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GERRITTOENNIES AND MARGARET ELLIOTT
[CONTRIBUTION FROM
THE
VOl. 59
LANKENAU HOSPITAL RESEARCH INSTITUTE]
A Polarimetric Method for the Determination of Water in Acetic Acid' BY GERRITTOENNIES AND MARGARET ELLIOTT Some recently proposed methods for the de- reactants (CH&!OOH)-suggested the probability termination of water in organic media are based of a strong thermodynamic tendency for the opon the acidimetric determination of acetic acid posite of reaction (2) formed from acetyl chloride2 or from acetic an- (CHsC0)pO CSHM(COOH)I.--t hydride.* Application of these methods to the 2CHsCOOH CeHdC0)nO (3) determination of water in acetic acid or similar This WBS found to exist: the bimolecular constanta liquids, although possible in principle, is obof this reaction in acetic acid is directly proporviously limited in accuracy and sensitivity by tional to the amount of strong acid present' and is, the large acidimetric blank value of the acetic in the presence of 0.001 M perchloric acid and a t acid. This limitation would be abolished if in about 25', of the order of 10-I; i. e., with equal the method of Toennies and Elliott3 the acetic amounts of catalyst, reaction (3) is estimated to anhydride could be replaced by an optically be lo4 times as fast as reaction (1). Therefore, active anhydride the hydration of which is measand since the acid-catalyzed reaction of acetic urable by an accompanying change in optical anhydride with moderate amounts of water in rotation. d-Camphoric acid and the correacetic acid is even more rapid than reaction (3), sponding anhydride differ widely in their efthe following principle for the determination of fect on po1a;ized light, but the hydration of water in glacial acetic acid suggests itself. camphoric anhydride In the presence of a suitable small concentraC8Ki4(CO)zO HzO +CsHir(CO0Hh (1) tion of a strong acid (perchloric acid) the acetic in acetic acid solution, although susceptible to acid solution of water is allowed to react with a acid c a t a l y ~ i s ,was ~ found to be too slow, with measured quantity of acetic anhydride, and the tolerably low acid concentrations (in presence unused amount of acetic anhydride is determined of 0.1 M HClOI k 2 j to be of practi by the decrease in optical rotation coincident cal promise. On the other hand, the complete with its reaction with a measured amount of absence of a conceivable5 interfering reaction d-camphoric acid. CsHi,(CO)*O 2CHsCOOH + The experimental investigation of the method CsHiz(C0OH)z (CHaCOIzO (2) has been confined to a demonstration of its -in spite of the large excess of one of the potential practicability and its minimum accuracy, while (1) Aided by the Robert McNeil Fellowship maintained by Mcit has not been extended either to a quantitative Neil Laboratories, Inc study of various factors that might add to its re(2) D. M Smith and W M D. Bryant, THISJOURNAL, 67, 841 (1935). finement (such as temperature, concentration or (3) G. Toennies and M. Elliott, ibid., 67, 2136 (1935). mutual effects on the specific rotations of d-cam(4) The catalysis of the hydration of organic anhydrides by strong acids is non-aqueous media, first suggested by observations of Orton phoric acid and d-camphoric anhydride) nor t o a and Jones [ J Chem Soc , 101, 1708 (19lZ)l on acetic anhydride and consideration of possible interfering substances confirmed by observations made in this Laboratory [Toennies and Lavine, J Biol. Chem , 100, 474 (1933); Laviae and Toeaaies, ibzd., (such as might conceivably be formic acid or 101, 732 (1933), ref. 31, has been studied by us on a number of an-
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hydrides of different types, and widely differing rates of catalysis have been found (results to be published). ( 5 ) The analogous reaction (CsHsCO)~O 2CHsCOOH 2CaH&OOH (CHsC0)zO has been described: R Kremann and W. Rdsler, Monotsh., 43, 3.58 (lOZ5), C A . , 17, 1426 /1923JJ
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(6) All velocity constants are expressed in moles per liter per minute. (7) G. Toennies and M. Elliott, abstracts of papers to be presented before the Division of Physical and Inorganic Chemistry at the Meeting of the American Chemical Sodety, Chapel Hill, N. C., April, 1937,
May, 1937
POLARIMETRIC DETERMINATION OF WATERIN ACETIC ACID
903
TABLE I REACTION BETWEEN d-CAMPHORIC ANHYDRIDEAND WATER Experimental Initial concentration of camphoric anhydride, b = 0.370 M ; of water, estimated from water content of acetic Reagents: Perchloric Acid Solution.-By dissolving a acid, as determined (u. infr.), and from water introduced by weighed amount of standardized concentrated perchloric 0.25 cc. of 10.9 M HC104, a = 0.362 M. Medium: acid in acetic acid* a solution of 1.00 M perchloric acid and acetic acid. Catalyst : 0.1 1 M HClOI. Temperature: H ~ in a 2-dm. tube. The 2.70 M water (exclusive of the water content of the acetic 24.0 * 1.5'. 01 = O ~ measured of the third column have been obtained by integraacid) was prepared. Repeated titration with an acetic acid values tions over the successive intervals of titne, and those of solution of piperidine, with crystal violet as the indicator,R the fourth column by multiplying each value of the third with the mean value of the water concentration for each showed complete stability. Acetic Anhydride.-The percentage of acetic anhydride interval. I n a control tube, without catalyst, ( b = 0.273 M,a = 0.068 M) the rotation remained, over a (x) in a concentrated preparation was calculated from the period of seven days, a = -0.07 * 0.02". average equivalent weight (e. w.) obtained by methylate Hours a k X 108 &(a - x ) X 108 titrationlOJ of a suitable aliquot dissolved in acetonitrile $1.41 0.75 2.47 19.5 100 - x loo 1.13 2.96 43.5 4-2.92 &+XiF== 1.59 3.21 67.5 +3.99 2.29 3.68 +4.71 90.0 The preparation used (Merck Reagent) showed 95.4 * f5.20 2.73 3.63 115.5 0.2% acetic anhydride. d-Camphoric Acid and &Camphoric Anhydride.-The the experimental conditions a reaction between camphoric camphoric acid used (Eastman) had a melting point of anhydride and acetic acid is thermodynamically impos187.5', corr. (Beilstein 187", corr.), and an equivalent weight, by methylate t i t r a t i ~ nof , ~100.1 ~ ~ ~ (calcd. 100 1). sible. The velocity of the reaction between camphoric acid Determinations of the specific rotationI1 in acetic acid gave and acetic anhydride (3) was investigated in the following [a]ggfl 53.9 * 0.1" in 0.25 M solution, and [a]z,il 4-54.25 * 0 1' in 0.05 M solution, while d-camphoric an- experiments. A solution of the initial concentrations 0.067 M Hg0, 0.688 M (CHaCO).lO, 0.216 M CsH14hydride in 0.25 M solution showed [ a ] H g -0.8 * 0.2" (COOH)2 and 0.001 M HClOk showed in the 2-dm. tube Ten cc. of acetic acid was found to dissolve a t room temthe following rotations: perature more than 0.5 but less than 0.6 g. of d-camphoric anhydride. 10 12 Minutes 7 14 Reaction Velocities.-The velocity of the acid-catalyzed 1.91' 1.60' 2.73' 2.22' ffHg reaction between camphoric anhydride and water is illus20 25 Minutes 16 32 trated by the experiment recorded in Table I The 0.90" 0.60" 0.32' a& 1.40' bimolecular constant, calculated over the range of 25 to 89% conversion of the anhydride, shows a uniformly inThe temperature was 25-28'. Calculation of the bicreasing trend which tends to disappear when it is assumed molecular constants for the points of 20, 40, 60 and that the velocity is inversely proportional to the water 80% conversion of the camphoric acid, obtained from the content of the medium (Table I , column 4). This assump- plotted curve and from the calculated initial and final tion may be justified as an approximation in view of the rotations $4.67 and -0.06", gives k = 0.135 * 0.007, fact that HzO, due to its basic properties in acetic acid,l**$ the assumption being made that the water present is will lower the acid potential of the catalyst eliminated a t once by reacting with the equivalent amount
acetone3), nor to an exploration of its applicability to other acid or non-acid media.
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