V O L U M E 23, NO. 2, F E B R U A R Y 1 9 5 1
343
acetic acid and seventy-five times that of the trichloroacetic Sample Taken First Wave Second Wave5 Error acid; for 0.1 millimolar concenTCA DCA lid TC.4 lid 0.94 lid lid DC.4 TCA DC.4 TCA DCA trations of the polychlorinated mM mM ,a. m.ll p ~ . w. pa. mM m.M m.W % % acids, the ratios are 2.5 and 0.098 0.100 0.76 0.093 1.57 0.71 0.86 0.106 -0,005 SO.006 -5.0 +6.0 ~ a .For 2 millimolar concen0,098 0.500 b 5.0 (4.9) 0.098 1.00 8.9 (9.0) trations of the polychlorinated 0,098 2.00 17.2 (17.1) 0.492 0.100 4bO0 0.487 4 56 3.76 0.80 0.099 -0.005 -0.001 -1 0 -1.0 acids the ratios are 12” and 0.492 0.500 4.00 0.487 7.98 3.76 4.22 0.516 -0,005 +0.016 -1.0 +3.2 I a. The presence of more than 0.492 1.00 3.95 0.481 11.98 3.71 8.27 1.01 -0.011 +O.Ol -2.2 +l.O 21.1 (20.4) a fivefold greater concentration 0.492 2.00 0.986 0.100 8.11 0.986 8.44 7.62 0.82 0.101 0.00 +0.001 0.0 +1.0 0.986 0.500 8.08 0.982 11.73 7.60 4.13 0.505 -0.004 +0.005 -0.4 +1.0 Of monochloroacetic acid 0.986 1.00 8.07 0.981 15.8 7.59 8.21 1.00 -0.005 +O.OO -0.5 0.0 affect the second wave of tri7.95 0.988 2.00 0.967 23.9 7.47 16.43 2.01 -0.019 +0.01 -1.9 +0.5 1.97 0.100 16.2 1.97 16.0 15.2 0.80 0.099 -0.00 -0.o01 0.0 -1.0 chloroacetic acid or the wave 1.97 0.500 16.1 1.96 19.2C 15.1 4.10 0,501 -0.01 +0.001 -0.5 +0.2 -0.5 0 0 Of dichloroacetic 1.97 16.1 1.96 1.00 23.3C 15.1 8.20 1.00 -0.01 0.00 1.97 2.00 16.1 1.96 31.5c 15.1 16.4 2.00 -0 01 0 00 -0.5 0 0 the beginning of the wave due Ratio of sixth roots of drop times was 0.94 for capillary used in making these measurements. to the reduction of monob Waves merged, but total wave is compared to that expected from calibration runs. c Galvanometer reading on limiting current plateau of second wave i3 not very steady, but diffusion current chloroacetic acid merges with can still be measured. these latter waves. The presence of more t’han a seventyfive times greater concentration of monochloroacetic acid will where lid and tid are the diffusion currents measured after the affect the first wave of trichloroacetic acid, because the wave due first and second wave increments a t -1.3 and -1.8 volts, reto monochloroacetic acid merges with the first trichloroacetate spectively, and it and z t are the drop times on the limiting current wave. portions of the first and second waves, respectively-Le., a t -1.3 and - 1.8 volts. With a mixture containing only the three chloroacetic acids, The weights or percentages of the two acids present are calcupolarographic analysis plus determination of the total acidity by lated as with the standard series technique of measurement. titration can be used to determine the amount of each acid. The difference between the total acidity and the sum of the dichloroacetic and trichloroacetic acids present gives the amount of DATA monochloroacetic acid. The polarographic reduction of other The effect of acetic and monochloroacetic acids on the diffusion polyhalogenated compounds is being studied. currents of dichloroacetic and trichloroacetic acids is given in Table I. SUMMARY A typical set of calibration data (Concentration us. diffusion Based upon their polarographic reduction in buffer solution of current) for the two acids is given in Table 11; linear relations pH 8, dichloroacetate and trichloroacetate can be simply and are obtained. The diffusion current constants, id/Crn’//‘ t’I8,a t rapidly determined when present either separately or in mixture. 29’ are 4.63 for dichloroacetic acid, and 4.64 and 4.63 for the Trichloroacetate gives two waves, the second, more negative one, first and second waves, respectively, of trichloroacetic acid. The of which is identical in characteristics with the one wave of dieffect of one acid on the other in mixtures is summarized in chloroacetate; the half-wave potentials are -0.84 and - 1.57 Table 111; blank spaces indicate sufficient merging of the two volts us. S.C.E. The diffusion currents of all three waves are waves to make separate measurement of the waves inaccurate. identical and are proportional to concentration; the diffusion With low concentration of the acids, no maxima were found; current constants are 4.61 and 4.63. The analytical results show slight maxima were obtained in most cases with concentration an accuracy of 2% or better. Excessive amounts of nionochloroexceeding 2 millimolar, but were never found to interfere with acetate interfere; acetic acid does not interfere. measurement of diffusion current. Table 111. Analysis of hlixtures of Dichloroacetic and Trichloroacetic Acids
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ACKNOWLEDG>MENT DISCUSSIOY
Although reference throughout this paper has been to the acids, a t pH 8.2 the concentrations of the undissociated forms of the acids are negligible and the behavior observed is that of the anions. The data obtained in these studies indicate that in samples (1) where the concentration of trichloroacetic acid is between 1 and 2 millimolar and the dichloroacetic acid is between 0.1 and 2 ndlimolar, the percentage error is 0 to 2% for each acid; (2) where the trichloroacetic acid is 0.1 to 0.5 millimolar and the dichloroacetic acid exceeds 2 millimolar, the first wave of trichloroacetic is not clear; and (3) where the trichloroacetic acid is less than 0.1 millimolar with any concentration of dichloroacetic acid, the plateau of the first wave becomes less flat and coincides with the second wave. If the amount of dichloroacetic acid exceeds the amount of trichloroacetic acid by a fourfold factor, it would be necessary to add known amounts of trichloroacetic acid to bring up the concentration in order to obtain optimum results. The real limitation of the technique described is that for 1 millimolar concentrations of the polychlorinated acids the molar concentration of the monorhloroacetic acid should not exceed five times that of the dichloro-
The authors wish to thank the Research Corp. for a Frederick Gardner Cottrell grant-in-aid upon which the equipment used was purchased, and the Atomic Energy Commission for a grant which supported this research project. LITER4TURE CITED
(1) Elving, P. J., and Tang, C. S.,J . Am. Chem. Soc., 72, 3244-G (1950). (2) Kolthoff, I. AI., IND. ENG.C H E X ,~ N A L ED., . 14, 198-8 ( 1 9 4 2 ) . RECEIVED May 22,1950.
Correction Attention has been called to an error in the article entitled “Principles of Precision Colorimetry. Xleasuring Maximum Precision Attainable with Commercial Instruments” [.ANAL. C H E K , 22, 1464 (1950)]. On page 1467 the correct range for the absorption-spectrum data on the copper (11) ammonium ion is 560 t o 760 mp. C. F. HISKEY
