总氮测定过程中空白吸光值偏高的原因分析

使用HJ 636-2012测定水质总氮时,空白吸光值往往偏高。为了寻找最佳的实验条件,使用多元线性回归模型定性评价过硫酸钾和氢氧化钠的纯度、实验用水、碱性过硫酸钾溶液的存放时间、消解条件、冷却时间、消解器皿等因素对空白吸光值的重要性。结果表明:过硫酸钾纯度对空白吸光值影响最显著。为了保证总氮空白吸光值小于0.030,测定总氮时应优先使用经过2~3次重结晶的过硫酸钾以及优级纯的氢氧化钠,碱性过硫酸钾溶液应现配现用;建议消解时间30 min,消解温度123℃,自然冷却时间1.5 h;实验应优先使用超纯水或无氨水;使用哈希消解管代替25 mL比色管。

Cause analysis of high blank absorbance in determination of total nitrogen

The blank absorbance was often too high in the determination of total nitrogen in water by using HJ 636-2012.In order to find the best experimental conditions,the importance of the purity of potassium persulfate and sodium hydroxide,experimental water,storage time of alkaline potassium persulfate solution,digestion conditions,cooling time,test vessel on the blank absorbance value were qualitatively evaluated by multiple linear regression model.The results showed that the purity of potassium persulfate had the most significant effect on the blank absorbance.In order to ensure that the blank absorbance of total nitrogen is less than 0.030,potassium persulfate after 2~3 recrystallization and guaranteed reagent sodium hydroxide should be used in the determination of total nitrogen,and the alkaline potassium persulfate solution should be used at present.It is suggested that the digestion time should be 30 minutes.The digestion temperature should be 123℃,and the natural cooling time should be 1.5 hours.Ultrapure water or non-ammonia water are preferred in the experiment.HACH digestion tube should be used instead of 25 mL colorimetric tube in the experiment.