游离硅酸对离子交换法测水泥中SO_3的干扰及误差分析

Interference and Error Analysis of Silicic Acid in Ion Exchange Method for Determination of SO_3 in Cement

针对离子交换法测水泥中S03的误差问题，采用ICP-AES测定和比较了GB／T176—2008和GB／T176—1976新旧两版标准中离子交换体系游离硅酸的溶出量，通过电位滴定和理论计算讨论了硅酸产生干扰的机理和消除干扰的措施。新版国标（GB／T176—2008）离子交换法提取SO3时，体系中游离硅酸含量增加到约15mg（以SiO2计），使得体系酸碱滴定化学计量点的pH降低至7．0以下，且终点的突跃范围缩小。变色域为8．0～9．6，显著高于化学计量点和突跃范围，从而导致滴定结果偏高，系统误差可达＋0．65mg（以SO3质量计），且终点变色不明显，不确定度增大。若选用变色点为pH=4．3～7．5范围的酸碱指示剂，可避免硅酸的干扰，SO3测定结果的误差可控制在±0．1％以内。

For the error problem of ion exchange method in cement, the paper measured and contrasted the dissolved silicate of the National standard GB/T 176-2008 and GB/T 176-1976 ion-exchange process of SO3 in cement through the ICP-AES, established the quantitative relationship between silicate and error, and discussed the interference mechanism of silicate by potentiometric titration and theoretical calculation. The results show that there will dissolve more free silica by the new national standard （GB/T 176-2008） ion exchange process to extract SO3,about 15 mg （SiO2）, which reduces system pH of stoichiometric point below 7.0 of acid-base titration, and the abrupt range narrow, when the phenolphthalein is to be as indicator（8. 0-9. 6） ,which is significantly higher than the stoichiometric point and the range, the silicate will participate in acid-base titration reaction, increases the titration results and uncertainty, the system error can he up to 0.65 mg（SO3 ）, resulting in the end color change is not obvious, and increasing the uncertainty. When selecting indicator color change between the pH is 4. 3-7.5, the interference of silicate can be negligible, and the SOa determination results of error can be controlled within ± 0. 1%.