There are abundant lithium ions in salt water system,which lead to serious interference in magnesium measurement by the complexometric titration method with EDTA, making titration results on the high side. In this paper,the authors choose n-butanol and absolute ethylalcohol with the volume ratio of 1:9 as masking agent. What' s more,we discuss the suitable quantity of the mixed alcohol added to the sample under the different Li+/Mg2+ mole ratio, concentrations and sampling volumes. Eventually, this paper indicates that the added quantity of the mixed alcohol has a bigger connection with Li+/Mg2+ mole ratios,but has no obvious connection with the concentrations and the sampling volumes,under the same Li+/Mg2+ mole ratio, the added quantity of the mixed alcohol has a suitable range. The measurement errors for this analytical method could be within 0. 3%. This paper is important to improve reliability and accuracy of the phase chemistry experiment containing lithium ion and magnesium ion in salt water system.%在水盐体系中,大量Li+的存在,对EDTA络合滴定法测镁有严重的干扰,使滴定结果偏高.采用体积比为1∶9的正丁醇与无水乙醇作掩蔽剂,探究其在不同Li+/Mg2+摩尔比、不同浓度、不同取样量下的适宜加入量,得出混合醇的加入量与Li+/Mg2+摩尔比有较大关系,与溶液浓度、取样重的多少均无明显关系；同时指出在同一Li+/Mg2+摩尔比下混合醇的加入量有个适宜的范围.该方法的相对误差不大于0.3％.对提高含Li+和Mg2+的水盐体系相化学实验研究的可靠性和精度具有重要意义.
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