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PART II. THE METHOD OF ANALYSIS.

CHAPTER I.

INTRODUCTION.

TERRESTRIAL matter, although consisting primarily of a definite number of elements, is found, as the student will now have learnt, in an almost infinite variety of combinations. These combinations or salts are formed, it will be remembered, by the union of two classes of bodies, which may be either simple or compound, and are distinguished as basic radicals, and acidradicals. To test these combinations, that is, to identify their basic or acid constituent, is easily accomplished by the employment of any appropriate reaction which may elicit some characteristic feature of colour, odour, insolubility, &c.; but to analyse such a combination, that is, to separate its basic and acid-radical either in an isolated condition or in a new form, is a more difficult task. And this difficulty is, of course, enhanced when the process of separation has to be performed upon a complicated mixture of salts.

Although complex bodies of mineral origin, almost without exception, as well as a vast majority of those derived from vegetables and animals, are reasonably believed to be definite saline combinations of basic and acid-radicals, still several substances which occur abundantly in organic structures, such as albumen, fibrine, starch, and gelatine, are composed of molecules so complex as to baffle all attempts made to catch a glimpse only of their constitution. These bodies, therefore, are not subjects for chemical analysis; at least, they cannot be resolved, like the saline combinations of compound radicals, into their proximate, but only into their several elementary constituents.

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Since, therefore, the processes of qualitative analysis to which the present work is confined have for their object the separation, recombination, and recognition of certain forms of elementary or of definite compound matter, i.e. the separate identification, in the bodies termed salts, of their acid- and basic radicals, it is obvious that all indefinite compounds, to which ultimate analysis only is applicable, are excluded from consideration. All salts commonly met with (using the term salt in its widest sense, as including bases, oxides, acids, &c.) may, however, be considered as fit subjects for qualitative chemical analysis.

It will be at once seen that the certainty of analysis depends much upon the simplicity or complexity of the bodies with which it has to deal. Thus, chemical analysis is most likely to be successful with bodies of mineral origin; for in them, generally speaking, the elements exist in the simplest forms of combination. These constituent elements of mineral substances are, moreover, possessed of most powerful chemical affinities, and, above all, are usually neither themselves gaseous at ordinary temperatures, nor do they yield gaseous products. On the other hand, the bodies derived from the animal and vegetable kingdoms are compounds in which, for the most part, the four elements, carbon, hydrogen, nitrogen, and oxygen, exist in large proportion. The ready decomposability of these organic bodies depends in great measure upon the gaseous character, i. e. the volatility of their constituent elements, and upon the remarkable tendency which carbon and hydrogen, or carbon, hydrogen, and oxygen present, to unite in endless and ever-varying proportions. Other causes combine to render these substances unstable, such as the great affinity of carbon and of hydrogen for oxygen (CO, and H2O being thus formed), and of nitrogen for hydrogen (NH, being the product): the gaseous character or ready volatility of the simpler compounds thus produced is also to be taken into account. But the causes of the instability of these organic substances is not now the object of discussion; it is merely adduced to show how small a hold upon them chemical analysis possesses, when we consider that its object is to separate a particular com

pound, which in these substances may slip, by a Protean metamorphosis, through the fingers of the experimenter in the very process of detection. The action of reagents, themselves the means of analysis, is sufficient to decompose bodies such as these.

But it is far otherwise with the elementary or simpler forms of matter which constitute mineral bodies. These are not so affected by reagents; and if isolated, there is in most cases no danger of their volatilization at ordinary temperatures, and none of their decomposition: neither is their presence so concealed as to be undiscoverable; for no known means, however energetic, can destroy their identity. It matters not whether they exist dissolved in mineral waters, crystallized as distinct substances in masses of mineral, or diffused in minute quantity through various soils; their recognition and separation is a matter of almost equal certainty. It is thus that mineral poisons are so much more easy of detection than those of vegetable or animal origin. The latter poisons are, in the majority of cases, easily destroyed, and often cannot be satisfactorily pronounced to be the poison supposed without a quantitative determination of the proportions in which the carbon, hydrogen, nitrogen, and oxygen are present. Arsenic, lead, and copper, on the other hand, are subject to no such accidents. Whatever stage of decomposition the body poisoned by these substances may have reached, these minerals can still be recognized and separated from the mouldering remains.

So far for the limits and the scope of qualitative analysis; we will now speak of its method.

The student will now have become acquainted with a certain number of tests, the application of which, either singly or combined, cannot fail to assure him of the presence or absence of every basic or acid-radical. To analyse successfully, there is, however, one point to which he must invariably attend; and that is, to preserve a well-ordered sequence in the application of the tests at his disposal. By the tables which have been appended to each subdivision of the basic and acid-radicals (in Chapters

VI. and VII.), the student has been somewhat prepared for this system, which, although at first sight it may appear to be a waste, is in reality an economy of time. The only true source of certainty in analysis consists, in fact, in this application of tests in a certain fixed order. The first step must always be to separate a group of substances (such as a subdivision of basic radicals) the members of which may always be removed together as one precipitate by the employment of a certain reagent. The labour is thus simplified: for if such a reagent fail to produce a precipitate, we know that an entire group of bodies is absent; if, on the contrary, a precipitate is produced, we have an assurance that it can only contain certain members, and know therefore the limit of our search in that direction. This accomplished, a second group-test is applied, and the same course followed, until, having exhausted group-tests, we have ascertained in what subdivision or subdivisions the substance or substances sought for exist. Then, by the application of less general tests, we continually subdivide the groups until the isolation of the individual members is accomplished. This principle applies to the analysis of simple salts or mixtures-to the detection of basic or acidradicals. And the student cannot be warned too early of the extreme folly of what may be termed analytical "angling"-of the promiscuous employment of tests which, when properly applied, are extremely effective in detecting individual substances. Complicated results may thus ensue, to unravel which may baffle all the ingenuity of the student; many substances, too, may thus be entirely overlooked, in consequence of the special test employed acting upon bodies other than the one sought for, and in a way not remembered.

To take an instance from a frequent occurrence in the laboratory. The student has a solution to analyse, the colour of which is green. He instantly concludes that the base is copper; and instead of employing the ordinary sequence of group-tests, he devises a short and easy method. Knowing that hydrate of ammonium gives a characteristic reaction with copper salts, he adds it a green precipitate is formed, and then redissolved; but the solution does not present the deep blue colour of cuprammonium salts. Thus he is disappointed; but, still under the impression that he is dealing with a

copper salt, he tries the action of hydrate of potassium: a green precipitate occurs, somewhat pale, it is true, but the student nevertheless regards it as confirmatory of his original supposition. Ferrocyanide of potassium is next added; and the green precipitate which follows is a new perplexity. In despair at these results so conflicting and so contradictory of his original idea, he adds sulphide of ammonium: the black sulphide formed confirms his first supposition; the doubts consequent upon the previous reaction clear away; and he definitely pronounces the solution to contain copper. Why has nickel been thus obviously mistaken for copper? Because the experimenter, in defiance of the conflicting evidence which the special tests have afforded, has entirely neglected all proper sequence of experiment, and omitted to apply the test which claims precedence of all-the group reagent, hydrosulphuric acid in an acid solution, by the employment of which he would at once have been able to decide the question about which so much time has been wasted.

The student, however, should be equally careful not to trust too implicitly to a mechanical familiarity with the routine of analysis, and should learn by repeated trials to assign to each indication no more than its due value. The accumulation of evidence is also of the highest importance.

Next to the accurate recollection of reactions characteristic of each basic and acid-radical, a scrupulous attention to the orderly application of tests is, as we have seen, most necessary to success in analysis; and so, in order that the student may be well practised in this method, it is advisable that he should at first test substances for basic radicals only, reserving for a second stage the analysis of salts with a view to the detection of their acid-radicals for these latter are, for the most part, identifiable only by a chain of circuitous evidence. And it must be remembered that, upon the knowledge of the basic radical present in a salt, the mode of testing for its acid constituent is frequently founded; indeed, the presence of the latter is often ascertained in the examination for the former constituent. We now proceed, therefore, to details.