L. 6. BASSETT, 1. H. HARLEY,' and 5. E. WIBERLEY Rensselaer Polytechnic Institute, Troy, New York
THEpast decade has witnessed a remarkable r e surgence in interest in analytical chemistry. That "rare bird," the analytical chemist, has at last achieved recognition as an indispensable member of the research team which is used today to attack a complex problem of modem chemistry in any of its branches. The increasingly important role of the analytical chemist in industrial chemistry has been discussed by C. Rosenblum ( I ) . The increased prestige of the analytical chemist may he attributed, in part, to his considerable success in solving the difficult analytical problems which have been presented by the sudden demand for formerly seldom used or unknown materials such as the antibiotics, and uranium and the newer elements formed from it by transmutation. Much of this success has been achieved by the development of new physical methods of analysis. These methods, commonly called instrumental methods because of the necessary emphasis on instrumentation, have opened a new field to the analytical chemist, and have greatly broadened his vision. The present widespread interest in instrumental methods is indicated by current analytical literature. A survey (2) for the year 1946 showed that 56 per cent of all research papers on the broad subject of chemical analysis dealt with instrumental methods. The annual reviews of analytical chemistry which appear in Analytical Chemistry, beginning with the year 1949, devote more than half of the total space to discussions of instrumental methods of analysis. G. Ewing (3) has an undergraduate recently described in THIS JOURNAL course in special methods of analysis involving the utilization of several instrumental methods. The colleges and universities have been quick to recognize the need for providing training in this rapidly growing field, hut they are confronted with difficult problems in fulfilling the need. It is not easy to fit new courses into an already crowded curriculum especially since it is recognized that training in the older, so-called wet methods, must not be abandoned. Then too, instrumental methods require instruments which are expensive, and which are so complex that they can rarely be made in the ordinary departmental shop. Educational institutions will require financial assistance if they are to supply adequate training in this field. The problem of financing modern instrumentation in academic institutions has been discussed editorially by W. J. Murphy (4). In spite of these difficulties
academic training is now being provided. A survey of the teaching of instrumentation made in 1947 by the Fisher Scientific Company (6) showed that in 167 institutions more than one-third already were giving a specific, separate credit course in instrumentation. An additional one-third stated that they fully expected to offersuch a course not later than the spring term of 1948. Since 1946 the chemistry department of Rensselaer Polytechnic Institute has offered a four-semester-hour course in instrumental analysis. Each week two hours are spent in the classroom and two three-hour periods in the laboratory. The course is required for undergraduate chemists (approximately twenty students) in the first semester of the senior year, and is offered as an elective for chemical engineers or others who have the prerequisites of two semesters each of analytical, organic, and physical chemistry. A similar course is required of first-year graduate students who have not had equivalent training in their undergraduate work. I t will be noted that the course is given at a time which will permit those interested to continue work in the field in the form of a research thesis. In addition it has been found that students doing a thesis in fields other than analytical chemistry have made use of the methods and techniques acquired during the course. Most of the research topics of the physical chemists, particularly in kinetics, require analytical measurements. The techniques of absorption spectroscopy are valuable to the synthetic organic chemist in determining the structures of the compounds that he has made. The experimental projects of the chemical engineer often require the use of analytical methods. Experience at Rensselaer has shown that once a student has had a course in instrumental analysis, he no longer approaches an instrument with fear and trembling, but rather fondly regards it as a possible solution to his particular problem. A course in instrumental analysis can conveniently be divided into three parts, absorption spectroscopy (including fluorometry), emission spectroscopy, and electrometric procedures. Each of these fields is considered in turn below. The actual experiments, the equipment used, and the results obtained by students are discussed in some detail. ABSORPTION SPECTROSCOPY
The first experiment is a qualitative study in the ultraviolet. A Beckman Model DU SpectrophoPresmt address: United States Atomic Energy Comrnis~ion, tomet,er equipped with a hydrogen-lamp source is used. New Yark, New York. The student is given an unknown sample of a simple
SEPTEMBER, 1951
467
aromatic organic compound such as toluene or benzene, and some iso-octane purified by the conventional silica gel treatment (6). Using solutions approximately 1 per cent by volume of the aromatic compound in isooctane, the ultraviolet spectrum is obtained from 200 to 400 millimicrons a t wave-length intervals dependent upon the absorption peaks obtained. Then, using the American Petroleum Institute series of ultraviolet spectra, the student identifies his unknown. The second experiment is a quantitative determination of manganese in steel by photometric measurements in the visible region of the spectrum. Each student is given an analyzed steel of manganese content unknown to him. Standard samples containing a half-gram sample of ingot iron and known amounts of a standard manganese solution are then prepared by the student. The unknown is treated along with these standards in 250-ml. pyrex volumet,ric flasks using the conventional treatment of solution in an acid mix consisting of equal parts of nitric acid, phosphoric acid, and water followed by oxidation with potassium periodate and suhseqnent dilution. The absorption spectra of one of the known samples and of the unknown solution are measured on both a Reckman Model B Spectrophotometer and a Coleman Model 108 Spectrophotometer from 350 to 1000 millimicrons. The wave length a t which the maximum absorbance occurs is noted and appropriate filters are selected for the Klett-Summerson and Evelyn filter photometers. Calibration curves are prepared on all four instruments, and the per cent of manganese in the unknown sample is determined. Colorimetric measurements of the unknown, and an appropriate standard are also made with a Duboscq colorirneter. The uee of several types of instruments (colorimeter, filter photometer, and spectrophotometer) enables the student to appraise the assets and defects of each type. Table 1 shows typical stndent results obtained on representaTABLE 1 Manganese Results on Steel Samples
-
llabmcy
Mas.O.30 Xlin. 0 . 2 5 Av. 0 . 2 8 Mar.0.48 hlin. 0 . 3 8 Av. 0 . 4 2 hIax.0.78 Mhl. 0 . 4 1 Av. 0 . 6 2 Mas.0.82 hlin. 0 . 0 8 Av. 0 . 7 4 nrit~.I . or hlin. 0 . 8 0 Av. 0 8ti Max.O.Qfi Min. 0 . 8 2 Av. 0 . 8 0
Manganese, lilell-
Yo----
tive unknown samples on the various instruments mentioned. The deviation b e h e e n the maximum and minimum values reported by the large number of students is surprisingly small for each of the instruments used except for the visual type Duhoscq colorimeter. In the third experiment, the st,udent measures the absorption spectrum of an organic compound in the infrared region using a Perkin Elmer Model 12B Recording Infrared Spectrometer. The wave lengths of
Figure 1. Student Obtainins en Infrared Spectrum Usins an Infrared Recoldin8 spertrometer
the absorption bands are obtained from the prism calibration curves, and the student t,hen identifies the functional group frequencies present hy referring to tables such as those published hy Barnes, Gore, Stafford, and Williams (7), Thompson (a),and Colthop (9). The organic compounds selected for the undergraduate student are ones which have several functional groups. In t,he case of the gradnat,e student, part,icularly an organic major, tshe student is allowed to select his own sample since often he has several unknowns which he is anxious to study. Such a method as this reveals to the student not only the benefits but also the limitations of infrared spectroscopy as a qnalitat,ive tool. The final experiment in ahsorpt,ion spectroscopy is a quantitative fluoromet.ric determination. Fluorescence is caused by the absorption of ultraviolet light and emission of this energy in the form of visible light. As a practical application of fluorescence in inst,rumental analysis, a determination is made of aluminum in steel usmg a method developed m t,his lahoratory. The instrument used is a Klet,t Fluorometer, Model 2070. Using a Vycor heaker, a one-gram sample of steel is dissolved in an acid mixture containing nitric acid, perchloric acid, and mat,er. The sample is heated until fumes of perchloric acid appear, and then fumed for approximat,ely five minutes, cooled, and diluted t,o 250 ml. An aliquot is selected dependent upon the suspected aluminum content, electrolyzed with a
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JOURNAL OF CHEMICAL EDUCATION
Figure 2. A Student Obtaining an Ultraviolet Spectrum
mercury cathode, and treated with 8-hydroxyquinoline and chloroform as described by Wiberley and Bassett (10). However, in place of the colorimetric measurement described in their procedure, a fluorometric measurement is made on the chloroform solution of the aluminum quinolate. This method has advantages over the colorimetric method in that a smaller sample may be taken for electrolysis, and small amounts of titanium and vanadium do not interfere. Some of the equipment used by the students in our absorption spectroscopy laborato~ymay be seen in Figures 1 and 2. EMISSION SPECTROSCOPY
Qualitative Analysis. The instrument used is a Baird 3-meter Grating Spectrograph. The student is given an unknown brass sample and a sample of R. U. powder. R. U. powder is a standard sample first made by J. W. Ryde and H. C. Jenkins of the Research Laboratories of the General Electric Company, Wembley, England, and is marketed in this country through Jarrell-Ash Company in Boston, Massachusetts. It consists of a base mixture of calcium, magnesium, and zinc oxides to which the proper amounts of 47 other elements have been added in such amounts that only a few of the persistent lines (Raies Ultimes) appear for each element in the spectrum of the powder. The spectra of the unknown brass sample and R. U. powder obtained with a d.-c. arc source and the spectrum of pure copper obtained with an a.-c. spark source are superimposed on an Eastman Kodak Spectrum No. 1 Analysis Plate. The student then identifies the elements present in his sample by comparing his plate with a plate vhich has an R. U. powder spectrum with most of the lines identified. This latter plate is prepared by the instructor. The student reports those elements with three or more lines ehown on the plate as definitely being present in the unknown sample. Quantitative Analysts. The same spectrograph as above is used together vith a Leeds and Northmp
Figure 3.
O e n e ~ a View l of the Emission Spestroscopy Laborator?
densitometer. Figure 3 shows two students using this equipment,. The samples used are aluminum samples which were obtained through the courtesy of the Aluminum Company of America. Each student measures three unknown samples and three standard aluminum samples using the conventional Petrey spark stand. Use is then made of the two-line calibration method as described by J. R. Churchill (11) to determine the percentages of the various elements present in the unknown samples. Table 2 shows typical student results obtained over a period of three years using this procedure. TABLE 2 Student SDectrosraDhic Results on Aluminum S a m ~ l e s Sample No.
SA-360 A h a " Av. of 6 studont results Av. dcv. of the mean SA-361 Alcoa Av. of 6 student results Av. dev. of thc mean SA-484 Alcoa. Av. of G student results Av. dev. of the mean SA-485 Alcoa Av. of 4 student results Av. dev. of the mean
0.04
0.02
0.03
..
" Aluminum Company of America analysis.
Flame Photometry. Finally, use is made of a Perkin Elmer Flame Photometer to complete the study of the field of emission spectroscopy. Figure 4 illustrates the equipment used. The samples analyzed are magnesium alloys with a lithium content varying from 1 to 20 per cent. The procedure used is as follows: Approximately a quarter-gram sample is weighed and dissolved in 5 ml. of 50 per cent hydrochloric acid. The excess acid is neutralized with ammonia until the solution is weakly acidic. The resulting solution is diluted
SEPTEMBER, 1951
dropode to adjust the applied voltage to 0.06 volt. A 0.025 N silver nitrate solution is standardized against pure sodium chloride amperometrically, and then titrated against the unknown sample, in a solution containing KNOI as an inert electrolyte, gelatin to coagulate the precipitate, and bromthyrnol blue as a maximum suppressor. The galvanometer readings on the Fisher Elecdropode are plotted versus the ml. of silver nitrate solution added. At the start of the titration there is little change in the current measured with each increment of silver nitrate added because the silver ions are continually removed by the reaction Agf C1- --t AgC1. Once all the chloride ions are removed, e --t AgQccurs at the dropping the reaction Ag+ mercury electrode and the current measured rapidly increases. The plot yields two sets of points, each set falling in a straight line, and the end point clearly occurs a t their intersection. Hence, the volume of silver nitrate required is obtained, and the percentage Figwe 4. A Student Making a M e s s u ~ e m e n with t a n a m e Photomete. of chloride in the unknown sample calculated. Using to an appropriate volume dependent upon the sus- this procedure the student results obtained have been pected lithium content. A series of standards of simi- found to he comparable in accuracy with those ohlar magnesium content and known lithium content tained by the Volhard method. Conductometrie Titration. For this experiment a are prepared and measured concurrently with the unknown sample using the flame photometer. A cali- Leeds and Northrup Wheatstone hridge is supplied bration curve is obtained by plotting the lithium con- with audio-frequency alternating current by a Central tent in p. p. m. versus the dial reading (per cent of Scientific Company Oscillator. The balance point is standard) obtained on the flame photometer. Using obtained by adjusting the bridge to minimum deviation this calibration curve and the dial reading for the un- from a linear trace on the tube of a Dumont Cathodeknown sample, the per cent of lithium in the alloy is Ray Oscillograph. The following procedure is used: A 50-ml. sample of determined. vanilla is diluted with water and titrated by measuring ELECTROMETRIC METHODS the resistance after each addition of a 2-ml increment Polarography. In this experiment the instrument of standard sodium hydroxide solution. The reciprocal used is a Sargent-Heyrovsky Recording Polarograph of the resistance value obtained (i. e., the conductance) (see Figure 5), and the samples analyzed are mag- is plotted versus the ml. of sodium hydroxide added. nesium alloys. The procedure used is a modification Initially there is little change in the conductance obof a method developed by Gull (12). A measure of two elements is obtained from each polarogram. Table 3 shows typical student results obtained on a few of the samples analyzed over a period of three years.
+
+
TABLE 3 Polarographic Results o n Magnesium Alloys Sample
No.
No. of student
% cad-
Dete7minations
mium
% zinc % couue?
8 Av. dev. of mean 10 .iv. dev. of mean
"
Av. dev. of mean
Amperometrie Titration. In this experiment an analysis is made of an unknown chloride sample (Thorn Smith) by titration with silver nitrate using the conventional amperometric assembly with a Fisher Elec-
Figure 5. A View of the Laboratory for Electrometric Methods Showmg a Student Obtaining a Polarogrem
An smoerometrir tltratmil assembly inhy be seen in the fareground. The oondlrctolnetrio rppsratus sppesrrs in the backproond
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JOURNAL OF CHEMICAL EDUCATION
tained and a straight line can be drawn through this each week, and must spend a minimum of two threegroup of points. Once the end point has been reached, hour periods in the laboratory. This course frethe unreacted hydroxyl ions causes a large increase in quently leads the analytical student to a choice of a the conductance of the solution, and another straight subject for his doctoral thesis. line can he drawn through this set of points. The end point is determined graphically from the inter- CONCLUSIONS section of the two straight lines obtained. The samThe experience of the past four years in teaching ples titrated are several varieties of ordinary household these courses has resulted in certain definite convictions vanilla, which contain vanillin, 3-methoxy-4hydroxy- or conclusious which may be of interest particularly benzaldehyde. to those who are contemplating the inauguration of The phenolic group of vanillin can be titrated with formal courses in instrumental analysis. sodium hydroxide and the amount of vanillin deThe courses should he primarily laboratory, not termined by this method. Since the solutions are demonstration, courses. This means that the numdark colored, an ordinary colored ilidicator method is ber of students must be limited by the number of useless whereas the conductometric method yields pieces of equipment which are available. The limitasatisfactory results. The student learns a t least two tion thus imposed may be severe, hut it is necessary basic facts from this experiment: the first being that if the student is really to profit from the opportunity an instrumental method may work where a conven- presented. The student who uses the equipment himtional one may fail, and secondly that the price of self is interested in his work; he acquires techniques vanilla is no criterion of its vanillin content. which will be useful to him in his future work; he Measurement of pH. The main purpose of this ex- recognizes the limitations as well as the advantages of a periment is to acquaint the student with various particular method or instrument; and he may be able methods of determining pH. The student first pre- to avoid disappointment in his future research since pares a series of buffers covering the range from 4.4 his choice of that research mill be based to some extent to 9.4 at intervals of one unit. Then he measures the on his own experience. pH of each of the buffers using a LaMotte Roulette, It is hoped that the student will acquire the idea that a Hellige Comparator, and finally a Beckman pH measurement of any property of a substance is a meter. He is then given an unknown solution to de- possible analytical tool, and that a knowledge of the termine its pH using each.of the three instruments. possible techniques will lead to the use of the best, Potentiometric Titration. Although this type of and most economical, method of analysis. titration is usually carried out in experiments in a The courses are liked by the students. This is physical chemistry course, the method is again used to particularly pleasing to the teacher of analytical chemillustrate its advantages. A determination of istry who is usually accustomed to having graduates in chromium in steel is made by oxidation of the chromium chemistry leave the institution with an openly avowed to the dichromate with ammonium persulfate and sil- distaste for analytical chemistry caused probably by a ver nitrate, and subsequent titration with a standard faint recollection of the sudden exposure in the sophosolution of ferrous ammonium sulfate. A total of more year to the strict discipline of the subject. three samples are run on each of the three following Finally, a course in instrumental analysis may be instruments, a Beckman Model G pH meter, a Fisher begun with but a few pieces of equipment. I t has Titrimeter, and a Garman-Droz Titrimeter. A plati- been the experience of the authors that, once begun, num electrode is used as the indicator electrode with a the course makes a place for itself, and more equipcalomel electrode as a reference electrode. The use of ment somehow becomes available. Indeed the probthree distinct instruments again serves to aid the lem before us now, at least in the first course, is how student in evaluating the assets and defects of each to choose which of the available methods and instrutype. The final phase of this experiment involves the ments should he used. titration of an unknown carbonate-bicarbonate mixture using a Beckman pH meter fitted with glass and calomel ACKNOWLEDGMENT electrodes. The authors wish to express their appreciation to the Metallurgy Department, and in particular to Dr. ADVANCED COURSE Arthur Burr for his cooperation in the use of the emisThe graduate student who is majoring in analytical sion spectroscopy equipment and to Dr. Augustus chemistry is required to take an additional semester Jones for his assistance in obtaining the flame phoof instruction in instrumental analysis. Other gradu- tometer. ate students who have had the first course or its equivaLITERATURE CITED lent may elect this additional training. (1) ROSENBLUM, C., Chem. Eng. News, 28,3578 (1950). The advanced course, which is given to only a few (2) STRONG,F. C., Ind. Eng. Chem., An.al. Ed., 19,968 (1947). students, is given in an informal manner. Each student chooses one of the three fields studied in the ~ ~ ~ ; ~ k J ~ ~ ~ first course for specialized and advanced training for a (5) ~ ~ b17, NO. ~ 2, ~Fisher~ Scientifio t ~CO.,717 ~ Forbes ~ ,st., semester. He consults with the instructor one hour Pittsburgh 19, Pennsylvania. (1948).
i:;
; ~
SEPTEMBER, 1951 (6) G u m , M. M., R. T.O'CaNNon, AND E. L. S m n , Znd. Eng. Chem., Anal. Ed., 16, 556 (1944).
(7) BARNEE,R. B., R. C. GORE,R. W. STABFORD, AND V. Z. W~LLIAMS, Anal. Chem., 20,402 (1948). (8) THOMPRON, H. W., J . Chem. Soe., 1948,32%-31.
471 (9) COI~THUP, N. B., J . Optical Soc. Am., 40,397 (1950). (10) WIRERI,EY, S. E., AND L. G. BASSETT,Anal. Chem., 21, 609 (1949). (11) C:HRRHILL, J. R . , i n d . Eng. Chem., Anal.Ed., 16,653 (1944). (12) Gul,~.,H. C., J . Soc. Chem. 2nd. (London), 56, 177 (1937).
