提高低质量浓度Cr(Ⅵ)检测准确度的方法

采用二苯碳酰二肼分光光度法,对两种超纯水机制备的超纯水、市售饮用蒸馏水、电镀废水等样品及其低质量浓度加标样品进行检测。结果表明,采用标准检测方法所用的单波长法和分析步骤,即样品中先加混合酸,后加显色剂进行显色及测定,低浓度纯水加标样品的回收率介于16%~92%;而采用先加显色剂,后加混合酸时,回收率介于94%~114%。推测其中机理,先加酸时,在酸性环境下,Cr(Ⅵ)与显色剂的显色反应和Cr(Ⅵ)对有色化合物的褪色反应同时存在,导致结果偏低;而在中性或弱碱性条件下,先加显色剂时,Cr(Ⅵ)与显色剂可先形成中间产物,中间产物在酸性环境下较完全地形成了紫红色化合物。以上方法结合参考波长为660 nm的双波长法可进一步提高检测的准确度。

New Method for Improving the Accuracy of Low Concentration Hexavalent Chromium Detection

Hexavalent chromium in two kinds of ultrapure water prepared by ultrapure water machine,commercial drinking distilled water,electroplating wastewater and low concentration spiked samples of them,were detected by diphenyl carbonyl hydrazine spectrophotometric method.Results showed that the recovery rate of the samples determined by standard detecting method,in which the mixed acid and chromogenic reagent were added in sequential order,was 16％ ～92％;the recovery rate of the samples detected by new method,in which the mixed acid and chromogenic reagent were added in reverse order,was 94％ ～ 114％.The mechanism could be speculated as follows:when acid was added to the samples firstly,the chromogenic reaction between hexavalent chromium and chromogenic agent,and the fading reaction between hexavalent chromium and coloured compounds,would existed at the same time under acidic condition,which would lead to the accuracy decline;while under neutral or weak alkaline conditions,when the chromogenic agent was adding firstly,hexavalent chromium and the chromogenic agent would form a intermediate product firstly,this intermediate product would fully formed purple compound in acidic condition.The detecting accuracy could be further improved by using this new method combined with dualwavelength method with 660 nm as reference wavelength.