An Introductory Course of Quantitative Chemical Analysis

An Introductory Course of Quant.i.tative Chemical a.n.a.lysis.

by Henry P. Talbot.

PREFACE

This Introductory Course of Quant.i.tative a.n.a.lysis has been prepared to meet the needs of students who are just entering upon the subject, after a course of qualitative a.n.a.lysis. It is primarily intended to enable the student to work successfully and intelligently without the necessity for a larger measure of personal a.s.sistance and supervision than can reasonably be given to each member of a large cla.s.s. To this end the directions are given in such detail that there is very little opportunity for the student to go astray; but the manual is not, the author believes, on this account less adapted for use with small cla.s.ses, where the instructor, by greater personal influence, can stimulate independent thought on the part of the pupil.

The method of presentation of the subject is that suggested by Professor A.A. Noyes' excellent manual of Qualitative a.n.a.lysis. For each a.n.a.lysis the procedure is given in considerable detail, and this is accompanied by explanatory notes, which are believed to be sufficiently expanded to enable the student to understand fully the underlying reason for each step prescribed. The use of the book should, nevertheless, be supplemented by cla.s.sroom instruction, mainly of the character of recitations, and the student should be taught to consult larger works. The general directions are intended to emphasize those matters upon which the beginner in quant.i.tative a.n.a.lysis must bestow special care, and to offer helpful suggestions. The student can hardly be expected to appreciate the force of all the statements contained in these directions, or, indeed, to retain them all in the memory after a single reading; but the instructor, by frequent reference to special paragraphs, as suitable occasion presents itself, can soon render them familiar to the student.

The a.n.a.lyses selected for practice are those comprised in the first course of quant.i.tative a.n.a.lysis at the Ma.s.sachusetts Inst.i.tute of Technology, and have been chosen, after an experience of years, as affording the best preparation for more advanced work, and as satisfactory types of gravimetric and volumetric methods. From the latter point of view, they also seem to furnish the best insight into quant.i.tative a.n.a.lysis for those students who can devote but a limited time to the subject, and who may never extend their study beyond the field covered by this manual. The author has had opportunity to test the efficiency of the course for use with such students, and has found the results satisfactory.

In place of the usual custom of selecting simple salts as material for preliminary practice, it has been found advantageous to subst.i.tute, in most instances, approximately pure samples of appropriate minerals or industrial products. The difficulties are not greatly enhanced, while the student gains in practical experience.

The a.n.a.lytical procedures described in the following pages have been selected chiefly with reference to their usefulness in teaching the subject, and with the purpose of affording as wide a variety of processes as is practicable within an introductory course of this character. The scope of the manual precludes any extended attempt to indicate alternative procedures, except through general references to larger works on a.n.a.lytical chemistry. The author is indebted to the standard works for many suggestions for which it is impracticable to make specific acknowledgment; no considerable credit is claimed by him for originality of procedure.

For many years, as a matter of convenience, the cla.s.ses for which this text was originally prepared were divided, one part beginning with gravimetric processes and the other with volumetric a.n.a.lyses. After a careful review of the experience thus gained the conclusion has been reached that volumetric a.n.a.lysis offers the better approach to the subject. Accordingly the arrangement of the present (the sixth) edition of this manual has been changed to introduce volumetric procedures first. Teachers who are familiar with earlier editions will, however, find that the order of presentation of the material under the various divisions is nearly the same as that previously followed, and those who may still prefer to begin the course of instruction with gravimetric processes will, it is believed, be able to follow that order without difficulty.

Procedures for the determination of sulphur in insoluble sulphates, for the determination of copper in copper ores by iodometric methods, for the determination of iron by permanganate in hydrochloric acid solutions, and for the standardization of pota.s.sium permanganate solutions using sodium oxalate as a standard, and of thiosulphate solutions using copper as a standard, have been added. The determination of silica in silicates decomposable by acids, as a separate procedure, has been omitted.

The explanatory notes have been rearranged to bring them into closer a.s.sociation with the procedures to which they relate. The number of problems has been considerably increased.

The author wishes to renew his expressions of appreciation of the kindly reception accorded the earlier editions of this manual. He has received helpful suggestions from so many of his colleagues within the Inst.i.tute, and friends elsewhere, that his sense of obligation must be expressed to them collectively. He is under special obligations to Professor L.F. Hamilton for a.s.sistance in the preparation of the present edition.

HENRY P. TALBOT

!Ma.s.sachusetts Inst.i.tute of Technology, September, 1921!.

QUANt.i.tATIVE CHEMICAL a.n.a.lYSIS

PART I

INTRODUCTION

SUBDIVISIONS OF a.n.a.lYTICAL CHEMISTRY

A complete chemical a.n.a.lysis of a body of unknown composition involves the recognition of its component parts by the methods of !qualitative a.n.a.lysis!, and the determination of the proportions in which these components are present by the processes of !quant.i.tative a.n.a.lysis!.

A preliminary qualitative examination is generally indispensable, if intelligent and proper provisions are to be made for the separation of the various const.i.tuents under such conditions as will insure accurate quant.i.tative estimations.

It is a.s.sumed that the operations of qualitative a.n.a.lysis are familiar to the student, who will find that the reactions made use of in quant.i.tative processes are frequently the same as those employed in qualitative a.n.a.lyses with respect to both precipitation and systematic separation from interfering substances; but it should be noted that the conditions must now be regulated with greater care, and in such a manner as to insure the most complete separation possible. For example, in the qualitative detection of sulphates by precipitation as barium sulphate from acid solution it is not necessary, in most instances, to take into account the solubility of the sulphate in hydrochloric acid, while in the quant.i.tative determination of sulphates by this reaction this solubility becomes an important consideration. The operations of qualitative a.n.a.lysis are, therefore, the more accurate the nearer they are made to conform to quant.i.tative conditions.

The methods of quant.i.tative a.n.a.lysis are subdivided, according to their nature, into those of !gravimetric a.n.a.lysis, volumetric a.n.a.lysis!, and !colorimetric a.n.a.lysis!. In !gravimetric! processes the const.i.tuent to be determined is sometimes isolated in elementary form, but more commonly in the form of some compound possessing a well-established and definite composition, which can be readily and completely separated, and weighed either directly or after ignition.

From the weight of this substance and its known composition, the amount of the const.i.tuent in question is determined.

In !volumetric! a.n.a.lysis, instead of the final weighing of a definite body, a well-defined reaction is caused to take place, wherein the reagent is added from an apparatus so designed that the volume of the solution employed to complete the reaction can be accurately measured.

The strength of this solution (and hence its value for the reaction in question) is accurately known, and the volume employed serves, therefore, as a measure of the substance acted upon. An example will make clear the distinction between these two types of a.n.a.lysis.

The percentage of chlorine in a sample of sodium chloride may be determined by dissolving a weighed amount of the chloride in water and precipitating the chloride ions as silver chloride, which is then separated by filtration, ignited, and weighed (a !gravimetric!

process); or the sodium chloride may be dissolved in water, and a solution of silver nitrate containing an accurately known amount of the silver salt in each cubic centimeter may be cautiously added from a measuring device called a burette until precipitation is complete, when the amount of chlorine may be calculated from the number of cubic centimeters of the silver nitrate solution involved in the reaction.

This is a !volumetric! process, and is equivalent to weighing without the use of a balance.

Volumetric methods are generally more rapid, require less apparatus, and are frequently capable of greater accuracy than gravimetric methods. They are particularly useful when many determinations of the same sort are required.

In !colorimetric! a.n.a.lyses the substance to be determined is converted into some compound which imparts to its solutions a distinct color, the intensity of which must vary in direct proportion to the amount of the compound in the solution. Such solutions are compared with respect to depth of color with standard solutions containing known amounts of the colored compound, or of other similar color-producing substance which has been found acceptable as a color standard. Colorimetric methods are, in general, restricted to the determinations of very small quant.i.ties, since only in dilute solutions are accurate comparisons of color possible.

GENERAL DIRECTIONS

The following paragraphs should be read carefully and thoughtfully. A prime essential for success as an a.n.a.lyst is attention to details and the avoidance of all conditions which could destroy, or even lessen, confidence in the a.n.a.lyses when completed. The suggestions here given are the outcome of much experience, and their adoption will tend to insure permanently work of a high grade, while neglect of them will often lead to disappointment and loss of time.

ACCURACY AND ECONOMY OF TIME

The fundamental conception of quant.i.tative a.n.a.lysis implies a necessity for all possible care in guarding against loss of material or the introduction of foreign matter. The laboratory desk, and all apparatus, should be scrupulously neat and clean at all times. A sponge should always be ready at hand, and desk and filter-stands should be kept dry and in good order. Funnels should never be allowed to drip upon the base of the stand. Gla.s.sware should always be wiped with a clean, lintless towel just before use. All filters and solutions should be covered to protect them from dust, just as far as is practicable, and every drop of solution or particle of precipitate must be regarded as invaluable for the success of the a.n.a.lysis.

An economical use of laboratory hours is best secured by acquiring a thorough knowledge of the character of the work to be done before undertaking it, and then by so arranging the work that no time shall be wasted during the evaporation of liquids and like time-consuming operations. To this end the student should read thoughtfully not only the !entire! procedure, but the explanatory notes as well, before any step is taken in the a.n.a.lysis. The explanatory notes furnish, in general, the reasons for particular steps or precautions, but they also occasionally contain details of manipulation not incorporated, for various reasons, in the procedure. These notes follow the procedures at frequent intervals, and the exact points to which they apply are indicated by references. The student should realize that a !failure to study the notes will inevitably lead to mistakes, loss of time, and an inadequate understanding of the subject!.

All a.n.a.lyses should be made in duplicate, and in general a close agreement of results should be expected. It should, however, be remembered that a close concordance of results in "check a.n.a.lyses" is not conclusive evidence of the accuracy of those results, although the probability of their accuracy is, of course, considerably enhanced.

The satisfaction in obtaining "check results" in such a.n.a.lyses must never be allowed to interfere with the critical examination of the procedure employed, nor must they ever be regarded as in any measure a subst.i.tute for absolute truth and accuracy.

In this connection it must also be emphasized that only the operator himself can know the whole history of an a.n.a.lysis, and only he can know whether his work is worthy of full confidence. No work should be continued for a moment after such confidence is lost, but should be resolutely discarded as soon as a cause for distrust is fully established. The student should, however, determine to put forth his best efforts in each a.n.a.lysis; it is well not to be too ready to condone failures and to "begin again," as much time is lost in these fruitless attempts. Nothing less than !absolute integrity! is or can be demanded of a quant.i.tative a.n.a.lyst, and any disregard of this principle, however slight, is as fatal to success as lack of chemical knowledge or inapt.i.tude in manipulation can possibly be.

NOTEBOOKS

Notebooks should contain, beside the record of observations, descriptive notes. All records of weights should be placed upon the right-hand page, while that on the left is reserved for the notes, calculations of factors, or the amount of reagents required.

The neat and systematic arrangement of the records of a.n.a.lyses is of the first importance, and is an evidence of careful work and an excellent credential. Of two notebooks in which the results may be, in fact, of equal value as legal evidence, that one which is neatly arranged will carry with it greater weight.

All records should be dated, and all observations should be recorded at once in the notebook. The making of records upon loose paper is a practice to be deprecated, as is also that of copying original entries into a second notebook. The student should accustom himself to orderly entries at the time of observation. Several sample pages of systematic records are to be found in the Appendix. These are based upon experience; but other arrangements, if clear and orderly, may prove equally serviceable. The student is advised to follow the sample pages until he is in a position to plan out a system of his own.

REAGENTS

The habit of carefully testing reagents, including distilled water, cannot be too early acquired or too constantly practiced; for, in spite of all reasonable precautionary measures, inferior chemicals will occasionally find their way into the stock room, or errors will be made in filling reagent bottles. The student should remember that while there may be others who share the responsibility for the purity of materials in the laboratory of an inst.i.tution, the responsibility will later be one which he must individually a.s.sume.

The stoppers of reagent bottles should never be laid upon the desk, unless upon a clean watch-gla.s.s or paper. The neck and mouth of all such bottles should be kept scrupulously clean, and care taken that no confusion of stoppers occurs.

WASH-BOTTLES