I. M. KOLTHOFF AND M. K. CHANTOONI, JR.
856
Conductometric, Potentiometric, and Spectrophotometric Determination of Dissociation Constants of Substituted Benzoic Acids in Acetonitrile1
by I. M. Kolthoff and M. K. Chantooni, Jr. School of ChemistTy, University of Minnesota, Minneapolis, Minnesota
55455
(Received October 4, 1966)
The two methods d e v e l ~ p e dfor ~ , ~the determination of the dissociation constant, KdHA, of uncharged weak acids in acetonitrile (AN) and also the methods for the determination , - , reliable results only when certain conditions of the homoconjugation constant, K f ~ ~ give are fulfilled. I n order to check the reliability of the methods the over-all dissociation constants, KdZHA, of 3,5-dinitrobenzoic acid (HDNB), salicylic, p-nitrobenzoic, m-nitrobenzoic, p-hydroxybenzoic, and benzoic acids have been calculated from the characteristics of the conductometric titration curves with weak bases and the constants KdHA and K f H A 2 from potentiometric measurements with the glass electrode. K d 2 of ~ ~HDNB and salicylic acids have also been calculated from spectrophotometric data with p,p'-dimethylaminoazobenzene as indicator. Under the specified conditions there is a gratifying agreement among the results of the three methods which are reported. Theh omoconjugation constant of the salicylate ion was found to be unexpectedly large.
Introduction I n previous studies it has been shown2 that the dissociation constant of an acid, HA, in acetonitrile (AN)
K
d
=~[H+][A-]fl/[HA] ~
fHt
=
f ~ =- f
(1)
can be found from the location of and the conductance at the maximum in the conductometric titration curve and also from the analysis of the initial portion of the conductometric titration curve of the acid with a suitable amine, provided the formation constant, K f ~ ~ of the honioconjugate is very large as compared to the
K * H A ~=- [HA2-I/[HA][A-l
HA,-
=
f ~ -(2)
ionic dissociation constant K d of the ~ salt ~ and ~ the mobility of the various ionic species are known. In the present paper are presented conductometric titration curves of substituted benzoic acids with various amines, B, and the dissociation constants of the acids K d H were ~ calculated,2knowing the ionic mobilities. It was desirable to check the reliability of the values Of the constants thus found by independent methods' I n a previous paper,3 it has been described how the dissociation constant K d of ~an acid ~ and the homoconjugation constant, KfHA2-,can be found from the shape of the potentiometric titration curve of HA with tetraThe Journal of Physical Chemistry
ethylammonium hydroxide using the glass electrode as hydrogen electrode. In order to avoid presence of water and impurities in the hydroxide we have measured the pa^ of a wide range of mixtures of an acid with its tetraethylammonium salt. Also, the over-all dissocia( L H + ~ H A ~ - / [HAI2 = tion constant of two acids, K d 2 = ~ ~ K d ~ ~ K f was ~ ~ 2determined - , spectrophotometrically using p,p'-dimethylaminoazobenzene (DMAAB) as a Hammett indicator (DIIAAB = I). The formation 2 -constant ,
was determined spectrophotometrically in buffer solutions composed of mixtures of DMAAB, picric acid, and its tetrabutylammonium salt. Since the homoconjugation constant of picric acid is negligibly small3
(1) This work was supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, under Grant AF-AFOSR28-65. (2) I. M.Kolthoff and M. K . Chantooni, Jr., J . Am. Chem. soc., 87, 1004 (1965). (3) I. M. Kolthoff and M. K . Chantooni, Jr., ibid., 87, 4428 (1965).
DISSOCIATION CONSTAKTS
OF
BENZOIC .ACID
IN
ACETONITRILE
857
Benzoic Acid. This was a National Bureau of Standards product, dried at 110" at atmospheric pressure: mp 122", lit.6122". pKdHp, being equal to 11.0 f 0.1.3 Picric Acid. This was purified as described previThe indicator method must be applied with caution ously.3 in the determination of pKdZHAin mixtures of an acid Salts. Sodium Salicylate, Sodium p-Nitrobenzoate with a large homoconjugation constant and its tetraand Lithium Benzoate. These were prepared by alkylammonium salt. Salts of amines with carboxylic neutralizing a 30% ethanol solution of the acid with 1 acids (BHA) are slightly dissociated; therefore, it M aqueous sodium hydroxide or 0.1 M lithium carbois anticipated that the ionic dissociation constant of the nate solution. Both bases were Nerck reagent grade indicator salt IHA will be small. For this reason the products. Phenolphthalein served as external indispectrophotometric method with an indicator base cator. The alcoholic solutions were taken to dryness, can be applied only to mixtures which contain a large and the residues were recrystallized from ethyl acetate excess of carboxylic acid over tetraalkylammonium and dried in vacuo at 90". salt. In such mixtures the concentration of A- is Tetraethylammonium Salicylate, Benzoate, p-Nitrovery small, and practically all of the anions are present benzoate, p-Hydroxybenzoate, and m-Bromobenzoates. as HA2-. The electroneutrality relation in these These were prepared by neutralizing the acid with 1 mixtures in the presence of the indicator base is M aqueous tetraethylammoniuni hydroxide solutions after dissolving the acid in a mininiuni volume of ab[R4AT+] [H+] [IH+] = [HA,-] (4) solute alcohol and adding 100 ml of water. PhenolphThe two acid systems studied spectrophotometrically, thalein served as external indicator. The resulting 3,5-dinitrobenzoic and salicylic acids, are extremely solutions were taken to dryness, and the residues were weak in AX; hence, [H+]
