,June, 196%
1187
NOTES
A KEW METHOD FOR THE DETERMINATIOK OF SELP-ASSOCIATIOK COSSTANTS OF CARBOXYLIC ACIDS IN
S0I;UTK)N BY SHERRIL D. CHRISTIAN^
AKD
XI. W.
DHARMAWARDHANA
c.
Departmerit of Chemoetry, University of Ceylon, Colombo, Ceylon Recezued October 17,1961
Graddon2 has reported recently on the spectra and solubility of a series of complexes between cupric carboxylates and the corresponding carboxylic acids in benzene, chloroform, dioxane, and other organic solvents. Cryoscopic, spectral, and magnetic data3.4 suggest that the complexes exist in the form CU*A~(HA)~ in non-donor solvents, where A represents the carboxylate radical. We have madle use of these complexes in developing a new method for determining self-association constants of carboxylic acids in solution. If the formation of the cupric carboxylatecarboxylic acid complex is represented by the equation6 Cu2A4(solid)
+ (HA)2 (in soln.) = Cu2h4(HA)i”(insoln.)
the concetitratiori of acid dimer may be related to that of the dissolved copper through the formation constant KX = [Cul/[(H-k)zl
-2
j
v
~
10
0
L 0 0.01 0.02
0.03 0.04 0.05
0.06 0.07 0.08
[Cu]”2. Fig. l.-SolubiIity plot for the system acetic acid-ohloroform-cupric acetate at 28.4”.
the acid. Actually, since the solution is saturated with water, hydration of the acid monomer and possibly the acid dimer may occur, so that [HA] and [ (HA)2] represent the total concentrations of monomer and dimer in both the hydrated and unhydrated forms. It should be observed that due to hydration RSas it is used here need not be equal to the dimerization constant of the acid in anhydrous solvent. Equation 1may be written in bhe form
2 From measurements of the solubility of the com[A”]/[Cu]*/z = (ILKX)-’/2+ (3 + E ) [CU]’/~ plex as a function of total acid concentration, it is possible to infer both K, and equilibrium constants for association of the acid monomer to form ( H A ) n . which indicates that a plot of [AT]/[Cu]l/z us. should be linear, with slope 3 2 / K x and Further, once K , and the acid association con- [CU]‘/~ Equilibrium constants may stants are known, the solubility method may be intercept (K$Y,)-’’z, used to Eitudy equilibria between dissolved car- be calculated from the relations boxylic acid species and water or other hydrogenbonding compounds. We report here the results of a study of the soluIn Fig. 1, solubility data at 25.4’ for the cupric bility of cupric acetate in chloroform solutions of acetate-acetic acid-chloroform system are plotted acetic acid, in the presence of sufficient water to as iiidicated above. Values of the constants desaturate the organic phase. To avoid possible termined from the straight line are K , = 0.0060 changes in carboxylic acid concentration during and Kz = 6.9 l./niole. This result for K Z agrees equilibration, we determined both the dissolved well with equilibrium constants obtained by copper (spectrally, using the absorption maximum Noel~yn-Hughes~and Brown and Mathieson8 a t 6800 A.) and total acid (by titration with KaOH) from partition data for the system water-chloroafter equilibrium had been attained. Since titra- form-acetic acid. Values of Kz calculated from tion with strong base results in the precipitation of their data are 7.0 and 11 l./mole, respectively, a t copper as the hydroxide, the total concentration of 2 5 O . titrable acid in the organic phase may be expressed Cupric carboxylate complexes should prove a S6 extremely useful as “indicators” for the carboxylic [AT,= [HA41 2[(HA)z] 6[CuzL14(H8)91 acid dimer. The procedure applied here to the or chloroform-acetic acid system leads to results entirely equivalent to those of the partition [ A T ] = (‘““l)’jz + 2[Cu]/Kx 4- 3[Cu] (1) method. However, the present method may be KZKX where K , represents the dimerization constant of employed with solvents that are anhydrous or contain arbitrary concentrations of water, whereas (1) Fulbright Grantee on leave of absence from Department of with the partition method the organic solvent must Chemistry, The University of Oklahoma, Norman, Oklahoma. be saturated with water. (2) D. P. Graddon, J . Inorg. & Nuclear Chem., 11, 222 (1961): 11,337 (19.59); Natura, 186,715 (1960). Preliminary results for the systems acetic acid(3) R.L. Martin and H. Waterman, J . Chem. Soc., 2545 (19571. chloroform-cupric acetate and propionic acid(4) R. L.Martin and A .Whitley. dbid., 1394 (19581. benzene-cupric propionate indicate that K , is ( 5 ) The complex formation reaction is written in terms of (H.4)2
+
+
+
rather than H.4, since t,he dimer is frequently the major solube species in solutions of carboxylic acids in organic solvents. (n) Sp
