To demonstrate the applicability of cyclic derivative voltammetry to the measurement of rate constants for rapid coupled chemical reactions, the: oxidation of ascorbic acid followed by a rapid hydration reaction was studied. This system has been studied at a variety of pH’s (13). The conditions chosen for study here were a pH of 7.2 in phosphate buffer at ambient room temperature. The rate constant for the succeeding chemical reaction was obtained from the measured ratio of the derivative anodic and cathodic peaks obtained at a scan (13) S. P. Perone and W. J. Kretlow, Anal. (1966).
Chem., 38, 1760
rate of 413 volts/second. The value measured for this rate constant was 1.3 X 103 second-1. This is in good agree ment with values obtained previously with conventional readout (13).
Received for review April 29, 1966. Accepted December 28, 1966. Work presented in part at the 152nd National Meeting, ACS, New York, September 1966. Work supported in part by the National Science Foundation, Grant No. GP6131. Partial support was also received from Public Health Service Grant No. CA-07773 from the National Cancer Institute.
Acid-Base Indicator Constants in Acetonitrile I. M. Kolthoflf, . School
K, Chantooni, Jr., and Sadhana Bhowmik
of Chemistry, University of Minnesota, Minneapolis, Minn.
pK^i in acetonitrile (AN) of sulfonated para-substituted phenylazonaphthol, neutral red (one pKYn of 6.0 and the other of 15.6 corresponding to color changes from blue to red and red to yellow, respectively) tropeolin 00, methyl orange, methyl red, azo violet, p-naphtholbenzein (one pKdm of 6.8 and the other of 23.4) and phenolphthalein have been determined spectrophotometrically at 25° C. Table III lists spectrophotometric characteristics and pKdHi of all the indicators studied so far in AN, the group covering a pKm range from 2 to 30. Limitations for paH determination of indicators of various charge types are discussed. The dissociation constants of diethylbarbituric acid and phenol have been determined, pKdHA values are 23.4 and 27.2 and K/Ha2- values of 4 X 101 and 1.1 X LO4, respectively, at 25° C. The indicator constants
In the past 20 years probably more than a thousand papers on acid-base titrations in nonaqueous solvents have been published. The best and most complete source of information on the progress in this field is the Biennial Reviews of Chemistry. Most of the work is being done Analytical potentiometrically, usually with the glass electrode as pH electrode. Other electrodes like antimony, platinum, and others have been found useful in many instances. In considerations of the acic strength it always has been tacitly assumed that the change of the glass electrode potential with paH corresponds to the theoretical value. Interesting semiquantitative information has been published on the relation between the half-neutralization potential (HNP) in several aprotic solvents and pKdHA in water. However, quantitative information on acid-base equilibria and determination of paH in aprotic solvents in still lacking. In many publications, indicators have been recommended for acid-base titrations. When the constant(s) determining the equilibrium of the system titrated and the pKdHi values are known, the most suitable indicators can be selected for acid-base titrations. Also paH can be determined spectrophotometrically when pKdni is known. In the last several years studies have been carried out in this laboratory on acid-base equilibria and dissociation constants of acids in acetonitrile (AN). It has been found (/) that the (1) I. M. Kolthoff and . 87, 4428 (1965).
K. Chantooni, Jr., J. Am. Chem. Soc.,
55455
glass electrode in this solvent responds reversibly to paH. Dissociation constants of a large number of acids, including several indicators have been obtained from conductometric,
potentiometric, and spectrophotometric measurements. In the present paper the constants of several indicators, not yet studied previously have been determined. In mixtures of weak acids and their tetraalkylammonium salts, the paH of which were measured with the glass electrode, the ratio of
JÍL [HI]’
I denoting the alkaline form and HI the acid form of the
indicator, was
was
determined spectrophotometrically.
then made of log
coefficient. The ionic strength
Tllf* -
vs.
-
LHIJ/hi was
paH, /denoting the activity
always kept small and the limiting
Debye-Hiickel expression: to calculate fi/fm-
A plot
-
log
/
=
1.51Z2
In the above plot pKdHi
=
s/µ —
was used
log
———
am
is equal to paH at the point where the log expression is zero. For the measurements at higher paH (23 to 30), mixtures of
diethylbarbituric acid and of phenol with their tetraalkylammonium salts were used. From the paH of these mixtures the dissociation constants of these acids and their homoconjugation constants KzHa2[HA2-]/[HA] [A-] were calcu=
lated (1).
EXPERIMENTAL
Acetonitrile was purified and dispensed as described previously (2). The various indicators, methyl red, methyl orange, tropeolin 00, azo violet, sulfonated para-substituted phenylazonaphthol, neutral red, p-naphthol-benzein, and phenolphthalein were obtained from different sources and were used without further purification. Stock solutions (0.1 % to 0.01 %) of the indicators were prepared in AN. The preparation and the purification of different acids
(2) I. M. Kolthoflf, S. Bruckenstein, and . J· Am. Chem. Soc., 83, 3927 (1961).
K. Chantooni, Jr.,
VOL 39, NO. 3, MARCH 1967
·
315
Table I. Ca,
in Mixtures of Diethyibarbituric Acid and Its Tetraalkylammonium Salt X paH M X 10s C„ 23.77 23.58 23.43
2.43 2.41 2.40 2.38 2.35 2.30 5.07 4.97 4.88 4.78 4.65 4.52 17.9
1.12 1.78 2.49 3.35 4.82 6.85
0.7 2.88 5.07 7.17 1.02 1.30 17.9
23.31 23.11 22.89 24.26 23.63 23.40 23.23 23.02 22.84 23.35
Figure 1. Absorption spectra of 1.3 X 10 5M neutral red in solutions of various paH
paH in Mixtures of Phenol and Its Tetrabutylammonium Salt
Table II.
X
C„
M X 103 2.88 4.32 5.76 7.16 9.79
Ca,
paH 28.96 28.0 27.0 26.53 25.89 25.19 24.16
103
6.2 6.1
6.0 6.0 5.9
5.7 5.2
14.8 24.8
and their tetraalkylammonium salts used for the buffer mixtures are described elsewhere (1, 3, 4). The techniques used in the determination of paH with the the spectrophotometric measureglass electrode (7) and in ments were the same as described previously (4). All ex25.0° C. periments were carried out at RESULTS
Dissociation Constants KVa of Diethyibarbituric Acid and Phenol. From paH measured with the glass electrode in mixtures of the above acids with their tetraalkylammonium salts the pKVx and also the homoconjugation constants ,
KVa,-
=
are given
[HA,-] [HA] [A
were calculated (7).
The paH data
J
in Tables I and II.
The following values were cal4 X 101 for diethyi1 X 104, respectively, for
23.4 and KVaculated, pKVa barbituric acid and 27.2 and 1.5 ± =
=
phenol. and pK'Vi of the IndiSpectrophotometric Characteristics cators. Azo Indicators. Methyl orange and tropeolin buffers with 00 have two red forms, one observed in picrate acid in and one stronger max at 520 and 543 mg, respectively, buffers or acid in 2,5-dichlorobenzenesulfonic media—e.g., On dilute perchloric acid—at 520 and 528 mg, respectively. the other hand, dimethylaminoazobenzene and methyl red, which structurally are very similar to methyl orange and media with in tropeolin 00, have the same red form the above 518 and mg, respectively. The absorption maxima at 510 acid forms of these of the molar absorptivities values of the in 0.05A7 methanedetermined were III) indicators azo (Table (3) I. M. Kolthoff and .
K. Chantooni, Jr., J. Phys. Chem., 70,
856 (1966).
and . (4) I. M. Kolthoff, S. Bhowmik, Nat. Acad. Sci., 56, 1370 (1966).
316
·
ANALYTICAL CHEMISTRY
K. Chantooni, Jr., Proc.
1 is in 5.0 X 10 -2Af perchloric acid; 2,3,4, and 5 are in 2,5-dichlorobenzenesulfonic acid buffers with paH 5.2, 5.7, 6.1, and 6.5, respectively; 6 is in 1.0 X 10-2M salicylic acid; 7, 8, 9 are in salicylic 15.2, 15.7,16.5; 10 is in diethylbarbiturate acid buffers with
buffer
sulfonic acid solutions in AN and those of the alkaline forms in benzoate buffers. In all instances Beer’s law was found to hold. The dissociation constants KVi+ of these azo indicators, except of tropeolin 00, were determined in picrate buffers at a constant picrate concentration of 1.3 X 10~3A7. pKVi+ of tropeolin 00 was determined in methanesulfonate buffers at a constant salt concentration of 3.4 X 10~SM. The dissociation constant of sulfonated para-substituted phenylazonaphthol in water has been determined by Reeves (5). In AN the red form of this indicator was found to have an absorption maximum at 552 mg in Q.2M perchloric acid. The molar absorptivity was determined in the same medium. The absorption maximum and molar absorptivity of the alkaline form of this indicator were determined in methanesulfonate buffer. This azo compound is a poor indicator, its color change being from red to orange red. Its pKVi+ has been determined in perchloric acid solutions in a concentration range between 0.01 and 0.001A7. Neutral red exhibits two color change intervals, one from 14.5 to 16.5) and the other from blue red (HI+) to yellow ( 5 to 7). In aqueous medium this indicator (H2I+2) to red (paH also has two color changes, one from red to yellow (paH 7 to 8.5) and one, not reported in the literature, from blue to red in strongly acid medium (5 to \M HC1). The absorption maximum of the red form in methanesulfonate buffer in AN was found at 532 mg, that of the yellow form in diethylbarbiturate buffer at 441 mg, that of the blue form in 2,5dichlorobenzenesulfonic solution, and in dilute perchloric acid at 608 mg. Beer’s law was found to hold for these various forms of neutral red. Figure 1 shows the absorption spectra of the three colored forms. Two isobestic points are observed, one at 465 mg and one at 563 mg, indicating simple acid-base equilibria between the various colored forms. The constant, pKVi+, was determined in salicylate buffers at a constant salicylate concentration of 4 X 10" 4M and salicylic acid concentrations varying between 10-3 and 10“2A7. In these media practically all salicylate ions (A-) are present in the form of the homoconjugate ions (HA2_) and formation of the ion pair IH+A~ is negligible. The constant, pKVi+2 (5) R. L. Reeves, J. Am. Chem. Soc., 88, 2240 (1966).
determined in 2,5-dichlorobenzenesulfonate buffers at a constant sulfonate concentration of 9 X 10™4M and acid concentrations varying between 6 X 10"4 and 6 X 10-3M. Undissociated indicator salt formation was negligible. Azo violet exhibits two color change intervals in AN. It is yellow at paH smaller than 18 and orange red at paH between 21 to 27. In 0.1 M tetrabutylammonium phenolate the indicator is present completely in the blue alkaline form which The orange-red has a sharp absorption maximum at 613 mu. form in a mixture of 2.0 X 10~3M phenol and 10~3M phenolate has two absorption maxima at 446 and 536 µ, rewas
Constants (pK*HI) and Spectrophotometric Characteristics of Indicators in AN
Table III.
Charge on
Indicator o-Nitrodiphenylamine (2) Sulfonated p-substituted phenylazonaphthol 3. m-Cresol purple (4) 4. Thymol blue (4) 5. o-Nitro-p-chloroaniline: 1. 2.
HI
o-Nitroaniline (2) Dibromothymolbenzeii
(4) 8. 9. 10. 11. 12.
Neutral red p-Naphtholbenzein Tropeolin 00 Thymolbenzein (4) Dimethylaminoazo-
benzene (3) 13. Methyl red 14. Methyl orange 15. Picric acid (1) 16. Bromcresol green (4) 17. Bromthymol blue (4) 18. Bromphenol blue (4) 19. Thymol blue (4) 20. m-Cresol purple (4) 21. Phenol red (4) 22. Neutral red 23. 2,4-Dinitrophenol (6)
Color change of I HI
552
430 506
2.2
3.54 3.19
533 553
0.74*
colorless red
yellow yellow
blue green red red red
red yellow yellow yellow yellow
2.31
yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow blue blue yellow
2.89 (560 µ)“ 2.44 0.56
0
red red colorless colorless colorless colorless colorless colorless colorless red colorless colorless
1
2+ + 1 + 1 + 1 + 1
1 1
+ +
0 0 0 0 0 0
0 1
+
0
0
colorless
nitrophenol (6) 28. 3,5-Dinitrophenol (6)
0
colorless
Dibromothymol-
0 0
benzein (4)
32. Bromthymol blue (4) 33. p-Naphtholbenzein 34. Phenol red (4) 35. Thymolbenzein (4) 36. m-Cresol purple (4) 37. Thymol blue (4) 38. Phenolphthalein 39. Azo violet
2.24 (585)“
+ +
1
yellow yellow
p-Nitrophenol (6) o-Nitrophenol (6)
(PK*HI) AN
0
10
10
11-
orangered colorless colorless
µ I
red red colorless
1
11-
30. 31.
I
HI
+ + 1 + 1
yellow
,
10 -4
colorless red
1
24. 2,6-Dinitrophenol (6) 25. Bromphenol blue (4) 26. Bromcresol green (4) 27. 2,6-Di-/erz-butyl-4-
29.
X
HI
e
+ +
1
(2) 6. 7.
spectively (curve 2 in Figure 2). Mixtures of the blue and the orange-red forms exhibit a well defined isosbestic point at 556 rnp as is illustrated in Figure 2. The yellow form in the salicylate buffer at about pnH 15 has an absorption maximum at 403 µ (curve 1, Figure 2). A rough estimate was made of the pKHI which was of the order of 19.7 in benzoate buffer mixtures. Apparently there is another yellow form of azo violet in stronger acid medium as is evident from the comparison in Figure 2 of curve 1 with curve 7, the latter in 0.01 M perchloric acid. For indicator purposes the color change from orange red to blue is much
1.2
7.58 4.39 5.2
2.8
1.82 2.01
-1.03
4.9
410 408
4.85 5.9
-0.29 ~1.06
5.1 6.9
608 625 528 536 510
532 425
6.0 6.8 8.0 9.9
~0.56 ~0.7 ~1.86 ~1.96
10.05
3.3
6.5 7.5 6.2 8.0 6.7
518 520 334
490 417
4.9 3.5 0.35
5.3 7.1 10.7
7.5 4.0 3.5 4.0 4.7 7.2
8.1 12.0 12.8 13.5 13.8 11.8
532
375 408 400 408 395 387 390 441
463 596 623 452
10.2 10.6 11.0 11.0 11.7 12.0 13.4 13.6 13.7 15.6 16.0 16.45 17.5 18.5 19.0
3.33 (526
µ)“
1.78
463
20.5
6.7
13.8
641
20.5
~6.7
13.8
415
20.7 22.0
7.1
7.2
13.9 14.8
1.2 2.5 2.01
15.1 8.05
1.0
3.7
408 408 318
0.33
0.26
336
yellow
redorange blue
colorless pale
yellow yellow
1.1
blue blue red blue blue blue red blue
7.22 1.20 3.54
yellow yellow yellow yellow yellow yellow yellow colorless orangered
403 410
7.22 2.5 1.65 1.04 3.54
6.22
1.04 1.65
3.0
306
2.08 4.63 10.9 6.45 1.45 4.80 11.1 7.3
400 425 390
(630
and
µ)“ “
3.88
541
3.0
4.95 4.6
410
0.55*
4.83
-2.94 -2.42*
2.6 3.4
0.54*
yellow
2.01
*
(pK*Hl)w
630 665 576 620 598 615 569 613
403 387 395
446
22.3 23.4 25.0 25.9 26.5 27.2
~29.2 30.5
15.2 14.3 17.1 16.9 18.2 18.3 19.7 18.5
7.1 9.1 7.9 -~9.06 8.3 8.9 ~9.5* —126
536
Molar absorptivity at wavelengths in parentheses. signifies estimated value. Phenolphthalein in water does not behave like
6 ~ *
a simple monoor diprotic acid (See discussion). ref. (5). Erroneous values have been reported in ref. (2), but correct values were used in the calculations.
* See *
VOL. 39, NO. 3, MARCH 1967
·
317
-
0.8 r
34,0
M?, [Hljfu
Figure 2. Absorption spectra of 5.4 X 10~6M azo violet in solutions of various paH 1 is in salicylic acid buffer (paH 15); 2, 3,4,5 and 6 are in phenol and tetrabutylammonium phenolate mixtures with paH 26.8, 30.1, 30.6, 30.8, and about 32, respectively; 7 is 1.0 X 10_2M perchloric =
acid
Figure 3.
Plot of paH
vs.
log
-
—
[HIJ/hi
Right hand ordinate refers to curves 8,9, and 10. 1, sulfonated -substítuted phenylazonaphthol, slope 1.03; 2, neutral red (red to blue) slope 1.2; 3, p-naphtholbenzein (yellow to green), slope 1.2; 4, tropeolin 00, slope 1.1; 5, methyl red, slope 1.08; 6, methyl orange, slope 1.0; 7, neutral red (yellow to red), slope 1.3; 8, p-naphtholbenzein (yellow to blue) slope 1.0; 9, phenolphthalein, slope 1.4; 10, azo violet (orange red to blue), slope 1.1 =
=
=
=
=
=
=
=
more pronounced than the change from orange-red to yellow and therefore the former transformation region has been studied in more detail. p-Naphtholbenzein like the other benzeins (4) has two transformation ranges in AN, one in acid solution from yellow to green (paH from 5.8 to 8.2) and the other from yellow to blue at high pH (paH from 22.4 to 24.4). The green form in 0.01 M perchloric acid has two spectral maxima at 625 mp and 460 µ (less sharp). The yellow form in salicylic acid buffer has an absorption maximum at 425 µ while the blue form in 1.0 X 10-1M tetrabutylammonium phenolate has a maximum at 665 µ. The pK6Hi+ in the acidic region was determined in methanesulfonic acid buffers, in which the tetraethylammonium salt concentration was kept 3.4 X 10""3 M and the acid concentration varied from 8.5 X 10-3M to 6.0 X 10“ 2M. The alkaline region was studied in diethylbarbituric acid buffer mixtures with constant salt concentration of 5.0 X 10"" 3M, and the acid concentration varying from 7.0 X 10-4M to 7.0 X 10_3M. The indicator constants in water of p-naphtholbenzein have not been reported in the literature, and we have determined them in the present study, using the spectrophotometric method. The indicator is virtually insoluble in water and for solubility purposes the determinations have been made in the presence of 0.5% methanol. (pKdHi)w values of —0.7 (green to yellow) and of 9.1 (yellow to blue) have been found. Phenolphthalein exhibits a color change at high pH from colorless to pink. The (apparent) indicator constant was determined in phenol-phenolate mixtures. Like in water the indicator does not behave as a simple monoprotic acid.
Figure
318
·
3
gives a
plot of log
ANALYTICAL CHEMISTRY
vs.
LHIJ/hi
paH of the various
=
=
indicators. The slopes are given in the legend of Figure 3. The pKdHi values and other indicator characteristics obtained in the present study as well as those obtained previously with Hammett indicators (2), sulfonephthaleins and benzeins (4), and nitropbenols (6) are summarized in Table III. The indicators are arranged in order of increasing . The values of molar absorptivities (c) refer to Xmax except in a few instances where the absorption of the other indicator form interfered. The values of given in parentheses in the column c X 104 refer to the wavelength at which the measurements were
done.
DISCUSSION The first 14 indicators in Table III are substances, the uncharged form of which is a base, and the acid form a cation HI+, except neutral red (blue to red) which is a divalent cation HI+2 in the acid form. The color change of neutral red from red to yellow corresponds to the protonation reIH+. For indicators, the basic form of action I + H+ which is uncharged, ApKdHi+ (pKdHi -)an ), ( (last column in Table III) is not constant. For the Hammett indicators it is 5 (2) for neutral red and (HI+ I) and thymolbenzein 8. For the other indicator bases studied it varies between about 6.5 and 8, except for methyl red where is 5.3. =
-
—
(6) I. M. Kolthoff, .
K. Chantooni, Jr., and Sadhana Bhowmik,
J. Am. Chem. Soc., 88, 5430 (1966).
study of the KáBH of a large number of base cations for aliphatic Coetzee and Padmanabhan (7) found a and piperidine varying between amines, pyridine, pyrrolidine, 7.2 and 8.3, the average value being 7.6. For the aromatic amines aniline and p-tcluidine pKdBn- was 6. The values reported in Table III for the indicators are in line with Coetzee's results. From a practical point of view it is of interest to mention that dibromofiymolbenzein, p-naphtholbenzein in their acidic region, and neutral red (red-blue) are outstanding indicators for the titration of carboxylates and many amines in AN with a standard solution of perchloric acid in glacial acetic acid. This will be the subject of a subsequent publication. Qualitatively the sulfonephthaleins differ in their behavior in AN from that in water. Benzeins and sulfonephthaleins in water exhibit two color change intervals, one from yellow to the strongly colored basic form and one from yellow to the strongly colored acid form. Benzeins in AN behave in this respect like in water; however, the sulfonephthaleins have three color change intervals in AN, one from yellow to the alkaline form, one from yellow i:o colorless (pK 11 to 13.6) and one from colorless to the strongly colored acid form. This explains that the sensitivity of (the yellow form) thymolbenzein for hydrogen ions to give the red form becomes about 10s greater in AN than in water. On the other hand, the formation of the red form from the colorless form of thymol blue in AN occurs at about the same pK as in water (from the yellow form). It has been postulated (4) that the colorless form is a sultone and that the three color change intervals are characterized by the following type of equilibria:
In
1
a
-
alkaline form (blue)
IV sultone
V red
The reaction of form III with hydrogen ions to give the red form V corresponds to the color change of benzeins to that of the strongly acid form. At the pH where the yellow form II of the sulfonephthalein has been transformed into the colorless sultone form IV, the concentration of III is negligibly small and its yellow color cannot be detected at the usual indicator concentration used. The sultone form (IV) seems to be quite stable in AN and is not readily hydrated by water. It was found that K^m (colorless to yellow) was not affected by making the water concentration as high as 0.3M. Thymolbenzein and dibromothymolbenzein and p-naphtholbenzein in their color change from yellow to the strongly colored alkaline form behave like monoprotic phenols. Their ’$ of 17.3 and 13.8 and 14.3, respectively, are of the expected order of magnitude. For example for phenol (pKdw 10.0) =
(7) J. F. Coetzee and G. R. Padmanabhan, J. Am. Chem. Soc., 87, 5007 (1965).
is 17.3 and equal to that of thymolbenzein. For p-nitrophenol 7.1) and 3,5-dinitrophenol (pKáw (pKá„ 6.7) values are equal to 13.85, the same as that of dibromothymolbenzein. It is surprising that the values of the diprotic sulfonephthaleins (yellow to alkaline forms) are not much values of monoprotic phenols which greater than the have similar pKáHA values as the sulfonephthaleins in water. =
=
For example, for bromophenol blue (pKdw 4.0) 12.8. 13.5, while for 2, 6 dinitrophenol (pKáw 3.6) For m-cresolpurple and thymol blue 18.2 while for 17.3. Although no data for the second disphenol sociation constant, pKd2, of diprotic acids in AN are found in the literature, it is evident from potentiometric titration curves that 2 for diprotic acids is of several orders of magnitude for monoprotic acids with (pKdi)w about greater than equal to (pKd2)w. The different behavior of the sulfonephthaleins is probably accounted for by the large distance of separation of the two changes in the divalent anion and the great stability of their anions as a result of resonance. Phenolphthalein in water is present mainly as the lactone. Its hydrated form, which is diprotic, is also colorless as is the univalent carboxylate anion. On the other hand, the divalent quinoid phenolate ion is red. The apparent dissociation constant of the carboxyl group is of the same order of magnitude as Ká of the phenol group. Hence in water, the simple =
=
-
=
=
=
=
[HI-]
[colorless]
—:——— K does not hold. It [I-2] [red] seems evident that the lactone form of phenolphthalein will be much more stable in AN than in water, which may account for the fact that the apparent indicator constant of phenolphthalein in AN is large (29.2) and that ^ is about 3 units greater than that of phenol. Indicators will be used rarely for the spectrophotometric determination of poH in AN, because the measurements can be done easily with a glass electrode. Indicators, the acid form of which is of the charge type HI+ or HI+2 must be used with discretion in the determination of paH. Where the unknown system contains a fair or large excess of salt MA over acid there will be formed an ion pair IH+A-, as the amine salts of acids with a large homoconjugation constant are slightly dissociated. The ion pair IH+A- in AN has the same color and spectrum asi H+. Without the correction for [IH+] be taken too large. On the [IH+A-] the ratio —would
relation [H+]
=
r-— K
=
other hand when the system investigated contains a fair or large excess of acid, and KfnA-2 is large, the error in general will be negligible, as K^h+ha-z in AN is large. For example, in the determination of pK6m of neutral red (15.6; red to yellow) salicylate buffers were used with a large excess of acid over salicylate (pKáHsai 16.7) and a normal behavior of the indicator was observed. In acid-salt systems, like picric acid-picrate, in which the anions of the acid are quite stable and have no or little tendency to homoconjugate the error by formation of IH+A- is negligible or very small when the concentration of the buffer salt is small. For example, the pKáHi+ values of dimethylaminoazobenzene, methyl orange, and methyl red have been determined in picrate buffers at a picrate concentration of 1.2 X 10-3M and no indication of HI+A- formation has been found. Another source of error with the above charge type of indicators may be formation of a heteroconjugate IH+—B, where the solution contains an uncharged base, B. However, in general, the error should be very small as the formation constant of IH+- -B is small (7). Indicators, the acid form of which is of the charge type HI =
-
VOL. 39, NO. 3, MARCH 1967
·
319
HI-, can give erroneous values of paH, when I- or I-2 has great tendency to heteroconjugate with the acid constituent HA in the solution, the heteroconjugate ion I-—HA having an absorption spectrum different from that of I- showing a maximum absorption at a smaller wavelength than I-. In an additional error is about heteroaddition, brought by conjugation of HI with the anion A- of the acid HA in the system. For example, the molar absorptivity of the p-nitrothe absorption maximum phenolate ion is 3.0 X 104 occurs at 415 mp. In an equimolar (6 X 10-3M) mixture of benzoic acid and its tetraethylammonium salt the values were 1.2 X 104 and 340 µ, respectively (6). Of the nitrophenols or a
567891011and
listed in Table III, only 2, 6-dinitrophenol, 2, 6-di-fert-butyl4-nitrophenol, and picric acid and their anions have negligibly small conjugation constants and are suitable for spectrophotometric paH determination. For titration purposes the limitations of paH determination are of no serious consequence.
Received for review November 7, 1966. Accepted December 30, 1966. This work was supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, under Grant AF-AFOSR-28-65, and by the National Science Foundation.
Voltammetric and Spectrophotometric Study of the Zirconium-Alizarin Red S Complex . E. Zittel and T. M. Florence1 Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. At a rotated pyrolytic graphite electrode alizarin red S (sodium alizarin-3-sulfonate) produces well-formed, The zir2-electron, oxidation and reduction waves. conium-alizarin red S complex is oxidized at a potential 0.35 volt more positive than the free dye, and a mixture of alizarin red S and its zirconium complex In the range exhibits two discrete oxidation waves. of acidities and zirconium concentrations studied, a stable, stoichiometric, 1:1 complex is formed with alizarin red S. Both the voltammetric and spectrophotometric data indicate that zirconium chelates with alizarin red S through the two phenolic oxygens, rather than through one phenolic oxygen and the quinone group. Spectrophotometric and kinetic studies showed that the reaction proceeds via a hydrolyzed species, probably Zr(OH)2+2. The equilibrium constant for the reaction, Zr+4
+
2
H20
+
H2A
