Effect of Water and Some Other Hydrogen Bond Donors and Acceptors on pdH in Acetonitrile of Mixtures of Acids and Their Salts I. M. Kolthoff and M. K. Chantooni, Jr. School of Chemistry, Unioersity of Minnesota, Minneapolis, Minn. An equation has been derived which allows the calculation of the effect of water on the pHa of mixtures of a weak acid and its tetraalkylammonium salt i n acetonitrile (AN) from the known dissociation constant of the acid, its homoconjugation constant, and the hydration constant(s) of the anion A - of the acid. The experimental values of paH, as determined with the glass electrode were found to be in excellent agreement with the calculated data up to a water concentration of 1M in mixtures of methanesulfonic, picric, 3,5-dinitrobenzoic, salicylic, and benzoic acids with their tetraalkylammonium salts. When hydration of the anion and acid are negligible, water, up to a concentration of lM, has no effect on the paH. This is the case, for example, with picric acid-picrate buffers. With the other acid-salt systems the effect of water increases with increasing ratio of concentration of salt (c,) to acid (c,,). For example, i n a benzoate system with ratio C~ t o c0 of 8.5 the paH decreased 3.0 units in the presence of 1.2M water. When the ratio was 0.034, the decrease i n paH at the same water concentrations was only 0.12. From the experimental data, it has been possible to calculate the hydration constants of the acids i n AN. A t the same molar concentrations, the effects of water, methanol, and Nbutanol on paH of a benzoate system are of the same order of magnitude, but i n a phenolate system water was found to have a considerably smaller effect than methanol and n-butanol, the latter two having about the same effect a t the same molar concentration. p-Bromophenol i s a much stronger hydrogen bond donor than water or alcohols and its effect on the paH of a benzoate mixture over a wide range of composition is much greater than the effects of water or alcohols (Figure 1). The titration curve of 3,5-dinitrobenzoic acid i n the presence of 0.5M p-bromophenol has the same shape as that of a weak acid i n water. The implications of the results on the effect of water and alcohols on potentiometric titration curves of uncharged acids i n AN have been discussed. Dimethylsulfoxide added to a benzoate system i n AN was found to markedly increase the paH where the ratio c n / c s i s large, but to hardly affect the paH when CJCis small. Hydrogen bonding of the acid to DMSO accounts for this effect.
INPREVIOUS PUBLICATIONS (1-6) the effect of homo- and heteroconjugation in acetonitrile (AN) of anions of a host of carboxylic and sulfonic acids and phenols o n the shape of their conductometric titration curves with weak bases and o n the shape of their potentiometric titration curves with tetraalkylammonium hydroxide in A N has been interpreted quantitatively. Normal titration curves, like those in water, were obtained only when no homoconjugation occurs because of (1) I. M. Kolthoff and M. K. Chantooni, Jr., J. Am. Chem. SOC. 85, 426 (1963). (2) Ibid., p. 2195. (3) Ibid., 87, 1004 (1965). (4) Ihid., p. 428. ( 5 ) Ibid., 88, 5430 (1966). (6) J . Phys. Chem., 70, 856 (1966). 1080
ANALYTICAL CHEMISTRY
55455
stability of the anions of the acid as a result of resonance plus intramolecular hydrogen bonding in the acid (picric acid ( 4 , 7), 2,6-dinitrophenol (3, 2,6-dihydroxybenzoic acid (8), or by a steric blocking of the ionic group (2,6-di-t-butyl phenol (9). The usual situation is that the anion A- homoconjugates with the acid HA, and in the presence of a hydrogen bond donor H R heteroconjugates with H R :
fA-)
(3)
Common hydrogen bond donors present during a titration of a n acid with tetraalkylammonium hydroxide are water and alcohols, which may greatly affect the paH of a mixture of a weak acid and its tetraalkylammonium salt. Consider, for example, the effect of water on the paH of a mixture containing a large excess of tetraalkylammonium benzoate over benzoic acid. In such a mixture in absence of water [HA] is much less than the analytical concentration ca because of homoconjugation (Equation 1). On the other hand the concentration of the anion [A-] is hardly affected by the homoconjugation, because it is present in such large excess. When water or a n alcohol is added to such a mixture [A-] will decrease: A-
+ xHzO
A-Zw
(~f. 2,3)
and consequently so does the paH of the mixture. As shown in this paper this effect of water or alcohol can be very large. When the acid in the mixture is in large excess over the salt, [A-] is considerably smaller than the concentration of the salt, cs, the salt being considered to be extensively or completely dissociated into its ions. When water is added to such a mixture, it must compete with the excess of acid for the anion, and the effect of water will be considerably less than in the mixture with excess of salt. As a matter of fact, the paH of a mixture with a large excess of acid may increase upon addition of water or alcohol, since these amphiprotic substances act not only as hydrogen bond donors, but also as weak bases, B, i.e. as hydrogen bond acceptors. They may react with a weak acid HA (7) J. F. Coetzee and G. Padmanabhan, J . Pliys. Chem., 69, 3193
(1965).
(8) J. F. Coetzee and G. Cunningham, J . Am. Chem. Soc., 87,
2534 (1965). (9) D. Bruss and G. Harlow, ANAL.CHEM., 30, 1836 (1958).
AH
+ O H R (B)
AH
,
. . . O H R (BHA)
and thus decrease the activity of HA. An example of this effect is observed in the addition of water to mixtures containing a large excess of salicylic acid over its tetraalkylammonium salt (see Table I). In a previous paper (10) the hydration constants (Equation 2 and 3, H R = HzO) of several anions have been determined. With the aid of equations derived below and the hydration constant(s) of the anion, it has been possible to calculate the effect of water on the paH of mixtures containing an excess of tetraalkylammonium salt over its parent acid. When the acid is not being hydrated, it is possible to calculate the effect of water on paH in any mixture of the acid and its salt. When (10) M. Chantooni, Jr., and I. M. Kolthoff, J. Am. Chern. SOC.,89, 1582 (1967).
the acid is being hydrated, the hydration constant of the acid (Equation 4, B = HzO) can be calculated from the effect of water on paH of mixtures containing an excess of acid over salt, knowing the homoconjugation constant and the hydration constant(s) of the anion. In the present paper the effect of water upon the paH of mixtures consisting of the following acids and their tetraethylammonium salts was calculated as described below and compared with the experimental values : methanesulfonic, picric, 3,5-dinitrobenzoic, salicylic, and benzoic acids, The effect of water and various other hydrogen bond donors (HR), methanol, n-butanol and the acceptor (B), dimethylsulfoxide, upon the paH of mixtures of benzoic acid and its tetraethylammonium salt and of mixtures of phenol and its tetrabutylammonium salt has been measured with the glass electrode, In all cases the additives HR and B are such weak acids and bases that acid-base interaction by proton exchange between H R and A- and between HA and B is negligible. The additives used likely are monomeric and solvated in AN.
Table I. Effect of Water on paH of Various Acid-Salt Mixtures CH~O M, [HtO] M paKobsd paHe31cil WzOI M PaHobsd PaHcalcd 3.0 X 10-2M HSal, 1.02 X 10-3M Et4NSal,KJAIV- = K ’ A ~ \ ~=- 4 2.2 x lO-3M CH3S03H, 3.5 X 10-3n Et4NCHaSOa K ~ A \ = ~ -3.6 (IO), K ~ ( H A = ) 0.6, ~ K’HA?- = 2 X lo3 (6), PK’EA = 16.7 (6), (IO), K ~ A ~ \= , . -8.0 (IO), K ~ ( H A = )1.4 ~ X lo‘, K’H.A- = 6.0 X lo3 16.9jQ,f 0.88 ( 4 ) , pkdrrA = 9.95a,f = 0.82 13.67 0 13.67 0 10.68 0 0 10.68 0.11 0.11 13.79 13.84 0.22 10.46 10.49 0.22 13.89 0.22 13.86 0.22 0.45 10.31 10.34 0.46 0.46 0.45 13.98 13.99 0.78 10.19 10.17 0.79 14.05 0.78 14.08 0.79 1.11 10.19 10.09 1.13 14.16 14.10 1.13 1.11 1.65 10.25 ( 10.00) 1.70 5.25 x lO-3M HSal, 1.92 x 10-ZMEt4NSal, p K d ~ . k= 16.8“, 4.75 x 10-2MHPi, 2.48 X 10-3MBu4NPi, K f ~ w (IO), =K~(HA)~ f = 0.71 = 0.5, K ~ H A *=- 2 (#), P K ~ H = A 11.0 (4, 10.93a,f = 0.83 18.56 18.56 0 0 0 9.57 9.57 0 0.22 0.22 18.12 18.07 9.59 9.57 0.06 0.06 17.82 0.34 17.92 0.34 0.114 9.57 9.57 0.114 17.60 17.48 0.59 0.57 0.336 9.56 9.57 0.336 0.80 0.78 17.31 17.23 1.04 1.02 17.10 17.02 9.56 x IO-3M HPi, 2.94 X 10-3M Bu4NPi, ~ K ’ H A= 10.99, 1.40 1.37 16.86 (16.77) f = 0.81 16.64 (16.65) 1.67 1.65 0 0 10.35 10.35 6.50 X 10-*M HBz, 2.18 X lOw3MEtcNBz, K ~ A , =~ -2.3, Kfaziv0.5 0.5 10.36 10.35 = 15.2, K ~ = 10.2, A K~ f o ~\v~ = ~0, KfH.+ ~ = 4.0 X lo3 (6), 1.2 1.2 10.28 (10.35) p K d ~ , + 20.7 (6) 2 0 . 6 ~f, = 0.85. 2.0 2.0 10.21 (10.35) 16.68 16.68 0 0 0.34 0.32 16.62 16.68 5.41 X 10-iAfHDNB, 3.47 X 10-aMEtaNDNB, K f ~ m -= 6.0 (IO), 0.63 0.60 16.65 16.66 K ~ A ? ,=~ -5.0 (IO), K ~ ( H A = )1.8, ~ K ~ H A=~ 1.0 - X lo4(6),P K ~ H A 16.63 1.02 1.02 16.58 = 16.9 (6), 16.94a,f = 0.82 1.54 1.50 16.53 (16.56) 0 0 13.00 13.00 3.85 X 10-3M HBz, 3.85 X 10-3M EteNBz, P K ~ H A= 20.80‘ 0.34 0.31 13.34 13.29 f = 0.80 0.55 0.51 13.49 13.47 0 0 20.71 20.71 0.77 0.70 13.61 13.59 0.34 0.33 19.97 20.10 1.11 1.06 13.74 13.77 0.59 0.57 19.61 19.75 1.43 1.43 13.84 (13.92) 0.79 0.78 19.32 19.48 1.18 1.15 19.02 19.12 5.95 X 10-3MHDNB, 2.14 X 10-’M EtaNDNB, P K ~ H = A 17.17a, 1.45 1.42 18.76 (18.90) f = 0.70 3.00 X 10-3M HBz, 3.05 x .10-2M Et,NBz, p K J a ~= 20.9a, 0 0 19.62 19.62 f = 0.66 0.11 0.11 19.23 19.20 0.22 0.20 18.88 18.88 0 0 23.58 23.58 0.34 0.33 18.50 18.55 0.13 0.11 23.26 23.26 0.57 0.55 17.98 18.14 0.34 0.34 22.36 22.42 0.80 0.76 17.60 17.85 0.63 0.57 21.72 21.71 0.81 0.78 21.20 21.21 PK~HAcalculated from paH in absence of water using the 1.18 1.15 20.64 20.56 value of K’HA~-entered in the table (see text). 1.45 1.42 20.19 (20.27) CH~O, M
Q
VOL. 39, NO. 10, AUGUST 1967
1081
n
When conjugation occurs, it is seen from Equation 8 that the shape of the neutralization curve in the presence of H R and B is characterized by the factor (c, c,) K f ~ ~ ? - , ' V W instead of (c, c,) KIH*,- when H R and B are absent ( 4 ) while the paH a t the midpoint is smaller by log (Vi W) unit in the presence of HR and B than in their absence. Also, according t o Equation 8 the addition of H R and B causes a decrease of paH a t the extreme ends of the neutralization curve bv log
0 and c,
-
/J
-71
I
I
.4 .8
0
-
0, respectively.
-
EXPERIMENTAL
-51
-6
+
+
ljaH-------
I
1,2
0
I
.4 ,8
1.2
I
Molarity of Additive Figure 1. Effect of additives on paH of benzoic acid-tetraethyl-ammonium benzoate mixtures
-.- -.-
Left hand figure, 3.6 X 10+M HBz-3.05 X 10-2M EtnNBz, paH at A paH = 0, 23.40. Right hand figure 6.5 X 10-2M HBz 42.18 X 10+M Et4NBz,paH at A paH = 0, 16.80. -x- dimethyln-butanol, water, -0- methanol sulfoxide as additive, and -A- p-bromophenol
Calculation of paH of Acid-Salt Mixtures Containing HR and B. I n a previous paper (4, an equation was derived which allows the calculation of the paH of mixtures of H A and its tetraalkylammonium salt, RINA, taking homoconjugation into account. It was assumed that the normal and homoconjugate salts R4NA and R4NHA2, are completely dissociated, exhibit n o acid-base dissociation and that CZH+
