V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 a
L
1117
r
r
containing lead, so a standard curve for such alloys was prepared using National Bureau of Standards samples 53c, lead-base bearing metal. The standardization curve is shown in Figure 3 and the results of check determinations are shown in Table 111. The maximum error was 0.75% of the amount present and the average error was 0.33%. ACKNOWLEDGMENT
This work was aided by a grant from the Carnegie Foundation Research Fund. Some of the data in this paper were taken from reports of undergraduate research by the junior authors.
z a d E
l
IO
I
Figure 3.
I
I
40
50
I
20 30 VOLUMES IN MU:
Standard Curves
the sulfuric acid is 2.0 N and the nitric acid is 1.5 X, these conditions were selected as the standard concentrations t o be used in determination of lead by the proposed method. The standardization curve is given in Figure 3, and the results of check determinations are shown in Table I1 for solutions containing only lead and anions. The maximum percentage error in these determinations is 0.73 and the average is 0.33. .4lloys containing tin and antimony gave high results when run by the usual procedure wing the standard curve for solutions
LITERATURE CITED
(1) Arrhenius, O., J . Am. Chem. Soc., 44, 132 (1922). ( 2 ) Craig, D. N., and Vinal, G. W., J . Research Natl. Bur. Standards, 22, 55 (1939). (3) Greene, H. S.,J . A m . Chem. Soc., 53, 3275 (1931). (4) Haslam, J., and Bieley, J., Analyst, 71, 223 (1946). ( 5 ) Kolthoff, I. hl., and Fischer, TT’. von, J . A m . Chem. Soc., 61, 191 (1939). (6) Kolthoff, I. >I., Perlich, R. TV,, and Weiblen, D., J . Phus. Chem., 46, 561 (1942). ( 7 ) Kolt,hoff,I. hl., and Rosenblum, Ch., J . Ani. Chem. Soc., 56, 1264, 11934). (8) Ibid., G, 597 (1935). (9) Wedding, Stah2 u.E k n , 7, 118 (1887). RECEIVED for review January 22, 1932. Accepted M a y 6 , 1952. Presented a t the Seventh Southwest Regional XIeeting of the AMERICANCHEMICAL Austin, Tex., December 6 t o 8, 1951. SOCIETY.
Chromatographic Determination of Purity of Chloroacetic Acid E D G i R D. SlIlTH, WM. A. MUELLER, AND L. N. ROGERS The Buckeye Cotton Oil Co., Memphis, Tenn. Present methods of analysis and specification for chloroacetic acid are based on the determination of melting point and chlorine content. Because, however, all commercial samples of chloroacetic acid may contain appreciable quantities of acetic and dichloroacetic acids without greatly depressing its melting point, such anal>-sesallow rather wide variations in the actual content of the monochlorinated acid. This work was undertaken to develop direct methods not only for monochloroacetic acid but also for the main acid impurities i n this material. By means of adsorption chromatography, acetic, chloro-
C
HLOROACETIC acid has recently berome of importance as an industrial chemical because of its use as an intermediate in the manufacture of many agricultural chemicals, insecticides, and sodium carboxymethylcellulose. Heretofore, no satisfactory method has been available for its analysis, and specifications have generally been based on melting point and total chlorine content. Because all commercial samples of chloroacetic acid may contain appreciable quantities of both acetic acid and dichloroacetic acid, such specifications may allow rather wide variations in actual chloroacetic acid content. A number of methods have been used for the separation of mixtures of organic acids-few of them quantitative. A review of these methods and their applicability is given in a recent article by Marvel and Rand8 ( 1 ) . These authors developed systems for
acetic, and dichloroacetic acids were quantitatively separated. The respective acid zones were located on the adsorbent column by use of acid-base indicator streak reagents. Quantitative analyses were obtained by mechanically cutting out these zones, then eluting and titrating each separately. Chloroacetic acid may be analyzed unambiguously and with reasonable accuracy in terms of its main component, monochloroacetic acid. With somewhat less accuracy, determinations may also be made of the principal impurities, acetic acid and dichloroacetic acid.
separating many of the water-soluble organic acids by means of partition chromatography. They were able to separate acetic acid from chloroacetic acid, but not chloroacetic from trichloroacetic acid. In addition, their techniques were unsuitable, in the piesent writers’ opinion, for rapid routine analysis. The present work describes an adsorption type of chromatographic procedure for quantitatively separating acetic, chloroacetic, and dichloroacetic acids, which is applicable to any mixture of these components. The separation is achieved by the use of a three-component developing solution on a single column of 2.5 to 1 silicic acid-Celite, the time required for this separation being only about 15 minutes. Quantitative determination of the three acids is completed by eluting each separately from the appropriate section of the developed chromatogram, and titrating with dilute bsse
1118
ANALYTICAL CHEMISTRY
Table I.
Effect of Varying Chloroacetic Acid Content on Analytical Properties
Composition, Weight % AcOH ClAcOH C12AcOH
Theoretical Analysis CI, % NaOH equiv.a
0
a
100 0 1.5 95 3.5 3.1 90 6.9 Defined a s grams of NaOH equivalent to
37.6 0.423 37.6 0.423 37.6 0.422 1 gram of indicated mixture.
Table 11. Uncorrected Titration Rasults of Duplicate Chromatographic Analyses of “Pure” Acetic, Chloroacetic, and Dichloroacetic Acids Sample
0.05 N NaOH t o Neutralize, hI1. AcOH zone ClAcOH zone ClzAcOH aone
Acetic acid
0.07 0 07 7.94
8.10 8.25 0.23
Chloroacetic acid
0 08 0 07
0.10 0.07 5.55
7.60
0.25
Dichloroacetic acid
Unfortunately, no good criterion of chloroacetic acid purity is available, aa chlorine content and total acidity can be varied, almost at will, by simply changing the relative amounts of acetic and dichloroacetic acids normally present as impurities. Examples of this are given in Table I, wherein actual chloroacetic acid content has been lowered from 100 to 90% without significantly affecting either chlorine content or sodium hydroxide equivalent. I
I
1
I
I
24
I /
/
/
0.20 0.20 6.00 a I n later work a sample of purified dichloroacetic acid was obtained on urhich this zone titrated only0.23 ml.
Table 111. Statistical Correlations between Calculated and Observed Chloroacetic Acid Analyses Concentration range, 7 0 No. of samples Correlation coefficient Significance level, % Slope of regression line Standard deviation about regressionline, %
Acetic Acid 0-20
Chloroacetic Acid 70-100 11 9 0.995 0.984 0.1 0.1 0.998 1.12
&O. 6
11.7
Dichloroacetic Acid 0-6 5-12 7 J 0,949 0.968 0.1 1.0 2.90 0.82 zkO.9
ztO.6
Eleven synthetic acid mixtures were prepared by weighing the appropriate quantities of each acid into a weighing bottle. The mixtures were melted and thoroughly stirred before sampling. Single analyses were then carried out on benzene solutions of these samples by means of the chromatographic techniques described. The results of these analyses were compared and correlated with the calculated analyses of these samples, with the results indicated by Table 111. The actual data points are given in Figures 1 to 3 with the calculated regression lines drawn through these points. I
0
4
Figure 1.
I
I
8 12 16 % Acetic Acid, Theory
I
I
PO
24
100
Calibration Curve for Acetic Acid Determination
Experience in these laboratories has shown that melting point is similarly a poor indication of product purity in this range of concentration. Repeated recrystallizations of a technical grade sample of chloroacetic acid gave a product which, when chromatographed, gave no visihle test for dichloroacetic or acetic acids. Redistillation of a practical grade of dichloroacetic acid, however, gave a product which still showed a definite zone for monochloroacetic acid. As this dichloroacetic acid was to be used only as a minor constituent in the synthetic standards t o be tested, and its only apparent impurity was chloroacetic acid (the major constituent of these standard samples), it was felt permissible to ignore this small amount of residual impurity. The theoretical analyses of the synthetic samples were therefore calculated on the assumption that all of the acids used u-ere 100% pure. As a test of the procedure, howevcbr, duplicate determinations of each of these individual acids were carried out, with the results shown in Table II. I t must be assumed either that both the chloroacetic and dichloroacetic acids used contained impurities, or that the titration of a blank zone is changed by the passage of an acid zone through it. In viely of the several careful recrystallizations performed on the chloroacetic acid sample, it was decided that thc latter ashumption nas the more reasonable one In calculating all subsequent results therefore, a blank correction of 0.07 mi. wm subtracted from the dichloroacetic acid titration, but a blank of 0.20 ml. was subtracted from all other zone titrations. The use of these corrections did not give any better correlation between theoretical and observed analyses than was found, for example, by using the same blank correction of 0.07 ml. for all zones; but the calculated values for the nearly pura materials were thereby made to appear more reasonablc
90
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P 80
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Y
g
70
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50 50
Figure 2.
60 70 80 % Ch1o:oecetic Acid, Theory
90
100
Calibration Curve for Chloroacetic Acid Determination
indicated by Table 111 and Figure 1, the slope of the regression line correlating observed and calculated acetic acid results for all samples is not significantly different from unity. I t is obvious (from the method of calculation used) that in the absence of dichloroacetic acid the observed chloroacetic results must also be identical with the theoretical ones within the limits of experimental error Such samples should consequently not fit the correlation indicated by the regression line in Figure 2, which was estahlishrd on the basis of three-component samples. This was experimentally verified in that inclusion in the chloroacetic acid correlation of the two synthetic samples that did not contain
1119
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 dichloroaeetic acid materially increased the standard deviation and decreased the slope of the regression line. These two samples were therefore excluded from consideration in Figure 2 and this “calibration” line should not be used when dichloroacetic acid is absent. The cause of this phenomenon appears to be that a small, and reasonably constant, amount of dichloroacetic acid remains in the chloroacetic zone. This is further brought out in Figure 3, from which it is seen that two straight lines of very different slopes were required to fit the data, depending on the concentration of dichloroacetic acid present, ( A quadratic relationship w~bstried, but the fit was somewhat less satisfactory.) The small quantity of dichloroacetic acid remaining in the chloroacetic acid zone cames the dichloroacetic acid results to be very much too low in the lower concentration range, but this effect becomes jess seriour: ~ b sthe total quantity of dichloroacetic acid increases. This same phenomenon, of course, also causes the chloroacetic acid results to be slightly too high when dichloroacetic acid is present, thus neccssitating t,he use o f the calibration line given in Figure 2. PROCEDURE
The only special apparatus required is a chromatographic tube mch as the No. 2 tube (19 mm. in diameter and 200 mm. long) obtainable from the Scientific Glass Apparatus Co., Bloomfield, N. J. These tubes need not be of the tapered type, but should be of uniform diameter, ao as to allow the wet adsorbent to he extruded easily.
lP
t
o
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P
4
6
a
1
0
1
~
1
4
% Dichloroacetic Acid, Theory
Figure 3. Calibration Curve for Dichloroacetic Acid Determination
The adsorbent is a mixture of approximately 2.5 parts by volume of silicic acid (Merck heavy powder) to 1 part of Celite. This material is thoroughly mixed and then packed uniformly, under full water pump vacuum, into the No. 2 chromatographic tube to a height of about 12 cm. Approximately 0.10 ml. of melted chloroacetic acid is pipetted into 10 ml. of benzenr, and 3 ml. of this solution are taken for analysis. The 3 ml. of benzene solution are poured on the packed adsorbent and allowed to suck completely into the top section of the column. Immediately after this solution disappears into the column it is followed by 30 ml. of a petroleum ether A solution containing 20% methyl amyl ketone, and 2% amyl alcohol. This “developing” solution is added in portions, if necessary, but the top of the column should never be allowed to dry out after the chloroacetic acid sample has been placed on it. When the last of the developing solution has entered the column, the suction is continued on the column of pecked adsorbent for 5 to 8 seconds to dry the column partially and thereby facilitate its extrusion. At the end of this time the suction is broken, the tube lightly tapped a few times to loosen the adsorbent. and the chromatogram extruded in one piece onto a clean white sheet of paper. The Beparated acid zones are located by streaking the column lightly with 1% aqueous solution of Congo red. The typical appearance of such a column and the identification of the zones are indicated in Figure 4. The dotted lines indicate
the approximate points a t which the adsorbent column should be cut in order to separate it into segments, each of which contains only one nearly purr acid. Cuts are made fairly close t o leading edges but well back of the characteristically diffuse trailing edges of the zones. The three acids do not all give equally strong tests with Congo red--l% of dichloroacetic acid can be easily detected, whereas about 5 % of acetic acid is required in order to obtain an unmistakable test. For this reason, it is always advisable to cut and analvze all segments of the adsorbent column as indicated, even though no color teat can be seen. Each segment of‘ the column is placed on a separate piece of paper and the lumps are broken in order to facilitate evaporation of most of t h e s o l v e n t . ACETIC After about 3 rninACID Utes’ standing, the individual acids may be quantitatively washed off each of these powC’iLOPOACETIC dered column secACLJ tions by packing it back into the empty No. 2 chromato_graphic tube and pouring over it about 2 volumes of DlCHLOROACErlC methanol. (A volACID ume represents the quantity of liquid required to wet Figure 4. Developed Chromatogram c~ompletely anv given amount df packed adsorbentin this instance 10 to 15 ml. are required. Ester formation between the acids and methanol did not lower the analytical results and actual tests showed that this solvent eluted the acids more completely than either water or acetone.) The filtrates from these “elutions” are collected in a clean test tube and transferred quantitatively to the titration vessel by rinsing thoroughly with water. These aqueous solutions are finally titrated to a pH of 8.5 as indicated by a pH meter, using approximately 0.05 N sodium hydroxide. (Indicator titrations can be substituted for the pH meter by using a large quantity, 1 to 2 ml., of phenolphthalein and continuing the titration until a pink color is obtained Thich persists for about 30 seconds or longer. If this latter procedure is used, however, it is usually necessary to also shave away the streaked portions of the column before elution in order to eliminate the interference of this indicator with the phenolphthalein color change. If the streaking has been done properly, only negligible quantities of the acids will be lost in this manner. ) ~
Using the procedure as outlined, the calibration curves given hold if 0.07 ml is subtracted from all dichloroacetic acid zone titrations, and 0.20 ml. from all other zone titrations. It is recommended, however, that these values be checked t o be sure that no foreign acidic impurities are present in any of the reagents or adsorbents used The calculations assume that equivalent losses of each of the acid components of the mixture are suffered and therefore the percentages of each are calculated on the basis of the total grams recovered rather than on the grams taken for analysis. For this reason, only the titration volumes m w t be noted precisely. The ueight of each acid component is calculated from these titration figures (grams of acid = corrected titration x normality x milliequivalent weight) and the calculated weights are totaled. From these figures the relative weight percentagep of each acid may be readily calculated. M ill
LITERATURE CITED
(1) Marvel, C . S., and Rands, R D.. Jr., J . Am. Chem. Soc., 72, 2642
(1950). RECEIVED for review June 28, 1951
.Iccepted RIey 8, 1952
