ANALYTICAL EDITION
264
than one-tenth of one per cent for the technical method, and would be less for the exact method. Literature Cited (1) Dennis and Nichols, “Gas Analysis,’’ p. 134, MacMillan, 1929. (2) Dennis and Nichols, I b i d . , p. 123.
’
Vol. 3, N o . 3
(3) devoldere and deSmet, 2.anal. Chem., 49,661 (1910). (4) Fuchs, 2. physik. Chem., 92, 641 (1918). (5) Kobe, IND. ENG.CHEM.,Anal. Ed., 8, 159 (1931). (6) Shepherd, Bur. Standards J . Research, 6 , 121-67 (1931). (7) U.S. Steel Corporation Chemists, “Methods for Sampling and Analysis of Gases,’’ pp. 62 and 98, Carnegie Steel Co., Pittsburgh, 1927.
Determination of Organic Acids
V-Applica tion of Partition Method to Quantitative Determination of Acetic, Propionic, and Butyric Acids in Mixture1a2 0. L. Osburn and C. H. Werkman DEPARTMENT OF BACTERIOLOGY, I O W A STATE COLLEGE, AMES,IOWA
HE differential distri-
The partition method has been extended to the of partition has yielded the bution of fatty acids quantitative determination of acetic, propionic, and most satisfactory results and between two immisbutyric acids in a mixture. A nomogram has been has been chosen for the desolvents has been emconstructed from which is read the percentage of each of termination of three acids. ployed in a partition method the three acids present after two values (percentage In former publications (2, for (a) the quantitative departition constants) characteristic of the mixture have 3, 4, 5 ) the partition value termination of t w o known been determined. characteristic of the mixture fatty acids in a mixture (2,3), of acids has been called the and (b) the provisional identification of two fatty acids in partition constant (P’) and was defined as the number reprea mixture (6). The method was developed to fill a need senting the cubic centimeters of 0.1 N alkali required to neuin fermentation studies for rapid and accurate determina- tralize 25 cc. of the aqueous phase to phenolphthalein, when tions of the lower fatty acids. For a consideration of the 30 cc. of the unknown 0.1 N acid solution were partitioned principles and advantages of the partition method, the reader with 20 cc. of the immiscible solvent. It is necessary in using is referred to Behrens (1) and to previous publications in this partition constants to adjust the unknown acid solution to series (2, 3, 4, 6). 0.1 N . If, instead of adjusting the acid solution to exactly The partition method is extended in the present paper to 0.1 N and determining the partition constant, the quantity the quantitative determination of acetic, propionic, and of acid distributed in the aqueous phase be calculated as the butyric acids in a mixture. The method has yielded excellent per cent of the total acid in an equal volume of the unknown results in this laboratory and with reasonable care will give solution, it becomes unnecessary to adjust the acid solution to results with an error of less than 5 per cent for each acid. exactly 0.1 N . It has been found necessary to adjust only Its most severe test is in the case of a mixture containing a to within the limits of 0.12 N and 0.08 N . Of course, the small proportion of one or two of the acids, final adjustment must be known accurately. This percentage I n conducting the quantitative determination according is termed the percentage partition constant ( K ) to differento the partition method, the unknown acid solution is parti- tiate i t from the partition constant (P’). tioned with isopropyl ether. If the isopropyl ether is of a The two values of K employed to determine the percentages commercial grade it should be purified by adding an excess of each of the three acids in solution are: of a solution of 5 per cent sodium hydroxide. After the K1equals percentage of acid in the aqueous phase when 30 mixture is shaken, the ether is decanted and dried by adding cc. of the unknown acid solution, adjusted to a normality calcium chloride. It is then distilled and the portion coming between 0.12 and 0.08, are partitioned with 60 cc. of isoover at constant temperature is collected for use. The propyl ether a t 25” C. K z is determined by using 30 cc. of the acid solution and 15 cc. of isopropyl ether. M reprecommercial product develops an acidity upon standing. I n determining the quantitative relationships of two acids sents the number of cubic centimeters of 0.1 N alkali required in a mixture, only one partition value characteristic of the to neutralize 25 cc. of the unknown solution. M-2 represents mixture is necessary, whereas two values are necessary in the number of cubic centimeters of 0.1 N alkali required to the determination of three acids. Such partition values neutralize 25 cc. of the aqueous phase after partition. may be obtained in a number of ways: (1) The unknown The solvents are shaken in a separatory funnel for 1minute, acids may be serially partitioned, a method suggested by and 3 minutes are then allowed for the phases to separate Behrens (1). I n this method one of the phases is again when 25 cc. of the aqueous layer are withdrawn for titration. partitioned and a second value obtained. Serial partition M2 has not been found satisfactory, Its use did not yield re= -jjx 100 (1) sults of the degree of accuracy or sensitivity desired. (2) Successive partition of the acids between water and a suitable The percentage partition constants ( K , 30 cc. acid, and immiscible solvent, and water and a second immiscible sol- 60 CC. ether) established for the three acids are: vent. This method has been employed in the provisional Acetic acid Ki = 91.5 identification of two acids in a mixture ( 5 ) . (3) Two sepaPropionic acid K1 a 7 0 . 5 rate partitions of the aqueous solution of acids using one K1 = 39.8 Butyric acid immiscible solvent but in different proportions. This type 1 Received February 26,1931. I n like manner, by extracting 30 cc. of each acid with 15 Supported by an appropriation from Industrial Research funds of a second ether at 250 “1 there is Of Iowa State College, as a part of the program for the study of the use of set of percentage partition constants: wastes by fermentation.
T
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9
“*
INDUSTRIAL AND ENGlNEERING CHEMISTRY
July 15, 1931 Acetic acid Propionic acid Butyric acid
Kz = 73.5 Kz = 38.8 K2 = 15.0
A
Throughout the following discussion, A will designate per cent of acetic; P , per cent of propionic; and B, per cent of butyric acid in the acid solution. Whatever the composition of the unknown solution of acids may be, it is evident from the two sets of constants established that
K1
=
0.915 A
+ 0.705 P Kz+ 0.395 B, and = 0.735 A + 0.388 P + 0.15 B
P
B
KI KP
=
265
402-0 665-0 = 20.1, p = L= 33.25, and 2 2 (t59.8 33.5) = 46.7 B = 33.5 2
+
-
With the foregoing facts a t hand, it is possible to construct a nomogram such as that shown in the accompanying figure, The accuracy of the nomogram can be checked. If any value of K1 is selected and straight lines are drawn through it and through the limiting values of Kz, then the lines should intercept the same limits of A, P , and B as those given by solution of the simultaneous equations. Analytical Procedure
A 0 5-
60: 507
40
30 20
About 100 cc. of the acid mixture should be a t hand, Titrate 25 cc. and adjust accurately to any normality between 0.12 and 0.08. Then partition two 30-cc. portions, one portion with 60 cc., the other with 15 cc. of the isopropyl ether a t 25" C. as described. Withdraw 25-cc. portions of the water layers, titrate, and calculate the values K1 and Kz. A straight edge laid across these two points on the nomogram intersects A , P , and B and indicates the per cent of each acid present. About 10 minutes are required for the complete procedure. Representative analyses determined by use of the nomogram are shown in Table I.
10 507
0
ANALYSIS
55 7
Table I-Representative Analyses ACID AMOUNT PRESENTAMOUNTFOUND
I
A P B A P
I1
B5 7
B
70 : 75
A P B P A B
I11
-
IV
80-
%
%
90 5 5 40 30 30 30 10 60
89.0
7; 25
7.5
4.0 40.5 28.5 30.5 29.5 10.5 60.5 70.2 3.0 25.5
85;
A, PI and B may also be evaluated by solving the three simultaneous equations by which the nomogram was constructed: 0.915 A 0.705 P 0.398 B Ki 0.388 P 0.735 A 0.15 B = Kz B = 100 P + A +
90195
++
Consequently, for each possible value of KI, there must be for each mixture a corresponding value of Kz. K z is calculated as given above. For example, if K1 = 60, then when A = 0, K P = 0.735 X 0
+ 0.388 X 66.5 + 0.150 X 33.5 = 30.6
and when for the same value of K1, A Kz
=
0.735 = 40.2
=
40.2,
+ 0.388 X 0 + 0.15 X 59.8 = 38.4
It is evident that for each value of K there is a minimum and maximum value of K2, and furthermore, for each intermediate ternary mixture, there can be one and only one value of K2, and this value must lie between the two limiting values of Kz. It is also true that if K z lies a t any fractional distance between its limiting values, then A , PI and B must also be a t the same fractional distance between their limits. For example, if K = 60, and the experimental value of KZ = 34.5-4. e., half way between the limits of K-then A, P, and B are fixed a t values half way between their limits and
++
The values of K1 and Kz for the pure acids vary with the relative concentration of the acids and with the temperature, and serious errors will result if the total normality of the acid mixture to be analyzed is not near 0.1 N (0.08 to 0.12 N ) . The temperature should be between 24" and 26" C. This method can be adapted to mixtures of these three acids a t any normality from 0.2 to 0.02 by establishing the variation of K1 and K2 with the concentration of the acids. These values can then be plotted on a graph as abscissas against volumes of ether as ordinates. In this manner it is quite simple to read off the volumes of ether to use with 30 cc. of the acids to obtain constants equal to those obtained. It is to be expected that in a solution containing a small per cent of butyric or propionic acid, the values of K1 and K z will be slightly higher than the true values. This has been noticed to a certain extent. Somewhat larger errors are apparent in Table I when P and B are in the neighborhood of 5 t o 10 per cent. Literature Cited (1) Behrens, Z . anal. Chem., 69, 97 (1926). (2) Werkman, IND. END.CHEM.,Anal. E d , 2, 302 (1930). (3) Werkman, Iowa State College J. Sci., 4, 459 (1930). (4) Werkman, I b i d , 5, l(1930). (5) Werkman,Ibid, 6, 121 (1931).
