log K = k’
1 n
- - pC, an average value is listed in the Table
and marked by the approximation symbol. A value of log K < - 3 signifies that the ion in quesion exhibited no appreciable interference over the range studied. In general, the extent of interference shows a qualitative relationship to the extractability of the quaternary ammonium salts, the best results being obtained when the electrode anion is highly extractable and the interfering anion is poorly extractablee.g., sulfate interference in the perchlorate electrode is minimal. Although the electrodes cannot be considered
as highly selective, a wide range of useful measurements can be made with them. ACKNOWLEDGMENT
The authors thank Ken Mueller for assistance in some of the measurementsRECEIVED for review February 27, 1969. Accepted April 30, 1969. Work supported by the U. S. Atomic Energy Commission.
Polarographic Studies of Chloroacetones Maynard E. Hall and Edward M. Harris1 Unioersity of Arkansas, Graduate Institute of Technology, Little Rock, Ark.
WINKELand Proske (1) were the first to study chlorinated acetones and they found that 1,3-dichloroacetone gave diffusion currents of only one-fifth that given by monochloroacetone instead of double the current as expected, The 1,3dichloroacetone was the only polychlorinated ketone they studied, and no explanation was given for the abnormally low diffusion current for the dichloroacetone. Pasternak and Halban (2) and Elving and Van Atta (3) studied the reduction of monohaloacetones and established that the C-Cl bond was reduced instead of the carbonyl group. No other reports could be found on polarographic studies of polychlorinated acetones. However, Elving and Bennett ( 4 ) studied the reduction of polychlorinated acetaldehydes and they found correspondingly less current for dichloroacetaldehyde than for chloroacetaldehyde, indicating the dichloroacetaldehyde to be more strongly hydrated. All of the above studies were made in aqueous solutions. Bell and McDougall (5) studied the hydration of some chlorinated acetones by measuring the decrease in absorbance in the 290-300 mp region, and more recently Hooper (6) reported the use of NMR to follow the hydration of chloroacetone. Because organic solvents are currently used in many polarographic analytical procedures, it appeared desirable to make a study of the polychlorinated acetones in acetonitrile containing various amounts of water, causing hydration of the carbonyl group, and to determine to what extent quantitative results can be affected by the presence of water. This report also includes the results and comparison of an ultraviolet study of the chlorinated acetones to the polarographic study. The compounds studied are monochloroacetone, 1,l -dichloroacetone, 1,3-dichloroacetone, and 1,1,3,3-tetrachloroacetone. No trichloroacetones were commercially available for study. 1 Present address, Chemistry Department, University of Arkansas, Fayetteville, Ark.
(1) A. Winkel and G. Proske, Ber. deut. Chem. Ges., 69,693 (1936). (2) R. Pasternak and H. von Halban, Helv. Chim. Acta, 29, 190 (1946). (3) P. J. Elving and R. E. Van Atta, ANAL.CHEM., 27, 1908 (1955). (4) P. J. Elving and C. E. Bennett, J. Electrochem. SOC., 101, 520 (1954). (5) R. p. Bell and A. 0. McDougall, Trans. Faraday SOC., 56, 1281 (1960). (6) D. L. Hooper, J. Chem. SOC.,B, 1967, 169.
EXPERIMENTAL, Apparatus. The polarograms were recorded with a Sargent Model XXI Recording Polarograph, with damping set on the number one position. The dropping mercury electrode was used with a 52.3-cm head of mercury, except in the experiments designed to determine the effect of mercury column height on the limiting current and to establish the type of process that controlled the reduction current. The drop time was measured in the CHICN-H~O (80-20%) system at an applied voltage of -1.70 V (the top of the reduction wave of monochloroacetone) us. the saturated calomel electrode, and was found to be 2.96 seconds, the flow rate being 1.69 mg/sec. From these values the capillary constant, mz’at1/6, had a value of 1.70 mgz’a sec-1’2. The drop time in the acetate buffer system at an applied voltage of -1.5 volts was 3.5 seconds. The solutions were contained in a polarographic H-cell in which the solution compartment and the SCE were connected with an aqueous agar bridge saturated with potassium chloride. The temperature of the cell was 24 + 0.5 “C. The ultraviolet spectra were obtained with a Bausch and Lomb Spectronic 600 double beam spectrophotometer in conjunction with a Sargent Model SRL Recorder. Absorbances were measured in 1-cm fused silica cells us. an appropriate blank solution. Reagents. Acetonitrile (Fisher Certified) was used as the solvent and contained 0.20% (volume) water by Karl Fischer titration. The monochloroacetone and 1,l-dichloroacetone were prepared in this laboratory and were estimated to be 99.35y0 and 99.23y0 pure, respectively, on the basis of gas chromatographic analyses and per cent chlorine analyses (7). The 1,3-dichloroacetone was obtained from Distillation Products Industries, and was used without further purification because its purity was estimated to be greater than 99% by gas chromatography and per cent chlorine analysis. The 1,1,3,3-tetrachloroacetonewas obtained from Eastern Chemical Corp., Pequannock, N. J. Supporting electrolytes were lithium perchlorate obtained from G. Frederick Smith Chemical Co. as LiC104.3H20, and tetramethylammonium chloride obtained from Distillation Products Industries. Nitrogen, used for removal of dissolved oxygen, was Matheson prepurified type. All other reagents used were analytical reagent grade. Procedure. Approximately 0.01M stock solutions of each chlorinated acetone were prepared in acetonitrile. Aliquots were taken and diluted with the acetonitrile-water mixtures to
.,
1130
ANALYTICAL CHEMISTRY
(7) W. Thornsberry, M.S. Thesis, University of Arkansas, 1968.
give a ketone concentration of approximately 0.1 or 0.4 m M for polarographic analysis. Each solution was 0.1M with respect to the supporting electrolyte, and both tetramethylammonium chloride and lithium perchlorate were investigated as supporting electrolytes. The procedure for studies using aqueous acetic acidsodium acetate buffer solutions was very similar to that used for the acetonitrile-water system. Aliquots of stock solution were diluted with the buffer solution (pH of 5.0) to give ketone concentrations in the 0.1 to 1.0 m M range. The buffer also served as the supporting electroiyte.
Table I. Polarographic Data for Chloroacetones in Acetate Buffer of pH 5.0
EI/Z Compound Monochloroacetone 1,l-Dichloroacetone 1,3-Dichloroacetone
(V us. SCE)
I
3.78 0.98 (1st wave) 0.98 (2nd wave) 0.97 (1st wave) 0.97 (2nd wave)
1.15 0.57 (1st wave) 1.16 (2nd wave) 0.83 (1st wave) 1.14(2nd wave)
RESULTS AND DISCUSSION
a supporting electrolyte, but because of low solubility, 0.1M solutions could not be obtained when the water content became less than 10%. All solutions were run within 2 to 3 hours after preparation. Attempts were made to obtain polarograms of 1,1,3,3-tetrachloroacetonebut the reduction currents were too small for reliable measurement, even when the water content was as low as 1.501,. Tetrachloroacetone reacted so readily with water that a completely anhydrous system would be necessary to obtain any appreciable reduction current for this compound. Table I1 is a summary of the polarographic data obtained for the chloroacetones in the acetonitrile-water system. The half-wave potentials given have been corrected for iR drop only for the 20% water solutions because these solutions were found most suitable for analytical work. The resistance measured across the cell for the 20% water content was 2200 ohms. Spectrometric Studies. The ketone concentrations used in the spectrometric studies were from 50 to 100 times greater than those used in the polarographic work because the ketones have low molar absorptivities. Solutions of each chlorinated acetone were prepared in acetonitrile containing water in the range 0.2-100%;;. Ultraviolet spectra were obtained in the 225-380 nm region within 2 to 3 hours after preparation, and the absorption data are given in Table 111. The polarographic data in general correlate with the spectrometric data for the two dichloroketones in that the diffusion current constant values and the molar absorptivities both decrease with increas-
Reduction in Acetate Buffer System. Table I summarizes the polarographic results obtained for the chlorinated acetones in the acetic acid-sodium acetate buffer system. The diffusion current constant ( I ) values for the dichloroacetones should be twice the I value for monochloroacetone because the total current of both waves of the dichloroacetones were used in calculating the I value, but the I values were only half that for the monochloroacetone. The half-wave potential of the second wave of the 1,l and 1,3-dichloroacetones is the same as for the single wave of monochloroacetone, and is in close agreement with the half-wave potential reported by Elving and Van Atta (3) for monochloroacetone using the same buffer system at a pH of 4.6. For monochloroacetone we found a diffusion current constant of 3.78 cs. 2.55 by Elving and Van Atta. This discrepancy in I values could be caused by difference in purity of monochloroacetone or by interfering side reactions. Reduction in Acetonitrile-Water System. The acetonitrilewater solvent system was investigated with water contents ranging from 1.0 to 100%. All water contents are expressed in terms of volume per cent. A completely anhydrous system was not investigated because poorly defined waves were obtained in low water concentrations and showed poor analytical promise. Furthermore, it would be impossible to keep a system anhydrous when using an aqueous agar bridge. The supporting electrolyte was lithium perchlorate (0.1M) for all solutions. Tetramethylammonium chloride was also used as
Table 11. Effect of Water Content on Polarographic Data for Chloroacetones in Acetonitrile-Water Solvent with 0.1M Lithium Perchlorate as Supporting Electrolyte E112(-V
Compound Monochloroacetone
1,l-Dichloroacetone
Volume 1 10 20 50 70 100 1 10
20 50
70 100
1,3-Dichloroacetone
1.5 10
20 50 70 100 a
HzO
I
1st wave
7.42 7.70 6.39 4.94 4.30 4.04 23.4 16.3 15.2 10.4 7.86 4.47 17.80 13.30 11.35 7.33 5.10 3.30
1.42 1.43 1. 445 1.41 1.36 1.22 1.09 1.19 1.1P 1.11 0.98 0.65 1.09 1.21 1 .20a 1.18 1.13 0.91
CS.
SCE)
2nd wave
1.42 1.45 1 .46a 1.43 1.38 1.20 1.43 1.46 1,460 1.44 1.40
1.22
Corrected for iR drop.
VOL. 41, NO. 8,JULY 1969
1131
Table 111. Spectrometric Data for Chloroacetones in Acetonitrile-Water Solvent Compound
Concn, M
A
e
L a x (mM)
Mono 131 1,3 Mono 191 1,3 Mono 1,1 1,3
0.0216 0.0101 0.0101
0.525 0.482 0.252
24.3 47.7 25.0
281 292 280
0.0216 0.0101 0.0200
0.514 0.466 0.427
23.8 44.1 21.4
278 290 280
0.0216 0.0101 0.0200
0.517 0.407 0.310
23.9 40.3 15.5
275 288 278
0.0216 0.0101 0.0200
0.509
23.6 34.8 12.6
277 288 278
0.0216 0.0101 0.0200
0.507
0.326 0.179
23.5 32.3 8.95
274 287 280
70
0.0216 0.0101 0.0200
0.497 0.269 0.107
23.0 26.6 5.35
275 285 280
100
0,0096 0.0104 0.0184
0.204 0.174 0.075
21.2 16.8 4.08
275 285 275
0.2
10
20
30
50
ing water content. The two methods, however, are not in agreement for monochloroacetone. This difference may be due t o the effects of varying viscosities on the diffusion currents and to the possibility of acetonitrile forming a complex with the monochloroacetone to affect the molar absorptivity. Bell and McDougall (5) reported that 33% of monochloroacetone and 21% of the 1,3-dichloroacetone become hydrated in water at room temperature as determined by ultraviolet absorbance methods. Their calculation was based on the differences in molar absorptivity in cyclohexane and water for each compound. The e value for monochloroacetone in cyclohexane was 30.7 and in water was 18.9 at 25 "C. This decrease of approximately one third in molar absorptivity in water was assumed to be due to hydration. Our ultraviolet data for monochloroacetone in acetonitrile shows very little decrease in molar absorptivity as the water content is increased. Also the molar absorptivity in acetonitrile is approximately 25 as compared to 30.7 in cyclohexane. These two observations indicate that acetonitrile interacts with monochloroacetone and can compete with the water. The ultraviolet spectrum obtained for 1,1,3,3-tetrachloroacetone showed that the presence of only 1% water nearly completely hydrated the carbonyl group as the absorbance in the carbonyl region (290 mp) was extremely small. It should be mentioned here that the spectrometric studies of monochloroacetone revealed the presence of mesityl oxide as an impurity in this compound. The concentration was only 2.5 X lOVM in the monochloroacetone, but mesityl oxide is a very strong absorber and interfered with absorbance measurements of very low concentrations of monochloroacetone. This interference was eliminated by adding the appropriate amount of mesityl oxide to the acetonitrile in the reference solution. The concentration of mesityl oxide was too small to interfere in the polarographic studies of monochloroacetone. Type of Current Process. Plots of the current vs. the square root of the corrected mercury column height were linear indicating the reduction process is diffusion controlled. 1132
ANALYTICAL CHEMISTRY
0.352 0.251
This is at variance with an equilibrium between the carbonyl and glycol forms. Quantitative Studies. Inasmuch as the optimum water content for well defined waves and greatest sensitivity was 20% a study was made at this level to determine what precision could be expected of quantitative measurements for all of the chlorinated acetones. A series of standard solutions of each ketone was prepared containing approximately 0.1, 0.2, 0.5, and 1.0 mmole of ketone per liter of acetonitrile (80 vol %)-water (20 vol %) solvent containing lithium perchlorate (0.1M). A pllot of the diffusion current cs. the concentration gave a good linear relationship over the concentration range studied. A statistical analysis of data obtained from known solutions was made using the range method of Dean and Dixon (8). The precision at 95y0 confidence limits was *4.357, and 0.97% for 0.1 and 0.6 m M concentrations, respectively, for monochloroacetone. For 0.1 and 0.5 m M 1,l-dichloroacetone the precision was + 7 . 9 5 z and +1.97%, respectively, and for the same concentrations of 1,3-dichloroacetone the precision was +0.98% and 5 1 . 4 7 z . Analytical methods of good precision by polarographic means have been developed for each of the chlorinated acetones separately, except for the tetrachloroacetone. Mixtures of the monochloroacetone and either of the two dichloroacetones could be analyzed but not mixtures of the two dichloroacetones. ACKNOWLEDGMENT
We are grateful to Edgar D. Smith of this laboratory for the preparation and purification of most of the chlorinated acetones used in this work. RECEIVED for review November 18, 1968. Accepted April 14, 1969. (8) R. B. Dean, and W. J. Dixon, ANAL.CHEM., 23, 636 (1951).
