Fan J, Shen X, Wang J
Department of Chemistry, Henan Normal University, Xinxiang, Henan, People's Republic of China.
Talanta. 1999 Jul 12;49(4):843-50. doi: 10.1016/s0039-9140(99)00092-2.
The concentration dissociation constants (pK(a)) of methyl yellow, MY (H(+)In) in mixed aqueous solvents of methanol, ethanol, iso-propanol, tert-butanol have been accurately determined from spectrophotometric measurements at 25 degrees C and a constant ionic strength of 0.1 mol l(-1). It has been shown that in these solvents, the pK(a) values decrease with increasing composition of the organic co-solvent. A linear relationship between pK(a) and the mole fraction (x(2)) of the co-solvent was observed in a limited range of the compositions for each of the solvent systems. The results have been discussed in the light of transfer thermodynamic properties of the species existing in the dissociation equilibrium, solvent basicity and solute-solvent interactions. Furthermore, it was also observed that with the change of the solvents, the absorption spectra of MY shifted apparently and the color transition changed accordingly. The solvent effect on the spectra has been attributed to the isomerization equilibria of MY. A simple application of MY was also shown to the sodium acetate-hydrochloric acid titrations in the mixed solvents.
在25℃和0.1 mol l⁻¹的恒定离子强度下，通过分光光度法测量准确测定了甲基黄（MY，H⁺In）在甲醇、乙醇、异丙醇、叔丁醇混合水溶液中的浓度解离常数（pK(a)）。结果表明，在这些溶剂中，pK(a)值随有机共溶剂组成的增加而降低。在每个溶剂体系的有限组成范围内，观察到pK(a)与共溶剂的摩尔分数（x(2)）之间存在线性关系。根据解离平衡中存在的物种的转移热力学性质、溶剂碱度和溶质-溶剂相互作用对结果进行了讨论。此外，还观察到随着溶剂的变化，MY的吸收光谱明显移动，颜色转变也相应改变。光谱的溶剂效应归因于MY的异构化平衡。还展示了MY在混合溶剂中对醋酸钠-盐酸滴定的简单应用。
