紫外-可见吸收光谱结合高斯多峰拟合技术测定甲基红酸离解常数

在一定pH值范围内,甲基红(MR)水溶液紫外-可见吸收光谱特征是酸式甲基红(HMR)最大吸收峰((530±15)nm)与碱式甲基红(MR-)最大吸收峰((435±20)nm)叠合在一起.本文用高斯多峰拟合技术实现了HMR和MR-叠合峰的分峰拟合计算.拟合计算输出两个吸收峰的积分面积即峰强度A1和A2,A1和A2之比与MR-和HMR浓度之比.进而计算甲基红水溶液酸离解平衡常数pKa.用本方法测量298.15K时的pKa值为4.76.拟合优度高,拟合度R2、拟合优度χ2分别达到0.998和10-5以下.深入探讨了表面活性剂十二烷基硫酸钠(SDS)、十六烷基三甲基溴化铵(CTAB)聚集行为对甲基红pKa的影响.与传统分光光度测量方法相比,紫外-可见吸收光谱结合高斯多峰拟合技术结果更可靠,测量步骤和数据处理过程更简单,更具有普适性.

Determination of Acid Dissociation Constant of Methyl Red by Multi-Peaks Gaussian Fitting Method Based on UV-Visible Absorption Spectrum

UV-visible electronic absorption spectra of methyl red(MR)aqueous solutions are characterized by the overlap of a principal peak atλ max((520±15)nm)with a shoulder peak atλ max((435±20)nm),which are assigned to acidic species(HMR)and basic species of methyl red,respectively.In this study,the spectra and the integrated absorbance of the MR-and HMR peaks(denoted A 1 and A 2,respectively)were interpreted using a new multi-peaks Gaussian fitting method.From the absorbance ratio A 1 /A 2 and the concentration ratio c MR-/c HMR,the average acid dissociation constant(pK a)was determined as 4.76 at 298.15 K.The goodness is high and the values of R 2(degree of fitting)andχ 2(chi-square test for goodness of fit) were 0.998 and below 10-5,respectively.The effects of aggregation behavior of sodium dodecyl sulfate(SDS)and cetylammonium bromide(CTAB)on pK a were also investigated via this method.The multi-peaks Gaussian fitting method was shown to determine pK a more reliably and simply than traditional spectrophotometric techniques.