光度法测定甲基橙和二甲基黄在甲醇-水混合溶剂中的离解常数

Spectrophtometric Determination of the Dissociation Constants of Methyl Orange and Dimethyl Yellow in MethanolWater Mixed Solvents

利用光度法测定了酸碱指示剂甲基橙（ＭＯ）和二甲基黄（ＤＹ）在０％～９０％（Ｖ／Ｖ）和３５％～８４％（Ｖ／Ｖ）甲醇－水混合溶剂中的离解常数ｐＫａ（２５℃，离子强度Ｉ＝０．１ｍｏｌ／ＬＫＣｌ）。实验结果表明：随着混合溶剂中甲醇含量的增加，两种指示剂的ｐＫａ均不断减小，当混合溶液中甲醇含量小于７５％时，ｐＫａ同甲醇的摩尔分数Ｘ２之间存在着良好的线性关系。甲醇的加入，指示剂在可见光区的吸收光谱有较明显的改变，其变色范围也有相应的移动。

n this paper,the dissociation constants(pKa) of two acidbase indicators,methyl orange(MO) and dimethyl yellow(DY),in methanolwater mixed solvents have been determined at 25℃ and I=0.1mol/L by spectrophotometric method.It has been shown that as the content of methanol in the mixed solvents increases,pKa of the indicators decreases gradually in the concentration range studied.Furthermore,it was observed that the pKa was a good linear function of the mole fraction of methanol(X2) in the mixtures up to 75% (V/V) methanol beyond which considerable deviations arise.The effect of solvent on the absorption spectra,color transition range and acidbase properties of these indicators were also investigated.