V O L U M E 2 3 , NO. 2, F E B R U A R Y 1 9 5 1 h t h indicators yielded a mean of 31.82 ml. of sodium hydroxide cquivalent t o 30.00 mi. of hydrochloric acid. Thus the indicator vtmr was negligible. In order t o study the behavior of the luminol indicator in the pwence of a highly colorcd component, 14 titrations of the hytltvchloric acid with thc sodium hydroxide were carried out, using thct luminal indicator in the presence of gentian violet. The concc~ntrationof gentian violrt uscd--0.003%-gave the solution a n intense color which in:idc it utterly impossible t o observe t,he end point of any of the ordinary acid-base indicators. The results of thwc titrat,ions, shown in Table 11, yield a mean value for the volume of sodium hydroxide required which is the same as that required in both titrations shown in Table Inamely, 31.82 ml. The deviat,ion from the mean, however, is somewhat larger. Its value-0.07 ml.-corresponds t o a precision of 2.13 parts pcr 1000. I n order to study more carefully titrations i n which luminol indicator is used, and to account, if possible, for the lower precision obtained with luminol as compared with phenolphthalein, five titrations of hydrochloric acid with sodium hydroxide were carried out potentiometrically in the presence of the luminol indicator. A glass indicator electrode, a saturated calomel reference electrode, and a Leeds and Northrup i66O-ii vacuum tube potentiometer were employcd. The temperature of the laboratory was 25" * 2" C. The average curvc obtained is plotted in Figure 1, A . The potentiometric titration of the hydrochloric acid with the sodium hydroxide in the abxnce of luminol indicator is plotted as B . The two curves cross very close t o the luminol end point. The inore gradual s l o p of the luminol curve accounts for the lower precision obtained with this indicator. Work is in progress t o modify the composition of the luminol indicator, so as to give a curve with a steeper slopc, t,hereby improving the possibility of oljtaining a higher degree of precision. If the titrations employing luminol in both Tables I and I1 are considered, the over-all precision obtained is 1.85 parts per 1000. Indicat,ions at the present time are that this can be improved upon. The more gradual slope of A as compared with B is t o be expected, in view of the fact that each component of the indicator functions a s a weak electrolyte. Because both luminol and hemoglobin combine with hydrogen ion, the p H values preceding the stoirhiomet#ricpoint will be higher than otherwise, and because
34 1
these same components supply protons t o hydroxyl ions after the stoichiometric point has been passed, the p H values obtained will be IoR-er than in the absence of these components. This effect is further accentuated by the presence of the weak acid hydrogen peroxide, which supplies protons t o hydroxyl ions in the neighborhood of the stoichiometric point. As an aid t o further study of the luminol indicator a potentiometric titration was carried out using luminol indicator from which the hydrogen peroxide had been omitted. The results are plotted as C in Figure 1. Part but not all of the buffering action of the indicator appears to be caused by the hydrogen peroxide. LITERATURE CITED
Abel, E., Molonatsh., 79, 457 (1948). Anderson, R. S., Ann. N. E'. Acad. Sci., 4 9 , 337 (1948). Bacon, R. G. R., Trans. Furuduy Soc., 42, 140 (1946). Baxendale, J. H., Evans, 11. G., and Park, G. S., Ibid., 42, 155 (1946).
Bernanose. -&., Bremer, T., and Goldfinger, P., B I L Lsoc. chim. Relges, 56, 269 (1947).
Drew, H. D. K., and Cross, €3. E., .I. Chem. SOC.(London),1949, 639.
Drew, H. D. K., and Pearman, F. H., Ibid., 1937, 588. Etienne, A , and Bichet, G., Compt. rend., 228, 1136 (1949). Haber, F., and Weiss, J . Proc. Rou. Soc., 147A, 332 (1934). Haber, F., and Willstaetter, R., Ber., 64, 2844 (1931). Hauroivitz, Felix, "Progress in Biochemistry," pp. 172, 281, New York, Interscience Publishers, 1950. Kautsky, H., and Kaiser, K. H., Natur~issenschufte?L,30, 148 (1942).
Kenny, F., and Kurts, R. B., IND.ENG.CHEM.,ANAL.ED.,23, 382 (1951).
Legge, J . IT., and Lemberg, R., "Hematin Compounds and Bile Pigments," p. 387, New York, Interscience Publishers, 1949. Lewis, G. K.,and Kasha, &I,, J . Am. Chem. Soc.. 66, 2107 (1944).
Mornan. L. B . . Truns. Faradau Soc.. 42. 169 11946). Thetrell, H., "Advances in Enzymology," Vol. 7 ; Kew York, Interscience Publishers, 1947. Waters, W. A,, "Chemistry of Free Kadicals," Oxford, England, Clarendon Press, 1946. Waters, W.A , , and Mera, J. H., J . Chem. Soc. (London), 1949, "
I
515,2427.
Weber, K., Ber., 75B, 568 (1942). Weber, K., Reaek, A , . and Vouk, V., Ibid., 75B, 1141 (1942). Zellner, C. K.,and Dougherty, G., J . A m . Chem. Soc., 59, 2581 (1937). RECEIVED June 19, 19.50
Polarographic Behavior of Organic Compounds Analysis of Mixtures of Dichloroacetic and Trichloroacetic Acids PHILIP J. ELVING AND CHING-SIANG TANG T h e Pennsylvania State College, State College, Pa.
E
LVING and Tang ( 1 ) reported that in the pH range of 6.8
t o 10.4 and the potential range of 0.4 t o - 1.9 volts, acetic :tiid monochloroacetic acids give no polarographic wave, while trichloroacetic acid gives two waves and dichloroacetic acid gives one wave; the latter wave is identical in characteristics with the more negative wave of trichloroacetic acid. The diffusion curicTnts of each of the two waves of trichloroacetic acid and of the o w wave of dichloroacetic acid, when corrected for the effect of the electrocapillary curve, are identical; the diffusion currents :LI'CL directlv proportional t o the concentration of the acids. The
waves are due to the successive removal of halogen whereby trichloroacetate is converted t o dichloroacetate, which can then be reduced t o monochloroacetate. In the procedure described for analyzing mixtures of dichloroacetic and trichloroacetic acids, the latter is determined from the diffusion current of its first wave, while the dichloroacetic acid can be measured by deducting the adjusted diffusion current of the first wave of the trichloroacetic acid from the total diffusion current of the second wave. The standard series method of calibration can be used.
ANALYTICAL CHEMISTRY
342 A means was sought for the determination of trichloroacetic and dichloroacetic acids in the presence of each other and of related compounds. Dichloroacetate and trichloroacetate, present singly or in mixture, can be determined simply and rapidly from the polarogram obtained at pH 8 in buffered solution. The trichloroacetate gives two reduction waves representing the successive removal of halogen to form first dichloroacetate and then monochloroacetate; dichloroacetate gives one wave which is identical in characteristics with the second wave
EXPERIMENTAL WORK
Reagents and Chemicals. All reagents used were of C.P. quality. Stock standard solutions of trichloroacetic acid (10 millimolar), dichloroacetic acid (10 millimolar), monochloroacetic acid (1.0 M ) , and acetic acid (1.0 M ) were prepared, and were standardized by titration with a standard sodium hydroxide solution, using phenolphthalein as indicator. Double-strength buffer solution of pH 8.2 was repared by adding concentrated ammonium hydroxide to a 1.0 i f f solution of ammonium chloride until the desired pH was reached; only a few drops per liter are necessary. When the solution is diluted in use, the electrolyte content is great enough so that the buffer solution can act as the base solution. A paratus. A calibrated Fisher Elecdropode was used in most of tRe work; all measurements were made a t 0.2, 0.1, or 0.05 of the galvanometer sensitivity. The deflection of the Elecdropode galvanometer scale was calibrated in microamperes by substituting for the polarographic cell Akra-Ohm resistances ranging from 10,000 to 100,000 ohms. Some current-potential curves were determined with a Sargent Model XXI polarograph. A Beckman Model G pH meter was used for the measurement of pH. All glassware used was borosilicate; all measuring apparatus was calibrated.
of trichloroacetate. Monochloroacetic acid does not affect the determination unless its molar concentration is greater than five times that of the dichloroacetic acid or seventy-five times that of the trichloroacetic acid. Acetic acid does not interfere. The method of polarographic analysis presented is sqitable for the rapid and simple determination of trichloroacetate and dichloroacetate individually and in the presence of each other. Substances polarographically inactive in the potential range covered do not interfere.
solution. The height of the second wave is corrected for the effect of the electrocapillary curve in the usual manner ( d ) , the capillary constants having been measured in the buffer base solution used. Although the polarographic measurements can be made a t any temperature, some type of constant temperature arrangement is desirable in order to ensure that the calibration and sample measurements are made a t the same temperature-e.g., 25" C.
Table 11. Calibration Data for Dichloroacetic and Trichloroacetic Acids Sample Taken First Wave Second Wavea mM. pa. pa. TC.4 0.098 0.80 0.79 0,492 4 04 4.00 0.986 8 11 8.04 1.97 16.21 16.05 DC.4 0.100 ... 0.81 0,500 ... 4.09 1.00 ... 8.20 16.35 2.00 ... Diffusion current for second wave corrected for effect of electrocapillary curve.
Table I. Effect of Concentration of Acetic and Monochloroacetic iicids on Diffusion Currents of Dichloroacetic and Trichloroacetic Acids Sample Taken DCA AICA m.M mM mM
-
TC.4
1 .. 0V 1I I 2.02 2.02 1.01
.. .. 1:0i 1:01
..
,, .,
.. 1 : 0c 21 1 .. 0c 2 1.02 1.02
i.i.0 1.50 1.5 0 29.0 I.,."
.
,
;,bo ~
7.50
.
.4.4
m.M
Trichloroacetic .4cid First Wave Second Wave" Fa. m.U Fa. m.M
b. ,, . ,, 16.30 16.30 IO0 8 . 1 2
,.
.
..
..
100 . , s'i4
b
I1.98
16.31
1.98 0.99
b
.. ..
8,lj
.. ..
::
DichloroAcid m.W
+a.
Calibration Procedure. In the experimental work or in calibration runs known volumes of the stock solutions of the acids are fpetted into a 100-ml. calibrated volumetric flask, 50 ml. of t e 1.0 M ammonium chloride-ammonium hydroxide base solution are added, and the contents are diluted to the mark. In this way, the resulting solution has known concentrations of acids and of base solution. The variation in pH of the mixture from the original buffer solution is negligible. The electrolysis is performed as subsequently indicated.
.. ..
.. ..
, ,
s:23
1:oo
ANALYTICAL PROCEDURE
..
8.22
1.00 1.00
Measure out a sample containing between 15 and 35 mg. of trichloroacetic acid, and between 1.5 and 25 mg. of dichloroacetic acid; the amount of monochloroacetic acid should not exceed 50 mg. if dichloroacetic acid is present and 750 mg. if dichloroacetic acid is absent. Transfer the sample to a 100-ml. calibrated volumetric flask; add 50 ml. of the 1.0 '34 ammonium chloride-ammonium hydroxide base solution; carefully adjust to p H 8.2 by the dropwise addition of concentrated ammonium hydroxide; and dilute the contents to the mark. Rinse the cell and electrode several times with the solution to be analyzed. Electrol ze the solution, using a quiet mercury pool or a saturated calomeyas the reference anode electrode, over the otential range of -0.4 to - 1.9 volts us. the saturated calomePelectrode. If the electrocapillary curve for the base solution is not known, note t , the drop time, a t potentials of - 1.3 and - 1.8 volts us. S.C.E. Run a similar curve on the base solution and correct the sample curve for the latter curve. Using the intercept method, determine the diffusion current for each of the two waves. The diffusion current for the first wave is used to calculate the amount of trichloroacetic acid present; the diffusion current used to calculate the dichloroacetic acid present can be determined from the following relation:
2.00 1.00
..
b
b
1 o.Y9 e 8.20 I55 00 TCA. Trichloroacetic acid. M C A . AIonochloroacetic acid. DC.4. Dichloroacetic acid. A S . Acetic Acid. a Diffusion current for second w a r e corrected for effect of electrocauillarv curve. b Beeinninn of wave-due t o monochloroacetic acid merged with waves due t o poiychloroLcetic acids. c Diffusion current a n d concentration for second wave of trichloroacetic acid are not reported, because in a mixture they are obtained by calculation from d a t a for first wave.
Basis for Procedure. The procedure followed in the polarographic study of trichloroacetic and dichloroacetic acids (1) was used as the basis for the analytical method. The half-wave potentials a t 25" us. the saturated calomel electrode are -1.57 volts for dichloroacetate, and -0.84 and -1.57 volts for trichloroacetate; these are constant over the pH range of 6.8 to 10.4. Optimum waves are obtained in buffer solution of pH 8, which is 0.5 M in electrolyte. Dissolved oxygen need not be removed; the residual current obtained with the base solution is subtracted from the limiting current obtained for the sample
V O L U M E 23, NO. 2, F E B R U A R Y 1 9 5 1
343
Table 111. Analysis of hlixtures of Dichloroacetic and Trichloroacetic Acids Sample Taken TCA DCA
lid
First Wave TC.4
lid
mM
mM
,a.
p ~ .
0.098 0,098 0.098 0,098 0.492 0.492 0.492 0.492 0.986 0.986 0.986 0.988 1.97 1.97 1.97 1.97
0.100 0.500 1.00 2.00
0.76
0.100 0.500
4bO0
b
m.ll 0.093
Second Wave5
1.57 5.0 8.9 17.2 4 56 7.98 11.98 21.1 8.44 11.73 15.8 23.9 16.0 19.2C 23.3C 31.5c
0.94 lid
w. 0.71 (4.9) (9.0) (17.1) 3.76 3.76 3.71 (20.4) 7.62 7.60 7.59 7.47 15.2 15.1 15.1 15.1
Error
lid
DC.4
TCA
DC.4
TCA
pa.
mM 0.106
m.M
m.W
%
%
-0,005
SO.006
-5.0
+6.0
0.86
DCA
acetic acid and seventy-five times that of the trichloroacetic acid; for 0.1 millimolar concentrations of the polychlorinated acids, the ratios are 2.5 and ~ a .For 2 millimolar concentrations of the polychlorinated
---
acids the ratios are 12” and I a. The presence of more than a fivefold greater concentration 8.11 0.986 0.82 0.101 0.00 +0.001 0.0 +1.0 0.500 8.08 0.982 4.13 0.505 -0.004 +0.005 -0.4 +1.0 Of monochloroacetic acid 1.00 8.07 0.981 8.21 1.00 -0.005 +O.OO -0.5 0.0 affect the second wave of tri7.95 2.00 0.967 16.43 2.01 -0.019 +0.01 -1.9 +0.5 0.100 16.2 1.97 0.80 0.099 -0.00 -0.o01 0.0 -1.0 chloroacetic acid or the wave 0.500 16.1 1.96 4.10 0,501 -0.01 +0.001 -0.5 +0.2 -0.5 0 0 Of dichloroacetic 16.1 1.96 1.00 8.20 1.00 -0.01 0.00 2.00 16.1 1.96 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. 1.00 2.00 0.100
4.00
3.95
0.487 0.487 0.481
0.80 4.22 8.27
0.099 0.516 1.01
-0.005 -0,005 -0.011
-0.001 +0.016
+O.Ol
0 -1.0 -2.2 -1
-1.0 +3.2 +l.O
Q
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 (1942).
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
