Page:The New International Encyclopædia 1st ed. v. 01.djvu/584

ANALYSIS. ANALYSIS, Chemical. The art of determining the chemical composition of substances. The derivation of the word analysis (see preceding article) suggests that chemical analysis necessary requires the breaking up of substances into their constituent parts. In practice the term is used in a wider sense, and is often applied to methods of testing that involve no processes of separation. In most eases, however, one or the other constituent is actually isolated, or some constituents of the original substance, which would interfere with the examination, are actually removed.

An analyst may restrict himself to merely determining what are the constituents of the substance submitted to him; in that case the analysis is qualitative. Or he may also determine the relative amounts of some or all of the constituents; then the analysis becomes quantitative. In some eases he can only state what elements are present, and in what quantities they enter into the composition of the given substance. The analysis is then said to be ultimate. In most cases, however, he further tries to deter- mine in what combinations and in what condi- tions in respect to their capacity of forming combinations the elements exist in the given sub- stance; and then the analysis is termed proxi- mate. The ultimate analysis of organic sub- stances is of great importance, and has been brought to high perfection. (See Cakbon Com- pounds.) On the other hand, the proximate analj'sis of organic substances is often a task beyond the power of analytical chemistry. At- tempts, however, have been made to treat this subject, too, in a systematic manner.

Pr^LIMINARY EXAJIINATION OF INORGANIC SuBSTA.xcES. When a substance is submitted for qualitative analysis, the chemist first notes its color and form — the latter with the aid of a simple magnifying glass. The substance is then usually sulijected to an examination by means of the blowpipe (q.v. ) or the non-luminous gas- flame. (See Flame.) Blowpipe analysis has been elaborated into a systematic scheme for the de- tection of all the important metallic and of some acidic radicals, and has proved of- gi'eat value, especially to the mineralogist. The chemist, as a rule, makes only a brief examination to deter- mine the general nature of the substance, and to answer such questions as whether water, or- ganic matter, silicates, complex cyanides, large quantities of an easily reducible metal, sulphur and arsenic, are or are not present, such con- stituents often rendering necessary a modification of the usual scheme of systematic analysis. Heating a small portion of the substance in a closed glass tube reveals the presence of most kinds of organic matter by the smell and separa- tion of carbon, and the presence of water by the drops which condense in the cooler part of the tube. Heating on charcoal with a reducing flame, sometimes with the aid of fluxes, shows the pres- ence of metals that give volatile oxides, the latter forming characteristic coats on the charcoal; and the same test makes it possible to detect any important quantity of an easily reducible metal, metals in the free state being readily identified l)y their lustre and physical properties. The behavior of the substance when fused with a bead of sodium metaphosphate or of sodium car- bonate shows whether a silicate or much silica is present, etc. Often additional special tests are made. For example, gently warming a small por- tion of the substance with concentrated sulphuric acid may serve to detect volatile acidic sub- stances, such as sulphurous acid and nitrous acid, which might be lost in the regular processes or appear in another form.

If the substance submitted for analysis is a liquid, its color and odor are noted, its reaction toward litmus is ascertained, a portion is evapo- rated to dryness, and the solid residue, if there is any, is subjected to the preliminary examina- tion as in the case of an}' other solid.' Qu.i.iT.TivE Inorganic Analysis. Before a systematic qualitative analysis of a solid sub- stance can be undertaken, the substance must be obtained in solution. Sometimes substances sub- mitted for analysis are found to be directly sol- uble in water. In most cases, however, substances cannot be dissolved unless transformed chemi- cally. Since most chlorides and most inorganic acids are soluble in water, the desired transfor- mation can usually be efl'ected by treating the finely powdered substances with aqueous hydro- chloric acid, which converts the metals or metal- lic oxides present into chlorides, while the acids originally combined in the substance aie set free. In ease metals (such as silver) are present, which form insoluble chlorides, or in case non-metals (such as sulphur or arsenic) are present, or in case hydrochloiic acid does not attack the sub- stance, nitric acid is used. By this the metallic compounds present in tlie substance are trans- formed into nitrates, and all normal nitrates are soluble in water; on the other hand, the non- metals present are mostly changed into the cor- responding oxygen acids, which are likewise soluble in water — sulphur, for instance, being transformed into sulphuric acid. Many impor- tant and familiar substances, however, resist the action of both of these acids. A few, as gold and platinum, will dissolve, forming soluble com- pounds in a mixture of hydrochloric and nitric acids, the so-called aqua regia, which, on warm- ing, gives off free chlorine. But other substances, such as glass, porcelain, and many natural silicates, resist the action of acids almost en- tirely. Such substances are usually broken up by melting them with carbonates of the alkali metals and potassium nitrate, or by treatment with hydrofluoric acid. Subsequent treatment with water and hydrochloric acid then usually yields the required solutions.

Let us suppose that we have obtained a clear solution in nitric acid, which may contain all the more familiar metals and is free from organic matter. To this solution we add hydrochloric acid; if we obtain a white solid substance, which does not dissolve in a moderate excess of acid, we know we must have present some or all of the three metals, lead, silver, or mercury in the univalent form, since, of all the more familiar metals, only these three form insoluble, or nearly insoluble, chlorides. The solid precipitate is separated from the liquid by filtration, and we have then on the filter a solid which may consist of any or all of the chlorides of lead, silver, and univalent mercury. A study of the properties of these chlorides shows that lead chloride is freely soluble in hot water, while the other two are not. Therefore, if the mass is treated with hot water, the lead chloride, if present, will dissolve, and can be filtered off while the other two remain behind. The liquid is then examined for lead, which is easily done, since all metals which could interfere with the test have been