碱量滴定法测定四价铈溶液中的游离酸

Alkalimetric determination of free acid in cerium(Ⅳ)solution

建立了快速测定Ce(Ⅳ)溶液中游离酸的碱量滴定法。在Ce(Ⅳ)存在的溶液中,Ce(Ⅳ)先被H2O2还原,然后以甲基橙作指示剂,用NaOH直接滴定;或者在H2O2还原后的溶液中加入Ca-EDTA溶液掩蔽还原生成的Ce(Ⅲ),以对硝基酚或溴百里酚蓝指示,NaOH滴定。H2O2还原Ce(Ⅳ)时生成等摩尔的H+,可依据试样中Ce(Ⅳ)的含量,从酸的测得值中扣除。研究表明,在20mL溶液中含有0.1mmol以上游离酸时,滴定终点敏锐,Ce(Ⅳ)被H2O2还原成Ce(Ⅲ)后不影响测定。方法用于含硝酸铈和硝酸试样中的硝酸测定,加标回收率在99.5%～104%,标准偏差在0.0012～0.0017mol/L。

A rapid alkalimetry for the determination of free acid in the presence of Ce（Ⅳ） is established. In a Ce（Ⅳ）solution, Ce（Ⅳ） is reduced with hydrogen peroxide,then titrated with sodium hydroxide,with methyl orange as indicator. In another way, before the titration, a Ca-EDTA solution is added to mask Ce（Ⅲ）and p-nitrophynol or bromothymol blue are used as indicator. The H^＋ with an equal tool as Ce（Ⅳ） presented, generated from the reaction of H2O2 with Ce（Ⅳ） ,can be evaluated from the contents of Ce（Ⅳ） in sample and total acid determined. The results for the study of the method indicate that in 20 mL of solution eontainning above 0. lmmol free acid,the titration end point is sharp and Ce（Ⅳ） which is reduced with H2O2 into Ce（Ⅲ） does not interfere in the determination. The method has been employed in the determination of nitric acid in the sample ery rates of 99.5%-104% and the standard containning cerium nitrate and nitric acid with the recov deviations of 0. 001 2-0. 001 7 mol/L.