JOURNAL OF CHEMICAL EDUCATION

QUANTITATIVE ANALYSIS-THE SCIENCE OF MEASUREMENT IN CHEMISTRY

DATA GATHERED by the writer at the University of Minnesota and at The Ohio State University show that not more than two per cent of graduates of university courses in quantitative analysis become professional an- alysts. We may, therefore, well ask ourselves what oh- jective should he sought in a beginning course in quan- titative analysis and whether or not we achieve it in courses as now taught.

Analytical chemistry has for some years distinguished between the terms chemical analyst and analytical chemist. The former term applies to one who knows the "hand and arm" operations of a few analyses. He is frequently called a technician and is usually employed on routine work. He is regarded and paid by his employers a t the subprofessional level. Usually he has had little formal training in chemistry. This group contains a large percentage of those who for one reason or another drop out of the university after one or two years. Such people often become highly skilled and their service to chemistry is great. The term analytical chemist applies to the person who has a greater knowledge of fact and theory and who can apply it in new situations. He is a professional person and usually has had graduate training in some univer- sity where graduate work in analytical chemistry is actively fostered. Such people will usually not engage in routine analysis hut will spend their t i e in the development of new methods and in the special proh- lems of the laboratory.

Industrial and other practical laboratories where much routine analysis is done are interested in using only those methods of analysis in which a large margin of safety exists. Unless an accident occurs, a relatively unskilled person can obtain an acceptahle result. This

WILLIAM MARSHALL MACNEVIN The Ohio State University, Columbus, Ohio

is of course as it should he for economic reasons. How- ever, the methods of many textbooks are either ex- tracted from or designed for practical settings. Per- haps the author wanted to meet the needs of the prac- '

tical analyst as well as the student and the text is in- tended as much for practical purposes as for teaching. Whatever the reason, the methods of most textbooks are designed to provide as large a margin of safety as possible with the consequence that the student, if he avoids an accident, need give very little consideration to the sources of error or to the reason for the particular choice of experimental conditions needed to get an acceptable result.

The educational value obtained from the performance of a procedure which, barring an accident, is certain to give an acceptahle result is limited. In the first place, experience has shown that most intelligent people can, by following the details of a procedure closely, analyze successfully. This was clearly demonstrated during the war when many high school students were taught only the techniques of quantitative analysis and ob- tained as good results as the university sophomores who used the same procedures and were also exposed to lectures on theory. A second challenge to the educational value of this kind of experience in quantita- tive analysis arises in the erroneous conception of success as a chemist which is acquired by students so exposed. They feel that because they can follow a procedure successfully they deserve the rank of chemist and professional recognition as such. They also feel that knowledge of the chemistry involved in the design of the analysis is completely unimportant. This state of mind is prevalent even among instructors of quantita- tive analysis who have been nurtured in such an un-

JANUARY, 1948 27

inspiring atmosphere themselves. Later, finding them- selves required to teach quantitative analysis in ad- dition to the subject of their real interest, they per- petuate this cook-book point of view in their students.

I t is not the purpose of this paper to suggest that we discard the common methods of analysis which are more or less the same in all textbooks. Rather, it is the purpose first to suggest that we try to present quantitative analysis to the student in a way that will have more educational value for him and secondly to make some suggestions as to how this can be done.

The practical need, especially the industrial need, for the analytical results of quantitative measurements has been and is so great that our study of quantitative measurement has of necessity been in the field called analysis. However, it must be admitted that quantita- tive techniques can be learned just as well in their application, for example, to the establishment of the laws of chemistry. In some respects, physical chem- istry covers the territory of quantitative measurement for other than analytical purposes. The strongly physical chemical flavor of some textbooks may there- fore be looked on as a compromise. The replacement, in a few schools, of the standard course in quantitative analysis by an elementary course in physical chemistry represents the trend away from the classical pattern.

Neither of the above extreme types of teaching pro- duces an analytical chemist. Certainly the cook-hook experience of quantitative analysis adds little to a student's education and, if nothing else, is definitely misleading. On the other hand, the demands of in- dustry for people with training in the techniques of quantitative analysis must be met. Somewhere in between lies the happy medium.

If one tries to state simply what quantitative an- alysis should accomplish, it would be, first, that it should teach quantitative analysis as "the science of measurement in chemistry." This implies an extensive knowledge of fact and accepted theory. Secondly, the course should also teach "how to analyze."

It is often said among analytical chemists that real ability in analysis is closely related to the ability to recognize and evaluate the errors of an analysis. This is a quality which experience develops. We could, however, make the beginning student of quantitative analysis more error-conscious than most textbooks do. This can be achieved in a number of ways:

1. The student can be asked to present a pre- analysis summary of the sources of error he antici- pates in the experiment he plans to do.

2. He can he asked to arrive a t a conclusion as to the probable magnitude of each of the errors and what their effect on the Enal result would be.

3. He can be asked to show why he takes the size of sample he does.

4. He can be asked to show what error can be expected in the result in view of his choice of con- ditions.

5. He can be asked to decide what special pre- cautions or techniques will have to be applied in order

to achieve the desired precision; likewise, what pre- cautions can be omitted.

6. He can be asked to justify the design of the experiment so that in view of the individual errors whose magnitude he has estimated or assumed he will get a result of a desired reliability.

If a student follows through such a plan of thought, if he carries through the experiment and gets a result which is as good as it should be, then he has learned far more than the student who strives for the maximum perfection in technique a t all t i e s , who uses time and technique not warranted by the precision desired in the result and who in the long run has no idea why his result is as good as it is.

The effect of errors on precision can be emphasized to the student if one or two experiments of higher than usual precision are included. For example, the pre- cision determination of chloride to 1 or 2 parts in 10,000 is not too difficult for a student of quantitative an- alysis toward the end of his course. It will require the application of vacuum corrections, of more accurate weighing techniques, and of corrections for losses through solubility. Appreciation of the effect of errors on precision is also gained by performing an analysis on a micro scale. At The Ohio State Univer- sity, the sophomore chemistry majors have included the micro Kjeldahl determination in their work for several years. They become acquainted with the microbalance and its errors, the application of weight corrections, titration errors, dissolving of alkali from glass and sampling errors.

In one sense it makes little difference whether a student gets a precision of one part per thousand or one per cent. The important thing is that he knows why his result is as good as it is. He should also be able to design the same experiment so that it will produce results whose precision is of any desired magnitude.

Accomplishment of this end means that instructors and textbooks. will have to discuss errors and their magnitudes more thoroughly than they now do. A quantitative determination will have to be presented to the student as a study of errors in a particular opera- tion. The performance of the analysis merely to get an acceptable result must be discouraged. Instmctors will have to discuss details of the laboratory exercises as part of the lecture work. They cannot he left to the textbook as is so often done.

The writer has for several years been presenting quantitative analysis to his students according to the ideas indicated in this paper. The student acquires quite an unusual regard for quantitative analysis. He not only becomes error-conscious, but he thinks more about the chemistry of the determination and develops an analytical sense which he may never get from the empirical approach to the subject. Students who have been thoroughly indoctrinated in this point of view seem to do superior work in the graduate school. They become better research workers, whether it be in analytical or another field of chemistry.