Wilson / Robinson / Staveley An Approach to Chemical Analysis

Its Development and Practice
1. Auflage 2013
ISBN: 978-1-4831-8573-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 384 Seiten

An Approach to Chemical Analysis: Its Development and Practice provides an overview of the development of chemical analysis and its application in solving analytical problems in chemistry. The text is comprised of 19 chapters that are organized into two parts. In the first part, the text covers the historical aspects of chemical. The book then proceeds to tackling methods for analysis in which the final measurement is preceded by one or more chemical reactions. The first two chapters of the second part discuss distillation and chromatography, respectively. Next, the title details the physical methods that only occasionally and incidentally need to be preceded by chemical reactions. The text will be of great use for students, researchers, and practitioners of chemistry.

Autoren/Hrsg.

Wilson, H. N.

Robinson, Robert

Staveley, L. A. K.

Irving, H. M. N. H.

Leseproben

CHAPTER 2

“CLASSICAL” ANALYSIS AND THE IMPACT OF PHYSICAL CHEMISTRY

Publisher Summary

This chapter describes the impact of physical chemistry on classical analysis during 20th century. The chapter also discusses the application of physical chemistry to analytical problems. Faraday’s law of electro-equivalence, 1833, is sometimes regarded as the beginning of physical chemistry. Faraday was also responsible for the words anion and cation, but electrochemical ideas of a qualitative go back to Berzelius, and soon led to the placing of the metals in order of increasing electropositiveness. Ostwald’s dilution law was used for calculating approximately the distribution of the various components involved in an equilibrium as a function of the hydrogen ion concentration. Of equal importance was the work of Arrhenius, who in 1877 first clearly expressed the idea that electrolytes in solution must be dissociated into free ions even when the solution was not subject to an externally applied electric potential.

(The author uses the word “classical” with some hesitation; it does not seem to carry the same meaning when used about analytical methods as when applied to art or literature, “ It also seems to imply a preference for long gravimetric methods of a respectable antiquity, and it is in that sense that it is used here.)

IF WE read the older textbooks, we find that up to about 1880 or even much later, the general aspect of “classical” analytical chemistry is rather like a vast patchwork quilt, some of the pieces being very old indeed and taken from the empirical practices of primitive and often secretive industries such as the alum industry, and others from recent discoveries of scientific workers. But—like the contemporary chemistry—all this information is ill-organized, and follows no discernible pattern, though the amount of information is already vast. Already in 1864—the year Fresenius’ was published—qualitative analysis is very similar to that taught 60 years later, though equations are hardly used, water is still HO and Professor Bunsen’s improved gas lamp is a novelty recommended for use with the spectral apparatus, “the best for the detection of calcium, strontium, and barium”. Part of the vast mass of facts contained in the Fresenius volumes or in Crookes’ was not quite accurate, but the analysts of the period certainly knew how to make the best of what they had, and they managed to advance both science and their bank accounts just as successfully as the later generations. Some of this earlier knowledge was subsequently forgotten; Sir J. J. Fox was asked in about 1940 whether he could recollect any method of separating cadmium and zinc, not involving the use of H2S. “No,” he said, “I can’t. But you go and look it up in the earliest edition of Fresenius that you can find, and you are sure to find something that will give you a lead,” and so it turned out. Most of the processes were lengthy, difficult and complex, and almost all the final determinations were gravimetric. Volumetric analysis in general was regarded as something rather commercial and low, and not very accurate, despite the evidence of Gay-Lussac’s silver method that it was potentially more accurate than any gravimetric method then known. Now, when we have a fuller understanding of dissociation constants, oxidation potentials, solubility products and so forth, it can be maintained that it is potentially more accurate than gravimetry. The initial error arose because was such an accurate operation, and it was felt that if the weight of the precipitate was accurately known, the result was accurate. Of course it is not, because only too often the precipitate is impure. This must have been accentuated in the early days by the imperfect separations then available, some derived from qualitative analysis, others from processes used in preparative chemistry. To these sources of error must be added the use of many imperfectly understood reactions. Crookes himself said that “analyses made and published by the most eminent chemists vary between 99.1 and 100.7 per cent; many analyses yield results between 97 and 102 per cent, while the rest never see the light at all”.

The root of the trouble was that improvement was hindered by lack of sound generalizations, could only be empirical, and it was not possible to proceed far by deduction or even by analogy. Progress was therefore by small advances on very narrow fronts, and so continued until the value of the concepts of physical chemistry became recognized by the analysts, when the new methods of thought led to new techniques and the improvement of old processes.

An early example is the electrodeposition of copper. Faraday had put forward his law of electrochemical equivalents in 1833, and copper was first deposited by electrolysis in 1837 (but not for analytical purposes). The first known analytical application was in 1867. The story as told by Crookes (Select Methods) is interesting. The copper ores mined at Mansfeld were very variable, and the directors of the mines, being dissatisfied with their analyses which were even more variable, offered a cash prize for a new method. A committee of three (two practical assayers and “the well-known Dr. Böttger”) decided that

(i) Lengthy processes, and those that required the operator to have too much scientific training should be excluded.

(ii) Processes in which the operator judged from the appearance of the ore how large a sample to take, should be excluded.

(iii) Evaporation of large volumes of acid, the evolution of quantities of SO2 or H2S, and potentially violent reactions to be excluded.

(iv) Expensive reagents to be excluded.

(v) Dry assay to be excluded, on grounds of too great expense and inaccuracy.

(vi) Precipitation of iron, alumina, etc., to be excluded as the precipitate always carries down copper.

These conditions have a strangely modern look; this might almost be an instruction for a modern chief analyst in a rather old-fashioned industry. (But he would be unlikely to win a cash prize for his efforts.) Item (vi) is particularly interesting, as this is one of the facts that has been repeatedly forgotten and rediscovered in the last 100 years. Out of sixteen processes submitted, two were chosen for further study. Both were electrolytic in nature. In the first the copper was precipitated direct from an acid solution by a rod of zinc fastened to a piece of platinum foil. After removal of the zinc rod, the precipitated metal was washed, dissolved in water and nitric acid, the solution made ammoniacal and titrated with potassium cyanide solution to the disappearance of the blue colour. In the second it was demonstrated for the first time that copper could be deposited from a nitric acid solution in a coherent form by “the galvanic current”. Mercury, silver and bismuth accompanied the copper, but were not present in interfering amounts. The copper was plated on to a platinum foil cathode, washed, dried and weighed as in present practice. But as the current used was small, the assay required ten hours to complete, so the referees chose the other method as it was rapid, needing only four hours. One would think that the much simpler iodide method (described by de Haën in 1854) would have been much quicker; perhaps the reagent was too expensive. It certainly was much used in copper mines for very many years, and could be completed in far less than an hour. By 1955 X-ray fluorescence spectrometry could do the work in about five minutes; today it could provide a continuous record of the copper content of an ore or concentrate—wet or dry—on a conveyer band or in a stream of sludge almost instantaneously.

THE ROLE OF PHYSICAL CHEMISTRY—FROM 1870 ONWARDS

The above account, bringing us to about 1870, also brings us to the application of physical chemistry to analytical problems—a development impossible to overestimate. In systemizing and explaining the mass of facts huddled together under the old roofs of “organic” and “inorganic” chemistry a new architecture was created, in which every stone has a proper part to play in support of its neighbours. One may think that some old courtyard or stable is complete, and perhaps it is; but discovery or application of some powerful theory or hypothesis opens out a new area, and development begins again.

This book is not a history of physical chemistry, but it is worth while recording some of the theories that had the greatest impact on chemical analysis. These are not always the most important; analysts most of the time get on very well without thinking about the phase rule for example, though one cannot imagine a phase rule investigation that does not need large numbers of analyses.

Modern inorganic chemistry perhaps began with the put forward by Mendeléeff in 1869, and perhaps we may regard the beginning of physical chemistry as Faraday’s law of electro-equivalence (1833). Faraday was also responsible for the words...