Quantitative Organic Elementary Microanalysis without a Microbalance J. B. NIEDERL, V. NIEDERL, R. H. NAGEL, AND A. A. BENEDETTI-PICHLER New York University, New York, N. Y.
The standard micromethods of quantitative organic elementary analysis allow a wide range in the amount of sample used, from about 1 mg. as the minimum to about 20 mg. as the maximum. The amount of sample to be taken for analysis depends upon the sensitivity and the precision of the balance employed. This relationship has now been established. Not only can microbalances of less than standard sensitivity and precision be used,
but an assay or even an ordinary analytical balance can be substituted for the standard microbalance, if its sensitivity and precision are not less than 0.025 mg. Minor changes in some of the microprocedures, but no changes in the standard microapparatus and equipment are necessary. Thus the lack of a microbalance need no longer delay the adoption of standard quantitative organic microchemical methods in teaching as well as in actual industrial practice.
F
ROM the very beginning of quantitative organic microanalysis the necessity of employing a microbalance has prevented the rapid spreading and universal adoption of standard microchemical methods in this field. There are several possible ways to eliminate the use of a microbalance. One possibility consists in changing the standard microchemical procedures as little as possible, using an assay or an ordinary analytical balance of high sensitivity, anrl increasing the amount of sample accordingly. This was successfully done by Prey1 (7) in his very first microchemical determinations, by Wise ( I S ) for the early Pregl microdetermination of carbon and hydrogen, and by Schmitt (9) for the vaporimetric microdetermination of molecular weight. Another possibility is to employ a regular analytical balance, showing the usual sensitivity of 0.1 mg., and to dilute the sample either with an inert solvent (6) or with the reagent (8). Still another possibility is to use the same type of balance and to discard the standard micromethods altogether and devise independent semimacro-, semimicro-, or mesomicromethods (2, 3, 10-12). The correlation of the precision of a given analytical balance with the general mathematical postulations governing the replacing of a microbalance of standard precision by a balance of lower precision, as given by Benedetti-Pichler ( I ) , made it possible to calculate beforehand t h e minimum amount of sample to be taken for a given determination. These postulations provided the basis for the systematic extension of the work of previous investigators (7, 9, I S ) to the entire field of organic microanalysis. Barring extreme cases [s > r; s = weight of sample; T = weight of reaction product ( I ) ] , rigid application of these mathematical postulations is not necessary for practical purposes. It is enough to determine the sensitivity and precision of a given balance and calculate the minimum amount of substance required to give weighing results within the limits of error of the micromethod in question. For general practice it appears advisable to adhere to the limits shown in Table I for the amount of sample to be taken for analysis. A regular analytical balance showing a sensitivity and a precision of 0.022 mg., both determined as described below, was selected for use, instead of a standard microbalance, in teaching the fundamental methods in quantitative organic
microanalysis. The procedures as described by Niederl and Nederl(4) were followed. It was found that with a balance of the sensitivity and precision of 0.022 mg., no changes whatsoever were necessary in the standard apparatus and equipment. The procedures also remained unchanged, with the exception of the time factor in some of the determinations (carbon and hydrogen; Dumas nitrogen). The results given below were obtained by several graduate students taking the regular course in quantitative organic microanalysis, and using the same equipment and apparatus as employed by the students assigned to standard microbalances. The results were not selected, nor were any omitted. They are given in the order actually obtained and include the very first analyses. The terms “micromethod”, “microanalysis”, or “microprocedure” are used in accordance with the definition given in INDUSTRIAL AND ENGINEERING CHEMISTRY [Anal. Ed., 11,111
(1939) 1.
I
TABLEI. LIMITSOF SAMPLE Pracieion of Balanoe
Amount of Sample Theqretical Practical minimum average
MQ.
MQ.
0.001
0.30-0.4b 0.6-0.5 1.5-2.0 3.0-4.0 6.0-8.0
0.002 0.005 0,010 0.020 ~ s < P . b s = r.
MQ. 3-5 4-6 5-8 6-10 8-12
The Balance The balance used in the determinations described herein was selected from a number of student balances available at this college in the laboratory for quantitative inorganic analysis. In order to perform the weighings in exactly the same manner as with a standard microbalance, the milligram markings on the beam were designated t o be read as follows: The original 5-mg. mark at the extreme left was regarded as the new zero mark; consequently the original zero mark became the new 5-mg. mark and the original 5-mg. mark at the extreme right became the new 10-mg. mark. The subdivisions,the 0.1-mg. marks, were ignored. The balance was set up alongside the standard Kuhlmann microbalances in the same room. A 5-mg. rider was employed. ADJUSTMENTAND DETERMINATION OF ZERO READING.The screws on the horizontal beam were then set so that the balance 412
ANALYTICAL EDITION
JULY 15, 1939
413
Determination of Neutralization Equivalent TABLE11. SENSITIVITY OF BALANCE Position of Sensitivity of Balance, Rider on the Rider Deflection Sum on Pointer Pointer Scale Scale Scale, Reading Units Reading Units
MQ. No load
0 1
10-gram load
0 1
a
b
47=
0 -47 ( 4 . 7 divisions) -10 (1 division) -35 ( 3 . 5 divisions)
45 b
TABLEIV. DETERMXNATION OF NEUTRALIZATION EQUIVALENT
1 unit on pointer scale equals 0.021 mg. 1 unit on pointer scale equals 0.022 mg.
gave a zero reading when the rider rested on the new zero mark on the rider scale (4, p. 15). The zero readings themselves gave extremely constant values and no corrections were necessary. DETERMINATION OF SENSITIVITY (4,p. 16). The sensitivity of the balance was determined with and without a load. The results given in Table I1 were obtained. Thus one division on the pointer scale corresponds to 0.220 mg. and one tenth of such a division, the "reading unit" in microchemical weighings, to 0.022 mg. (22 micrograms). DETERMINATION OF PRECISION. The precision of the balance (4, pp. 17-18) with a 10-gram load on both weighing pans, was determined by two series of weighings, performed during a period of 2 hours. After each weighing the rider was raised and placed as exactly as possible on the same mark. The weights on the pans were lifted and put back again as close to the original position as possible (Method A). Then a second series of weighings was carried out in which the weights on the pans were always placed considerably off the center of the weighing pans (Method B). The results (average deviations) were as follows:
0.7 1.2 1.0
0.07 0.12 0.10
Potassium bitartrate Sodium oxalate
Weight of Substance
Weight of Residue
Mu.
MQ.
%
%
%
11.31. 11.58 10.82 9.63 9.04
5.20 5.32 4.91 10.24 9.63
20.63 20.61 20.36 34.43 34.49
20.79
-0.16 -0.18 -0.43 +0.10 +0.16
-MetalFound Cslcd.
34.33
Error
1.063 1,059
TABLEV. DETERMINATION OF AMIKOIDNITROGEN
Myristamide
TABLE111. DETERMINATION OF METALS
9.57 7.91
Volumetric Determination of Aminoid Nitrogen In the determination of aminoid nitrogen by the Kjeldahl method (4, pp. 51-8; 6),the precaution observed was to take a small enough sample so that no more than 10 m1.-the capacity of the buret-of standard 0.01 N acid is required.
MQ.
The first procedure was the determination of the percentage of metal in metallo-organic salts (4, pp. 41-3).
Factor0
MI.
Weight of 0.01061 N Neutralization Equivalent Acid NaOH Found Calcd. Error Mg. MI. -2.0 7.77 146 148 Cinnamic acid 12.00 12.13 7.82 146 -2.0 Salicylic acid 13.78 9.54 136 138 -2.0 12.55 8.54 139 -1.0 a = X 0.01. Found b y dividing theoretical number of m!. of 0.01 N alkali required b y sample, b y number of ml. actually consumed in neutralization. There are two separate factors, one for phenolphthalein and another for methyl red (4, p. 47; 6 ) .
0.015 0.026 0.022
Determination of Metals
0.01 N NaOH
Substance
p-Toluamide
Since during the actual weighing the distribution of mass on the balance pans will sometimes follow that of the first series and sometimes that of the second, all figures obtained were averaged and the practical precision of the balance under these conditions was calculated as shown above. This indicates that weighings on this balance can be duplicated within j= 0.022 mg. (22 micrograms). The practical weighings were carried out according to the method of weighing on a microbalance (4, pp. 14-21). Thus each division on the pointer scale was mentally divided into 10 units. The rider was moved whole milligram divisions on the rider scale, and the milligram fraction was obtained by multiplying the deflection sum on the pointer scale by 0.022.
Substance
Weight of Acid Mg. 12.42 10.23
Substance
Pointer Scale Readings Divisions Method A, 14 weighings hlethod B, 12 weighings Average, 26 weighings
The second procedure was the standardization of the 0.01 N sodium hydroxide solution by means of benzoic acid, and then the determination of the neutralization equivalents of cinnaqic and salicylic acids (4, pp. 44-50; 5 ) . In this method the weight of material taken is limited by the capacity of the microburet, which is 10 ml.
Weight of SamDle
0.00998 N 0.01068 N HK(1Os)i NaOH . .
----NitrogenFound Calcd.
Error
Mi.
MI.
MI.
%
%
%
8.46 8.35 9.07 7.96 9.53
9.44 9.21 9.41 8.63 9.13
2.96 2.75 5.17 4.82 4.67
10.37 10.49 5.98 6.04 6.06
10.37
-0.00 $0.12
6.17
-0.19 -0.13 -0.11
Gasometric Determination of Nitrogen (Table VI) The next procedure tested was the gasometric determination of nitrogen by the Dumas method (4, pp. 60-78). After several attempts it was found that to obtain satisfactory analyses the speed of the gas bubbles throughout the entire period of analysis had to be reduced to one bubble per second. Consequently the time of the first and the second combustion, as well as the washing out process (4, pp. 72-3), had to be doubled. Thus the entire analysis required about 2 hours. It was also found advisable to double the amount of fine copper oxide in the mixing procedure (4,p. 70). Determination of Carbon and Hydrogen The carbon and hydrogen determination (4, pp. 80-115) required several changes before satisfactory analyses were obtained. The combustion was carried out a t such a rate that the substance distilled or vaporized slowly and evenly into the filling of the combustion tube. This was brought about as follows: The boat was placed 7 t o 8 cm. instead of 5 cm. in front of the combustion tube filling, and the furnace was kept at a temperature of from 700" to 750" C. The heating was started as usual 5 cm. in front of the boat, and the boat was approached by the flame at the usual rate till the substance started to distill or decompose. The burner was halted at this point till all the material distilled out of the boat. Then the burner was slowly advanced till it was directly under the boat, and was ke t there until the material had distilled into the combustion tube Rlling in the furnace.
INDUSTRIAL AND ENGINEERING CHEMISTRY
414
TABLE VI. GASOMETRIC DETERMINATION OF NITROGEN Weight of Vol. of Nz, Sample Corrected
Substance Azobenzene
2,4-Dinitrobenzoic acid
Pressure
Mo.
M1.
Mm.
8.66 8.44 7.37 7.51 8.02 9.48 8.93
1.155 1.109 0.999 1.024 1.082 1.077 1.056
754 753 758 758 757 765 764
----Nitrogen--, Found Calod.
Temp. C.
%
26 30 26 26 26 24 31
%
Error
%
15.11 15.38 -0.27n 14.67 -0.710 15.44 + O . 06 15.53 +0.15 l6,34 -0.04 1 3 . 1 4 13.21 -0.07 13.34 + O . 13
Research Substances CioHizOzNSCl CzoHziNOz
9.13 0.468 12.07 0.460 8.85 0,347 CizHi6ONSCl 7.98 0.374 a These two analyses were performed before been determined.
764 31 5 , 7 8 5.72 +0.06 762 22.5 4.42 4 . 5 6 -0.14 761 23 4.53 -0.03 764 29 5.33 5 . 4 3 -0.10 proper duration of period of combustion had
If at any time during the combustion the rate of flow of the water from the Mariotte flask decreased because of choking of the capillary by water, the burner was halted until the pressure had returned to normal. This precaution was particularly necessary for substances which distilled rather than decomposed. By this procedure the first combustion period was increased from 15 to 25 minutes. After the substance had vaporized, as observed by the disappearance of the distillate, the gas burner was advanced as usual. During both the combustion and sweeping out period, with the latter extended to 30 minutes, the capillary end of the combustion tube and the constrictions of the water-absorption tube required mechanical heating with a heated file to drive over the water which tended to deposit in the capillary (total time, 60 minutes; 300 ml. of oxygen).
Substance
Weight of COz
Weight of --CarbonHzO Found Calcd.
Error
Substance
39.9 24.1
109.7 112.4
110.2
-0.5 +2.2
Azobenzene
2.39 2.36
22.63 22.18
23.3 23.2
181.8 182.2 183.6
-0.4 +1.4
Naphthalene
2.93
21.53
42.2
129.1
+1.1
--Hydrogen--Found Calcd. Error
M9.
%
%
%
%
5.80 6.15
36.28 36.20
36.79
-0.51 -0.59
6.72 6.84
7.09 -0.37 -0.25
d-Glucose
9.13 10.08 9.78 10.80 9.96 12.94 10.26 9.82 11.89
13.56 14.78 14.26 25.89 23.65 31.15 25.67 24.85 30.15
5.46 5.85 5.54 4.80 4.36 6.39 4.54 4.11 5.29
40.50 40.00 40.01 39.80 65.38 65.45 64.79 65.66 68.30 68.88 68.99 69.15
+0.50 $0.01 -0.20
+0.21 -0.58 +0.11 +0.27
6.67 6.52 6.34 5.00 4.90 5.53 4.94 4.68 5.00
6 . 6 9 -0.02 -0.17 -0,35 5.45 -0.45 -0.55 +0,08 4.92 +0.02 -0.24 +0.08
10.41 10.70
36.82 36.61
5.96 5.98
93.84 93.34
93.75
+0.09 -0.41
6.40 6.26
6.25 $0.15 -0.00
10.16 12.22 10.79 10.40 9.96 8.24 8.82
21.74 26.20 28.51 27.42 25.63 22.29 15.78
5,89 6.37 8.31 8.16 4.70 5.40 4.52
58.39 58.47 72.06 71.96 70.18 73.78 48.77
58.42
-0.03
6.48 5.84 8.61 8.78 5.28 7.33 5.73
6.56 - 0 . 0 8 -0.72 8.43 +0.17 +0,35 5 . 5 2 -0.24 7 . 6 2 -0.29 5 . 7 1 +0.02
Research Substances CiaHzsBrOs C20H2804 C19H1806 CZlHZeOZ CirHzaBrzOz
72.29 69.93 73.68 48.57
+0.05
-0.23 -0.33 $0.25 4-0.10 +O.ZO
Determination of Molecular Weight (Table VIII) The CrYOSCOPiC method (4,PP. 171-4) had to be slightly modified as follows:
-Molecular Weight, Found Calcd. Error
Mg.
Mo.
Naphthalene
A
21.65 24,62
12.85 13.42
Benzoic acid
Weight of Weight of Sample Camphor lug.
Mo.
-0.65
(1) Benedetti-Pichler, A. A., Mikrochemie, 25, 390 (1938); IND. ENG.CHEM.,Anal. Ed., 11, 226 (1939). (2) Dennstedt, M., “Anleitung zur vereinfachten Elementaranalyse”, Hamburg, V. Meisner’s Verlag, 1919. (3) Holscher, F., L. Gattermann’s “Laboratory Methods of Organic Chemistry”, 24th ed., pp. 46-87, New York, Macmillan Go., 1937.
2.37 1.66
9.66 10.11
-0.07
Literature Cited
Resorcinol
Sulfonal
Resorcinol
A melting point tube was used having an inside diameter of 3 mm. a t the base instead of the usual 2 mm., in order to facilitate mixing of the substance with the greater amount of camphor; the thickness of the walls of the tube was increased because of the increase of the internal pressure.
TABLE VIII. DETERMINATION OF MOLECULAR WEIGHT
TABLE VII. DETERMINATIOK OF CARBON AND HYDROGEN Weight of Sample
VOL. 11, NO. 7
%
%
128.0
(4) Niederl, J. B., and Niederl, V., “Micromethods of Quantitative Organic Elementary AnalySIS”, New York, John Wiley & Sons, 1938. (5) Niederl, J. B., Niederl, V., and Eitingon, M., Mikrochemie. 25. 143 (1938). Niederl, J. B., Trautz, 0. R:, and Saschek, W. J., Ibid., Emich Festschrift, 219 (1930). Pregl, F., E. Abderhalden’s “Handbuch der biochemischen Arbeitsmethoden”, Vol. V, Part 11, pp. 1307-56, Berlin and Vienna, Urban and Schwarzenberg, 1912. (8) Saschek, W. J., IND. Exa. CHEM.,4nal. Ed., 3, 198 (1931). (9) Schmitt, R. B., Bull. Assoc. Jesuit Scientists, 14, 69 (1936). (10) Sucharda, E., and Bobranski, B., “Halbmikromethoden zur automatischen Verbrennune organischer Substanzen”, Braunschweig, Vieweg und Sohn, 1929. (11) Sucharda, E., and Bobranski, B., “Semimicromethods for the Elementary Analysis of Organic Compounds”, tr. by C. W. Ferguson, London, A. Gallenkamp and Co., 1936. (12) Ter Meulen, H., and Heslinga, J., “Neue M e t h o d e n d e r o r g a n i s c h- c h e m i s c h e n Analyse”, Leipzig, Akademische Verlagsgesellschaft, 1927. (13) Wise, L. E., J . Am. Chem. SOC.,39, 2055 (1917).
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PRESENTED by R. H . Nagel before the Metropolitan Microchemical Society, and before t h e Division of Microchemistry a t the 97th Meeting of the American Chemical Society, Baltimore, Md.
