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在分析化学中所使用的大部分络合配位基,多为中强碱,在一定的pH下可以质子化。金属离子与已质子化的配位基形成络合物而同时释放出质子,因此,测量滴定过程中体系pH的变化,根据酸碱平衡与络合平衡,可测得络合物的稳定常数。苹果酸为二元弱酸(H_2L),与Yb(Ⅲ)络合形成[YbL~+],[YbL_2~-]和[YbL_3~(3-)]络合物。根据电荷等衡与质量等衡,可计算出在不同中和度a下,体系中游离配位基的浓度[L~(2-)]及平均配位数n,按照实验获得的各个[L~(2-)]和n值,绘制-log[L~2]和n的关系曲线,由半值点法确定各级稳定常数。为了获得更为精确的数值,可用松弛法对实验数值进行校正。苹果酸与Yb(Ⅲ)所形成络合物的第一级稳定常数K_1,在研究了测定体
Most of the complexing ligands used in analytical chemistry are mostly moderately basic and can be protonated at a certain pH. The metal ions form a complex with the protonated ligand and release protons at the same time. Therefore, the pH of the system during titration is measured. Based on the acid-base balance and the complex balance, the stability constant of the complex can be measured. Malic acid is a weak binary acid (H_2L) complexed with Yb (Ⅲ) to form complexes of [YbL ~ +], [YbL_2 ~ -] and [YbL_3 ~ (3-)]. According to the charge balance and the mass balance, we can calculate the concentration [L ~ (2-)] and the average coordination number n of the free ligand in the system under different neutralization degrees a. According to the experimental [L ~ (2-)] and n values, plot the relationship between -log [L ~ 2] and n, and determine the stability constants at all levels by the half-value point method. In order to obtain more accurate values, relaxation values can be used to correct experimental values. The first-order stability constant, K_1, of the complex formed between malic acid and Yb (III)