INORGANIC QUANTITATIVE

ANALYSIS

CHAPTER I

SCOPE AND NATURE OF SUBJECT

1. — Quantitative Analysis has for its object the determination of the quantities of constituents present in a given amount of compound or mixture. For convenience it is usual to divide the subject of Quantitative Analysis into two branches: the one dealing with the analysis of the various compounds of the metallic and non-metallic elements, this is known as Inorganic Analysis; the other dealing with the analysis of the compounds of carbon with hydrogen wherein the hydrogen may be replaced entirely or in part by oxygen, nitrogen, chlorine, or sulphur, this is known as Organic Analysis. It is with the former branch of the subject that this book will deal, namely, with Inorganic Quantitative Analysis.

2. The substances which are generally encountered in this branch of the subject are acids, bases, salts, alloys (ferrous and non-ferrous), minerals, ores and occasionally gases. With respect to the frequency with which the several elements are encountered the following table will serve to give a fair idea, although it must be pointed out that the classification is only relative since the determination of an element may become a frequent matter because of some local reason or because of some technical or commercial application. Thus the determination of tungsten and vanadium fall in the frequent class today whereas two decades ago it was a rare matter to encounter either of these elements.

Yttrium

3. In general, to effect an analysis the procedure of the analyst is to prepare a representative sample and then effect its solution by appropriate means; from this point on the further work of the analyst really divides itself into two quite distinct steps, the first having to do with the isolation of the desired constituent from such of the others as might interfere with its subsequent determination, the second having to do with the actual determination of the desired constituent after the previous removal of the interfering constituents.

Of these two steps the first is often a very complicated matter involving a considerable knowledge of theory and technique, while the second is, at least in many cases, relatively a simple matter. This difference in the order of difficulties cannot be too strongly emphasized; it might seem that the comprehensive schemes of separation which have been worked out for qualitative analysis would obviate the troubles, but this is only partially true

1 Elements in this column are those usually considered in an elementary course of Quantitative Analysis.

because the qualitative schemes in the main are not precise enough. The general tendency has, therefore, been rather to devise special methods, such methods depending not only upon the particular constituents present but also upon the absolute amounts of the constituents; hence it not infrequently happens that a whole quantitative procedure will have to be abandoned or seriously modified because a sample contains a constituent that has not been provided for by the particular scheme.

An intelligent quantitative analysis presupposes, then, not only a knowledge of qualitative analysis but also more or less specific information as to the approximate composition of the sample. Very often it is possible to gain the requisite information in this latter regard by access to proper references if only the general identity of the sample can be established. Thus if the sample is a known mineral or ore, some work giving the analyses of minerals, like Dana's Mineralogy,2 should be consulted; if an alloy, some work on the composition of alloys, particularly the Reprint of the American Society for Testing Materials, entitled A List of Alloys, prepared by William Campbell;3 if no clue whatever as to the general identity of the sample can be had, then recourse to a qualitative analysis must be made.

In view of the foregoing considerations pointing out the greater difficulty of separations in contradistinction to determinations, it seems preferable for pedagogical reasons to treat of them in inverse order. We shall therefore begin by taking up simple determinations in which interfering constituents are absent, and then gradually work into cases where the necessary separations become a matter of increasing difficulty.

4. Principles Involved. — In developing the subject along these lines it seems advisable not to adhere to the traditional treatment of classifying quantitative methods according to the technique employed, as either gravimetric or volumetric, but rather to arrange them according to the physico-chemical principles which are involved. On this basis it will be found that the

2 James D. Dana, A System of Mineralogy. John Wiley & Sons, New York, 1903, 6th ed. This work gives the analyses of about 5500 minerals.

'Reprinted by American Society for Testing Materials, Philadelphia, from its copyrighted proceedings, Vol. 22, Part I. 1922. This work gives the composition of about 1600 non-ferrous alloys.

important principles around which we may group practically all our quantitative methods are those concerning

1. Neutralization

2. Solubility Product

3. Oxidation-Reduction

4. Colorimetry

5. Evolution and Measurement of Gases

This is the order that will be pursued, and the determinations which have been chosen for the laboratory exercises have been selected with the idea of illustrating these principles, along with the supplemental idea that the exercises introduce the student to something new in laboratory technique each time.

5. Precision. The utmost precision that is attainable in quantitative analysis, outside of atomic weight determinations, may be stated as one part per thousand. The reason that this is the limiting precision is to be found in the fact that every analytical method is attended with certain sources of error, such as incompleteness of precipitation, contamination of the precipitate by impurities, incompleteness of oxidation or reduction, etc., and it is not possible to reduce such errors below the limit named except by an unwarranted amount of time and labor. Relatively there are only a very few analytical methods which show a precision of one part per thousand; the greater number of methods show a less precision than this, a few even showing such a poor precision as twenty to thirty parts per thousand. It must also be mentioned in this connection that the precision obtainable by any given method depends also upon the amount of constituent being determined, the smaller the amount of constituent the poorer the precision. For further details about the factors which enter into precision the reader must be referred at this juncture to Chapter IV.

6. Literature. The habit of looking up original articles and consulting the literature pertaining to the subject should form just as much an integral part of an analyst's training as his laboratory work. As a guide in this direction we have appended at the end of the chapter a list of textbooks, reference works and journals that will be of service not only to the student but also to the man engaged in analytical practice.