W. H. J. STORK,G. J. M. LIPPITS,AND M. MANDEL
1772
Association of Crystal Violet in Aqueous Solutions by W. H. J. Stork, G. J. M. Lippits, and M. Mandel* The Gorlaeus Laboratories, Department of Physical Chemistry 111, The University of Leiden, Leiden, The Netherlands (Received July 16, 1971) Publication costs borne completely by The Journal of Physical Chemistry
The association of crystal violet in aqueous solutions has been studied using spectroscopyin the visible region. For concentrations below M this association can be described by a dimerization equilibrium Characterized by an association constant K d = 6 x lo21. mol-' at 20". At concentrations above M , higher aggregates are formed. Degrees of association for crystal violet in aqueous solutions obtained from vapor pressure measurements were in qualitative agreement with the interpretation of spectroscopic data.
We wish to report some spectroscopic results on the association of crystal violet (CV, [4-[bis [p-(dimethylamino) phenyl] methylenel- 2,s- cyclohexadiene-lylidenlammonium chloride) in aqueous solutions, obtained during an investigation of the binding of this dye to poly(methacry1ic acid).l*z A few papers on the association of CV have already been published. I n some of them3-5 it was concluded from the concentration dependence of the CV spectrum that several aggregation equilibria occur simultaneously, though Krasnov and Shilova6interpret their spectroscopic data by a simple dimerization equilibrium. The aggregation has also been investigated by polarographic measurements; high degrees of association were reported.'P8 The results of the present paper were obtained with CV of commercial origin (U.C.B.), purified by repeated crystallization from u-ater. The purified and dried samples, dissolved in water at 20", had an extinction coefficient E 0.98 X 105 1. mol-l at the absorption M), maximum 16,900 cm-l (for a concentration of in agreement with literature v a l ~ e s . ~ ~ 6Absorption ~*~~0 spectra in the range between 14,000 and 23,000 cm-' were measured with a Zeiss-PMQ-I1 spectrophotometer; very dilute solutions only were investigated with a Unicam SP 700 recording spectrophotometer. Adsorption of the dye, especially to ground surfaces and luted interfaces, was found to be a source of experimental error for the dilute solutions. A reduction of the influence of this effect was achieved by pretreating all glassware with solutions of adequate Concentrations. CV solutions at 26 different concentrations CO ranging from 2.5 X to 2.5 X -44 were investigated. For each concentration at least two independent measurements mere performed. Some results are represented in Figures 1-3. At Co < 10-5 M the spectra are characterized by a peak at 16,900 em-1 and a shoulder at 18,000 cm-l. A decrease of the dye concentration results in an overall lowering of the intensity of the spectrum without change of its shape (Figure 1). The rise of the extinction coefficient in the uv region upon dilution, as reThe Journal of Physical Chemistry, VoE. 7 6 , No. 13, 197B
ported by Schubert, et aZ.,4 was carefully investigated but not confirmed. Buffered solutions (potassium acid phthalate, 0.005 PI)of CV within the same concentration range also exhibited an analogous, although somewhat smaller, lowering of the intensity in the visible region with decreasing CO. Therefore, we are rather inclined t o ascribe this effect primarily to adsorption phenomena as has been done for other dyes (e.g., ref 11) instead of to hydrolysis of the dye.6 As a systematic study of the adsorption at these low concentrations is very difficult, no final conclusion can be reached, although it seems unlikely that association processes, of the kind to be discussed at higher concentrations, are involved in this concentration range. For Co > M, a peak a t 13,500em-' rises at the expense of the peak a t 16,900cm-'. All spectra intersect in a small region near 18,000 cm-' for
