Part 1 (1/2)
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.