Sidney Siggia
Olin Mafhieson Chemical Corp. New Haven, Connecticut
Perspectives on the Role of Chemistry in Analytical Chemistry
There has been much discussion in recent years about the disappearance of chemistry from analytical chemistry. There is nothing further from the truth. I t should suffice to say that one cannot deal with chemical materials in any way, analytical or otherwise, without a knowledge of chemistry. "Chemistry" still means that the chemical properties of materials are characteristic. The rest follows automatically since characteristic properties are very desirable in analysis. However, the development of physical instruments has shifted emphasis to the taking of measurements on a substance as it is presented, without an intervening reaction. To the nonanalyst (i.e., physicist or physical chemist) it appear that now reactions are no longer needed. Even though we nlav accept the premise that reactions are no loiger needed for analysis, the materials being examined are still used in chen~ical reactions. We must understand these reactions to know what to look for analytically. Without a knowledge of chemistry, one cannot know which physical instrument or other approach to use. Also, one does not get full value of what the data mean if the basic chemical problen~is not clear. The function of an analysis is to guide the chemist in his selection of the critical experiment. The analytical chemist must not only have knowledge of the analytical methods but also he must be able to anticipate an anomalous product. This requires more than a fleeting acquaintance with chemistry. The reasons for the statement that chemistry is going out of analytical chemistry is that reactions are no longer required with the modern tools. This is true to a degree, but the people making these statements do not apparently recognize how much more can be, and is being, done with these modern physical tools if chemical reactions are applied along with the physical measurement. The buret-balance-pipet set of tools basically gives mass-volume data. This in itself provides little insight into characteristic qualitative or quantitative composition. However, when one does some chemical reactions and obtains mass-volume relationships, one can get characteristic qualitative and quantitative composition. The chemistry in this case is absolutely essential and beautiful quantitative and qualitative analysis was done using only this approach. I n the case of the instrumental or physical methods of aualysis, one measures light, electrical, magnetic properties, or other properties which alone shed light on composition. However, these devices still leave room for doubt in many cases. For example, absorption spectroscopy measures light absorbed. Although this measurement alone can be related directly to qualitative and quantitative composition, it is never absolute. However, a 624
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Journal of Chemical Education
subsequent chemical reaction with reexamination of the derivative by the same device or by other devices confirms the conclusions. Some reactions commonly used with instrumental approaches to verify identities are shown in the table. The list is not exhaustive, but suggestive. The reader will observe that most of these are the same classical reactions which were used when analysis was all "chemical" in nature. The following series of exan~plesshow that the application of chemistry to instrumental systems was essential to solving - the uroblem. 1. Mass spectrometry, infrared, and nuclear magnetic remnance we, a11 used on a system. Resultant data showed two n o s a i h l e ...-.-stn~d~ to ~henroqant .--~ ~r.-.q-....
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HsCZ
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HsC6
/c=c \ HJ!.
"c=c
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/CaHa
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This impasse was resolved by a.permangrtnrtte oxidation and infrared examination of the product. Benzophenone was found which indicated that structure (A) was the correct one 2. Traces of an racetylenic compound had to he determined in its ethylenic counterpart. Neither compound would form characteristic colors, nor were they polarographically active, nor did they abaorb in the UV region. However, reaction of the -CSwith mercuric acetate resulted in
which did absorb UV light making a characteristic quantitative analysis possible. 3. Mass spectrometry, infrared and NMR show two possible structures on a system
A sodium borohydride reduction resulted in a secondary alcohol (as determined by NMR) whirh verified structure (A). 4.
(A) \ CClCHpBr and (B) \ CBrCHnCl were two possible
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structures arrived a t by means of instrumentrtl techniques. De(confirmed by IR). hydrohalogenation yielded 'C=CHB~
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This indicated that structure (A) was correct.
There are many other examples which demonstrate how little things have really changed. At one time we precipitated and weighed the precipitates to make certain analyses. Now we can precipitate and then make
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X-ray fluorescence ineasurelnents on the precipitate. Not only is this rapid, but it also allows multiple analyses in one operation. This technique also adds selectivity and sensitivity to the X-ray approach. At one time, we developed a visible color and measured it by eye or with simple colorimeter; now the colors we develop go into the ultraviolet and infrared and we have developed fluorescence methods as well. Where we once prepared a derivative and got a melting point, we now prepare a derivative and look at it with IR, X-ray, mass spectrometry, and/or NMR. Reaction Oxidation Sehiff base farmstian Esterification and smidification Reduction (hydro~enation or hydrides) Hydrolysis Halogenation Dehydrohalogenation Urethane formation Deutemtion Oxime formation Metallic derivatives
Type of Compound Beinp. Examined Unsaturated, aldehydic, hydrazines Carbanyl, amino Hydroxyl, amino Nitro, am, hydrmo, carbonyl~,unsaturation; peroxides Esters, amldes, nitriles, urethanes, wocvanates. water ~nsiltbation' Halogenated Isocyanates, hydroxy Active hydrogen Carbanyls Acetylenes
A further answer to the individuals who feel that chemistry is going out of analytical chemistry is the fact that the classical volumetric, gravimetric, and combustion methods are still very much in evidence and heavily used. Some current problems can best or only be solved using "wet" methods. In addition, many of our instrumental methods depend on calibrations arrived a t by the classical chemical methods. In summary, to make an analysis, we take measurable parameters of the compound and use them if we can. If there are not enough measurable parameters, me can react the compound to get more measurements. At one time, the only measurable parameters were volume of gases or liquids and nature and mass of precipitates. Now, however, we have optical, electrical, thermal, magnetic, or other properties as well. These are applied with and involve the same chemistry. Without this chemistry, all our physical parameters would he of little analytical value. How can we handle chemical materials and solve chemical problems with purely physical approaches? The answer is that we cannot do so and maintain effectiveness. Test tubes, flasks, beakers, and burets still belong in the analytical laboratory.
Volume 4 7 , Number 7 7 , November 1 9 6 4
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