at the ordinary temperature. On the other hand, however, more than half of the acid-radicals of common occurrence, and almost all the rarer ones, are compound bodies, which are characterized by great diversity of physical condition, several being gaseous, some liquid, and others solid. The fact of a body being compound frequently adds to our facilities of recognizing it; for, in addition to any peculiarities which it may exhibit as a compound, we have other clues to its presence in the characteristic properties which its constituents may individually manifest when, by its decomposition, they have been set free. In fact, those acid-radicals which are complex cannot, with but few exceptions, be obtained in the isolated condition, but are resolved either into other and generally simpler compounds, or into their elementary constituents. Whilst, therefore, we indubitably prove the presence of an elementary acid-radical by obtaining it in the free state and examining its properties, we no less distinctly show the existence of a compound acid-radical when we observe some of the well-ascertained features of the decomposition which it undergoes when liberated.

To illustrate the foregoing cases, we will take two examples. When chlorine acts upon a bromide, decomposition ensues, with separation of the element bromine, the peculiar features of which, such as the yellow colour which it imparts to a solvent, or the orange tint which starch-paper exposed to its vapour assumes, or its peculiar suffocating odour, may be immediately recognized

MBr+Cl=MCI+Br. But if, on the other hand, we deal not with an elementary, but with a compound acid-radical, such as that existing in the oxalates, by treating the salt in such a manner as to decompose it into new and characteristic products, the latter frequently furnish us with good proof of the previous existence of the compound acid-radical. These products, in the case of the oxalates, are the two gases carbonic anhydride (CO2) and carbonic oxide (CO); and they are obtained by the action of sulphuric acid upon oxalates, the decomposition being aided by heat, thusM,C,04+2H, 50,=M, SO4+H, SO4, H, 0+CO,+CO.

hydrate of sulphuric

acid. The more easily recognizable gaseous product, or at least the more characteristic, is carbonic oxide (CO), which, when the carbonic anhydride (CO,) has been removed by an appropriate agent, may be readily kindled on the application of a light, burning with a blue flame.

The preceding hints will perhaps suffice to show the general principles upon which the detection of acid-radicals is based, and to exhibit the chief points in which their reactions resemble, and also those in which they differ from the reactions of the basic radicals. We now proceed to detail these reactions, adopting the subdivisions which have already been chosen in the description of the acid elements. The classification of the acid-radicals is thus entirely founded upon the chemical characters of each group, and not upon their similarity in an analytical point of view. 1. Salts containing the acid-radicals of Subdivision I. (page 17). SALTS OF CHLORINE, BROMINE, IODINE, AND FLUORINE, AND OF

THE CHIEF COMPOUND ACID-RADICALS INTO THE COMPOSITION
OF WHICH THEY ENTER.

2. Salts containing the acid-radicals of Subdivision II. (p. 18). SALTS OF OXYGEN, SULPHUR, SELENIUM, AND TELLURIUM, AND

OF THE CHIEF COMPOUND ACID-RADICALS INTO THE COMPOSI-
TION OF WHICH THEY ENTER.

3. Salts containing the acid-radicals of Subdivision III. (p. 18).

SALTS OF CARBON, BORON, SILICON, TANTALUM, NIOBIUM,

PELOPIUM, AND TITANIUM, AND OF THE CHIEF COMPOUND
ACID-RADICALS INTO THE COMPOSITION OF WHICH THEY ENTER.

4. Salts containing the acid-radicals of Subdivision IV. (p. 18).

SALTS OF NITROGEN, PHOSPHORUS, ARSENIC, AND ANTIMONY,

AND OF THE COMPOUND ACID-RADICALS INTO THE COMPOSITION
OF WHICH THEY ENTER.

SUBDIVISION I. SALTS OF CHLORINE, BROMINE, IODINE, AND FLUO

RINE, AND OF THE CHIEF COMPOUND ACID-RADICALS INTO THE COMPOSITION OF WHICH THEY ENTER. The three first-named members of this Subdivision present the most striking resemblance to each other, both in the salts which they form by simple union with the basic radicals, and also in those in which they exist as a part only of a compound acidradical. Fluorine differs in many respects, and in none more decisively than in not forming compound acid-radicals similar to those which the other members of the group are known to yield.

In the further arrangement of the present Subdivision, as also in the arrangement of those which follow, we take advantage of the differing chemical constitution of the members, and not of any analytical characteristics which they may present. In accordance with this plan, we now proceed to divide the present group into three Sections, as follows:

ifferingsement of angement of of the gron

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