JOURNAL OF CHEMICAL EDUCATION
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THE TEACHING OF QUANTITATIVE ORGANIC MICROANALYSIS JOSEPHB. NIEDERL Washington Square College, New York University, New York, New York
W I T H the introduction in 1925 (SO) of quantitative organic microanalysis a t the Washington Square College, New York University, the teaching of quantitative organic microanalytical methods was simultaneously taken up and was incorporated in 1928 in the regular Graduate School chemistry curriculum. In the following a ~dsum6of the teaching experience since gained is attempted. The course at New York University was first organized along the lines practiced in Pregl's laboratory at the University of Graz (Austria), where the student, usually a postgraduate, often a chemistry teacher or industrial chemist, was taught individually the principles of weighing on a microchemical balance, then the more simple methods, and finally the carbon and hydrogen determination and the more special methods of determining molecular weight and molecular and atomic groupings. There mas no special supervision or control or time-schedule and it was up to the individual to make the best of his time, usually from 4 to 6 weeks (24 t o 36 periods), approximately 8 hours a day for the entire course. The course in quantitative organic microanalysis is a graduate course a t New York University; consequently, only graduate students are accepted, each holding either a BSc. or Ch.E. degree. Quantitative inorganic analysis is made prerequisite, but it is of no importance whether the student is majoring in organic or any other branch of chemistry, as it was found that
the training is beneficial for any student of the experimental sciences, whether physics, biology, or chemistry, since the student very soon realizes the importance of exactness, cleanliness, and the painstaking following of experimental procedures. Deviations from these requirements certainly will prove fatal to obtaining acceptable results. The student gradually acquires a feeling for what Pregl called the "chemical asepsis." The course is given in the spring'and lasts from the beginning of February until about the middle of May, about 14 weeks. Each student is required to reserve one full day a week for the course, which thus gives each student 14 periods. For each period there is a definite assignment of work which has to be completed, taking into consideration an eight-hour day, which may be extended according to the individual requirements of the student. Each student is assigned to his own microchemical balance, so that there is no interference from other individuals. The equipment furnished each student is simple: a micro-spatula, a camel's hair brush, a pair of forceps, and a clean towel. The following table illustrates a typical working schedule. At present the course is limited to four students a day and four or five days a week. The balances are set up on a stone table in the middle of the room, away from all immediate sources of heat. No other special precautions are taken. Since it is preferable when weighing on a microchemi-
MARCH, 1950
Period
IS1 Kind of delmination
Allowable error, %
Metals Carboxyl Nitrogen (Kjeldrthl) Nitrogen (Dumas) Carbon and hydrogen Halogen Sulfur Mol. wt. (Pregl, Rieche) ($4, 85,27) Mol. wt. (Rast) ($8) Mol. wt. (Xiederl) ($1, $8)
-0.2 *0.5 *0.3 *0.3 10.3 10.2 10.2 *KO *50 15.0
Kind of determination
1st and 2nd
Technique of weighing; determination of the sensitivity of the bdanee Determination of metals 3rd 4th to 6th Kjeldahl nitrogen Dumas nitrogen 7th and 8th 9th, loth, and 11th Carbon and hydrogen Molecular weights 12th 12th to 14th Examination unknowns
cal balance to use the rider exclusively, avoiding the handling of weight,^, amounts of from 1to 10 mg. will be desirable, with quantities of from 3 to 5 mg. the most convenient. It is therefore quite obvious that wherever there is a microchemical balance available, these "milligram processes" (Pregl methods) are the logical application. This viewpoint has been used by the author in teaching microanalysis in preference to resorting to so-called "semi-micro" methods (deca-milligram procedures) of analysis in use in some educational and industrial laboratories (8, 9, 14, 16, $8). For the metal, carboxyl, and the Rast (26) molecular weight determination, as the simplest types of quantitative microchemical procedures, individual equipment is provided for all the four students, all of them doing the same without interference. For all the other determinations the apparatus is available in pairs and the students then work together, two of them doing the Kjeldahl determinations while the other pair is working on the Dumas or the carbon and hydrogen apparatus, and vice versa. Since in Pregl's book ($4, 25) the actual working procedures are rather obscured by historical narratives, experimental working procedures have been worked out in a systematic way for every determination taught (19). In most cases the original Pregl method has been retained and changes have been inaugurated only by force of necessity. The changes have been most pronounced in the carbon and hydrogen determination. One apparatus is set up precisely as described by Niederl and Roth (XI), having only one combustion gas line, a preheater, and electrically heated combustion tube with side-inlet, filled according to Pregl. The other apparatus is identical, with the exception that it contains silver wool instead of the lead superoxide, thus eliminating the objectionable features of this reagent (6, 12, 29), but also limiting the use of this second apparatus to compounds not containing nitrogen. By a strict separation and use of an aspirator for the absorption train any defects due either to improper technique or to unfavorable atmospheric conditions or faulty tubes and reagents are easily detected. The determinations must he repeated until satisfactory results are obtained. The following precision allowed is shown in the following table. As soon as this precision is attained, research substances are handed out. In this way a large number of research substances are analyzed. The following table shows the results of the very first determinations a t the beginning of the course, carried out by students with no previous experience in microanalysis but with a fairly uniform and average
undergraduate college education, in comparison with the results obtained a t the end of the course. Results Obtained
Voy first analysis, %
At the end of course, %
20
32
Good (results within the limitations of the method) Fair (resultsoff 0.3%'000.5%) cresults off 0.5% to
30 40 10
42 28 6 These results represen the average of about 400 students registering for the graduate course in Organic Microanalysis (Chemistry 202) at this University.
F2ei~
The table given below illustrates the results and accomplishments with the teaching arrangement outlined before: Organic
Microanalms
Performed During Spring
T e r m of 1934
Known substanees Number of students 16 TO~,I 1185 741(62.5%) Analysis per student 74 46 Kinds of analyses Known Research Residue substances substawes . 81 31 79 46 carboxy1 Nitrogen (Kjeldahl) 199 78 159 97 ~ ~ ~ ~ ~ " , ~ 143 { ~ ~ 178 g e n MOI. wt. (Rast) 32 8 M01. wt. (xiederl) 32 .. Halogen 6 16 741 444
.
Research substances 444(37.5%) 28 Total 112 125 277 256 321 40 32 22 1185
From the above table another important fact may be observed, namely, that a course of this type, aside from its educational value, constitutes a definite asset to any institution pursuing organic chemical research, inasmuch as it affordsthe analysis of a large number (in the above case, 444 or 37.5 per cent of the total number of analyses) of research substances. Examination unknowns are handed out for the last two periods of the course. They consist of the total analysis of one unknown substance, usually a research substance, requiring, in addition to quantitative carbon and hydrogen determination a t least one, but not more than two, further, independent determinations,
JOURNAL OF CHEMICAL EDUCATION
such as nitrogen, metals, carboxyl, or molecular weight. As time permits, any one of the other standard quantitative microdeterminations (halogens, sulfur, alkoxy, or alkimide, etc.) may be taken up and practiced. In this discussion the case of the special student, with a higher educational background or of broader chemical experience, is omitted. In summarizing, then, it can be said that in all cases where balances allowing a reproducibility of weighing8 within t0.002 to 0.010 mg. are available, there is no need to resort to so-called "semi-micro" methods (8, 9, 14, 16, 88), but the classical quantitative "milligram" procedures of Pregl (84, 25), Ernich (10) and others (6, 12, 17, 18, 29) can be taught and practiced under conditions as found a prim. in any average quantitative chemical laboratory. A course in organic quantitative microanalysis has, besides, the often repeated typical advantage of a course in microtechnique (1, 9, 3, 4, 6, 7, 9, 10, is, 14, 15, 28), such as: development of "experimental asepsis," saving of time and materials, and the exceptional feature that it is also of invaluable aid to organic chemical research. REFERENCES (1) ALBER,H. K., "Mikmkemi," P. A. Nomtedt and Sonem, Stockholm, 1933. (2) BEHRENS,H., "A Manual of Microohemid Analysis," The Macmillan Co., London and New York, 1894. (3) BEHRENS,H., AND KLEY, "Mikrochemische Analyse," L. Vass, Leipeig and Hamburg, 1915. (4) BENEDETTI-PICHLERAND SPIKE% "Introduction to the Microtechnique of Inorganic Qualitative Analysis," Microchemioal Service, Douglaston, L. I., N. Y., 1935. (5) BOETIUS, M., "ijher die Fehlerquellen beider mikroanalytischen Bestimmung des Kohlen- nnd Wasserstoffes nach der Methode van Fritz Pregl," Verlng Chemie, Berlin, 1931. E. M., J. CAEM. EDUC., 5, 9, 258, 536 (1928); (6) CHAMOT, Ind. Eng. Chem., Anal. Ed., 4 , 7 (1932). E. M., am MASON,"Handbook of Chemical (7) CH~MOT. Mic~scopy," John Wiley and Sons, Inc., New York and London. 1931.
(8) C ~ h a g rE. P., ASSOC. OfiEial Agric. Chem., 16, 255, 414, 575 (1934). C. J.. m W. SCHILLER. J. CHEM. Enuc... 9.. (9) . . ENDELDER. 1636 (1982). . (101, EMICH. F.. "Mikroehemisches Praktikum." J. F. Berema&, Munich, 1931; "Lehrbuch der'Mihochemie;;' J. F. Bergmann, Munich, 1926; Z. angew. C h . , 44, 725 (1931); Microchemze, 10, 205 (1932); 13, 283
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(1911)
(11) EMICH AND ~ C B N E ~ E B "Microchemical , hbomtary Manual," John Wilev and Sons, Inc.. New York, 1932. FRIEDRICH,A., 'mie Praxis der Quantitativeu Mikroanalyse," F. Denticke, Leipzig and Vienna, 1933. GARNER,"Indu8tzial Microswpy," I. Pitman and Sona, London, 1932. GRASSNER, F., Microchemie, 10,255 (1932). GREY, 'iPracticaI Chemistry by Micro-methods," W. Hefferand Sons, Cambridge, 1925. G ~ E I TG., , Helu. Chim. A&, 12, 829 (1929). LIEB m~ BENEDETTI-PICHLEE, "Mikrachemische Analyse," Berl-Lunge, "Chemische Technische Uutereuchungsmethoden, Vol. 1," J. Springer, Berlin, 1931. NIEDERL,J. B., Z. a d . C h . , 77, 169 (1929); 89, 57 11932)~ - - - - ,. (19) NIEDERL,J. B., m~ N I E D E ~"Micromethods , of Qumtitative Organic Micro Analysis," J. Wiley and Sons, New York, N. Y., 1942. (20) NIEDERL,J. B., m ROTH,I d . Eng. Chem., Anal. Ed., 6,272 (1934). (21) NIEDERL,J. B., AND ROTH,Mimochemle, 11,251 (1932). ibid.. 11. 237 (19321. (221 NIEnERL. J. B.. AND SASCAEE. , (23j NIEDERL; J. B.; TRIUTZ, AN/SASC&EK; ibid., Emieh Festschrift, 1 9 3 0 , ~227. . (24) P m a ~m ROTE, "Die quantitative organische Mikroanalyse," J. Springer, Berlin, 1935. L FYLEMAN,"Quantitative organic microanal:-(25) P ~ G AND sis," P. Bakiston's Son and Co., Philadelphia, 1930. (26) %ST, K., Bw., 55,1051,3727 (1922). (27) RJECHE,A,, %bid.,59, 2186 (1926); Microchemie, 12, 12!1 (1932). , (28) SPOERRI,T. E.,J. CKEM.EDUC.,10,491 (1933). (29) WEYGAND,C., "Quantitative Analytisohe Mihomethoden der Organischen Chemie in Vergleichender Darstellung," Akad. Verlagsgesellschaft, Leipzig, 1931. "Microohemie," New York University Cen(30) ANONYMOUS. tennial, 1932, p. 11. \
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