1774
ANALYTICAL CHEMISTRY
(8) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., and Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 205, Wiley, New York, 1953. (9) Ibid., p. 478. (10) Hodgman, C. D., “Handbook of Chemistry and Physics,” 34th ed., p. 1559, Chemical Rubber Publishing Co., Cleveland, 1952-3. (11) lime, L.,2. physik. Chem., A146, 41 (1930). (12) “International Critical Tables,” vol. IV, p. 53, McGraw-Hill, New York, 1926. (13) Kolthoff, I. hl., J . Phys. Chem., 36, 860 (1932). (14) Kolthoff, I. M., and Sandell, E. B., Ihid., 37, 723 (1933). (15) Prutton, C. F., and Maron, S. H., “Fundamental Principles of Physical Chemistry,” pp. 6 5 2 4 , Rlacmillan, New York, 1944.
(16)
Salutsky, M., Stites, J. F., and AIartin, A. R., ANAL.CHEV..
25, 1677 (1953). (17) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 562, Interscience, S e w York, 1950. ~. 25, 1519 (1953). (18) Stine, C. R., and Gordon, L., A 4 i v ~ 4CHEM., (19) Wahl, A. C., and Bonner, X . A,, “Radioactivity Applied to Chemistry,” pp. 105-7, Wiley, New York, 1951. (20) Weaver, B., A N i L . (21) Ibid.,p. 479.
CHEX.,
26, 477 (1954).
(22) . , Willard, H. H., Ibid., 22, 1372 (1950). RECEIVED for review r e b r u a r y 26, 1955. .\crepted .Tuli 19. 1955. Presented before the Division of Analytical Chemistry a t tile 127th meeting of the SOCIETY, Cincinnati, Ohio, A\Iarcti IEA. AMERICAN CHENICAL
Titrimetric Assay of Trichloroacetate W. A. SCHNEIDER, J ~ . , a n d L. E. STREETER The Dow Chemical Co., Midland, Mich. A simple and accurate assay method based upon the decarboxylation of trichloroacetate is presented. A known amount of standard acid is added to the neutral sample, the solution is refluxed for at least an hour, and the residual acid is titrated. The acid consumed is a direct measure of the trichloroacetate content. Contaminants usually present in commercial grades of the acid or salt do not interfere and need not be determined. The proposed method requires much less time than the classical alkaline hydrolysis procedure.
T
H E accepted method for the assay of trichloroacetate is essentially a chloride determination corrected for the chloride contributed by the major contaminant, dichloroacetate. The total chloride is determined following a Parr peroxide bomb decomposition or an alkaline hydrolysis. The latter process also converts any dichloroacetate present to oxalate, which is then determined by the usual calcium oxalate precipitation and subsequent permanganate titration. This method, which mav be credited to Pool ( 4 ) , has been largely developed by industrial laboratories without publication. In this laboratory, it has been noted that different analysts could not always agree on trichloroacetate assay, the usual point of difference being the dichloroacetate determination. A study by Dalin and Haimsohn ( 1 ) discussed the errors of dichloroacetate determination as applied to the assay of monochloroacetate, but these authors found that low results were related to the quantities of reagents used. However, in the present work, high values of dichloroacetate were the major concern.
Table I.
Substance I Added None
Table 11. Effect of Contaminants ’$ of NaTCA Taken,“ NaTCA Found,
Mixture
G.
%
G. Recovery ... 2.053 2.051 99.9 ... 2,492 2.492 100.0 2.492 2 492 100.0 NaHCOJ 1.98 2.508 2,510 100.1 4.63 2.508 2.510 100.1 N~~HPOI 1.07 2.492 2.495 100.1 2.05 2,508 2.510 100.1 5.43 2 508 2.510 100.1 NaDCA 11.6 ‘ 2.053 2.051 99.9 31.9 2,053 2.049 99.8 50.2 2,492 2.473 99.2 NahICA 9.5 2.053 2.049 99.8 19.2 2.492 2.484 99.6 40.5 2.492 2.468 99.0 51.7 2.492 2,380 95.5 Weights of KaTCA taken were computed on the basis of assayed salt purity of 99.6%.
...
of trichloroacetate in aqueous solution. I n some of these, the reaction rate was followed by titration of the residual acid (8, 3). The present method is essentially a refinement of this principle and is defined by the following equation:
Although the procedure described was developed specifically for the assay of technical sodium trichloroacetate, the general method is not restricted to the salt, as it has been used with equal success for the acid and the ethyl ester.
Standardization of Decarboxylation Method
Standard Sample Trichloroacetic acid Ethyl trichloroacetate Sodium triohloroacetate Sodium trichloroacetate
% Compound Found 99.4,99.8,99.5,99.7 100 2 , 9 9 . 6 , 9 9 . 9 , 9 9 9,99 6 99.7,99.6,99.7,99.6 99.7,99.7,99.6,99.5
Samples of trichloroacetic acid, known to be free of dichloroacetate by infrared and freezing point data, invariably showed a dichloroacetate content of from 2 to 3%, which was found to be a function of the sodium hydroxide concentration used for hydrolysis. The conclusion reached was that the alkaline hydrolysis also converts some trichloroacetate to oxalate, thus causing spuriously high values for dichloroacetate. Clearly, a new assay method was needed for trichloroacetate. Several kinetic studies have been made of the decarboxylation
APPARATUS AND REAGENTS
Flasks, 250 ml., flat-bottomed, short-necked T24/40. Condenser, 50-cm., water-cooled, with glass joint $24/40. Methyl red indicator, 0.1% solution in 95% ethyl alcohol. Sulfuric acid, approximately IN solution. Sodium hydroxide, 1N standard solution. Dioxane, freshly distilled. PROCEDURE
Dissolve a 25-gram sample of sodium trichloroacetate in water and dilute to 100.0 ml. ( I t is best to take such a relatively large quantity t,o ensure a representative sample.) After dissolving, select an aliquot which will cause the final tit,ration volume to be about half of the blank titration volume. Pipet duplicate 10.00-ml. aliquots of sample solution into 250ml. reflux flasks, add 1 drop of methyl red, and neutralize by
V O L U M E 27, N O . 11, N O V E M B E R 1 9 5 5 titrating with I N sulfuric acid. As the solution is usually alkaline due to bicarbonate, it is important to titrate to a distinct orange-pink, which is about pH 5.5 to 5.3 and indicates neutralization of the bicarbonate. The usual requirement is less than 0.15 ml. of 1X sulfuric acid. In the case of an acid sample, neutralization is accomplished with 1N sodium hydroxide, and the end point (methyl red is used) is the transition from light orange to yellow. 9 d d 25.00 ml. of LV sulfuric acid, 35 ml. of dioxane and a few glass beads. Boil under reflux for a t least 60 minutes. This time is not critical, but less than 60 minutes may not be sufficient for complete reaction. To the cooled solution add 2 drops of methyl red, and titrate with standard 1A- sodium hydroxide. The end point is taken at the transition from light orange to yellow. In this titration, the indicator is influenced by the relatively basic solvent, and the orange transition color appears long before the end point; however, a sharp change from orange to yellow clearly marks the proper end point. Because of acid contained even in freshly distilled dioxane, run duplicate blanks with each series of determinations. These are identical to the test solutions except that they contain no sample. This practice also bases the determination on a single standard solution, the sodium hydrox,de. The sources of error in this method are primarily those which are common to all volumetric methods: calibration and drainage of glassiTare, estimation of end points, and buret readings. In addition, low results mag be caused by incomplete reaction due either to insufficient reflux time or nonvigorous boiling, which. is essentially the same thing. C.4LCULATION
Expressed as per cent sodium trichloroacetate:
% NaTC-4
=
(net titration volume) X 0.1854 X 100 aliquot weight
RATE OF DECARBOXY LATIOY
The first trials of this method LT ere conducted in water solutions. Using 0.25-gram aliquots and 0.1N reagents in a total volume of 50 ml., the rate of reaction was such that refluxing for 1 hour seemed to be sufficient for completion. However, when applied to recrystallized trichloroacetic acid (99.57 mole %), results were found to be low by about 3%. With larger samples and a higher sulfuric acid concentration, even 3 hours of refluuing time was not enough. A review of the literature revealed that the reaction rate is a function of the trichloracetate ion concentration and is increased in a solvent such as dioxane. According to Salmi and Korte ( 5 ) , the rate of decarboxylation of trichloroacetic acid is most rapid in 62% dioxane-water solutions. However, the actual concentration of dioxane does not appear critical. Since in this work identical and rapid rates were found in both 40 and 50% solutions, the 62% concentration is considered unnecessarily large. For the procedure, 50% dioxane was selected, because of a secondary influence on the rate. The reaction product, chloroform, is not expelled during boiling under reflux. With samples as large as 2.5 grams in 70 ml. of solution, the accumulation of chloroform as a second liquid phase seriously cools the system, thus slowing the reaction rate. In water alone, the effect of the chloroform is very dramatic, practically stopping active boiling. A solution of 40% dioxane is barely enough to keep the chloroform dissolved, but 50% solutions remain single phase throughout the procedure. Data were obtained comparing reaction rates for two alternate sample sizes, 0.25 and 2.5 grams. The smaller sample with 0.1N reagents would seem preferable, because only 30 minutes are needed for completion; however, this advantage is offset by a greater uncertainty in selection of the end point and by a potentially greater sampling error. As a control procedure, the IN reagents are better; but for special work, good precision and accuracy can be obtained with small samples and 0.1N reagents.
1775 In the latter case an electrometric detection of the end poilit is recommended. STANDARDIZATION O F PROCEDURE
The procedure has been tested with pure samples of trichloioacetic acid, ethyl trichloroacetate, and sodium trichloroacetate (each entry in Table I is an independent single determination). The acid, prepared by recrystallization six times from carbon tetrachloride, was dried in a vacuum, and contamination of atmospheric moisture was avoided. Freezing point data indicated a purity of 99.74 mole %; total chloride determination showed 99.7%. By virtue of careful preparation and fractional distillation through a 40-plate column, the ethyl ester \vas thought to be nearly 100% pure, and infrared analysis showed that is was free of ethyl dichloroacetate. A freezing point determination reported 99.45 mole %, and a saponification titration a t 15' C. showed 99.9%. This substance seems to be an inadequate standard in that the total chloride determinations were unsatisfactory. By Parr peroxide bomb, the assay was 99.2%, whereas hydrolytic chlorides ranged from 98.7 to 99.4% giving about the same average. In terms of precision, the proposed decarbouylation method is superior, in this case, to the chloride method., and the results agree with those of the freezing point and saponification determinations. Two different lots of sodium trichloroacetate were purified by repeated crystallization from mater a t temperatures beta cen 30' and 0" C. This was a drastic procedure because of the solubility of the salt. From 10 pounds of the salt in the first solution, about 0.5-pound yield was obtained. The products were dried, ground fine, and then dried in a vacuum desiccator for a t lrast 10 days to remove water of crystallization. By total chloride determination, the purity of each was 99.6%. EFFECT O F CONTAMINANTS
Technical grades of sodium trichloroacetate, containing bicarbonate and dichloroacetate, also may contain disodium phosphate in concentrations up to 1% of the dry salt as an added corrosion inhibitor. hlonochloroacetic acid is not usually encountered in trichloroacetic acid preparations, but its behavior in this procedure is of interest. The results of the influence of these four substances are listed in Table 11, but large concentrations of bicarbonate and phosphate were not considered, as the procedure is presented as an assay method. However, it is probable that concentrations greater than lo%, especially of phosphate, would seriouPly affect the titration end points. I t was expected that dichloroacetate (NaDCA)andmonochloroacetate (SahlCA) would cause low results by yielding acid in hydrolysis, but this laboratory found that these can be tolerated up to concentrations of 32 and 9.5%, respectively, which is an important point in favor of this new method. ACKNOWLEDGMENT
The authors are indebted to Francis E. Hance, principal chemist, the Hawaiian Sugar Planters Association, whose helpful criticism of the accepted analytical methods motivated this work. LITERATURE CITED
Dalin, George, and Haimsohn, Jerome, ANAL.CHEM.,20, 470 (1948).
Johnson, P., and Moelwyn-Hughes, E. A., Proc. Roy. SOC. ( L o n d o n ) , A175,118 (1940).
Kappanna, A. N., 2. p h y s i k . Chem., A158, 355 (1932). Pool, J. F. A , , P h a r m . Weekblad, 42, 165 (1905); J . Chem. Soc., 88, 425 (1905).
Salmi, E. J., and Korte, Raymond, Ann. Acad. Sci. Fennicae. A54, No. 10,22 (1940);Chem. Zentr., 1942 I, 328. RECEIVED for review February 11, 1955. Accepted July 11, 1955.
