蒸馏预处理和温度对纳氏试剂比色法测量氨氮结果的影响

Effects of distillation pretreatment and temperature on test data of Nessler's reagent colorimetric method for ammonia measurement

在采用纳氏试剂比色法测氨氮时,水样的预处理和温度直接影响到结果的准确性。通过试验对蒸馏预处理的馏出液量做敏感性分析,并分析温度对纳氏试剂显色反应的影响。结果表明:在蒸馏预处理中,水样在馏出液体积量达到70～100 mL(不含初始的50 mL硼酸)时氨氮已被蒸馏完全,无需再按国标(GB 7479—87)要求使馏出液达到150mL;温度控制在25～27℃时,纳氏试剂显色完全,读取的吸光度最可靠。此成果不仅可以使工作效率得以提高,而且确保了测量数据的准确性。

Nessler＇s reagent colorimetric method is the most common laboratory test for ammonia recommended by the National Standard, which is characterized by simple operation and high sensitivity. During measuring ammonia by Nessler＇ s reagent colorimetric method, the water sample pretreatment and the temperature control are particularly critical, which directly affect the accuracy of the results. Through experiments, we analyzed the sensitivity on the distillate volume of distillation pretreatment and the influence of temperature on the color reaction to the Nash reagent, and explored the relationship between distillate volume and absorbance and the relationship between temperature and color. The results of the experiment show that, in the process of distillation pretreatment, at low concentrations the ammonia in the water sample has been distilled completely when the distillate volume amount to 70 -90 mL （excluding the initial 50 mL boric acid） ; at high concentrations the ammonia in the water sample has been distilled completely when the distillate volume amount to 90 100 mL, for the need of the test. Consequently there is no need to distillate 150 mL according to the national standard（GB 7479--87）, in order that the time of operation can be shortened and the steps of distillation can be optimized. The color of the water sample at high concentrations of ammonia fluctuates slightly with the temperature, which presents basically a variation of the parabolic trend. On the other hand, the color fluctuation of the water sample at low concentrations of ammonia with the change of temperature is obvious in a particular range and the absorbance, at first, increases and then declines roughly. When the temperature is controlled between 25 ℃ and 27 ℃, The Nash reagent presents the color completely and the accurate amount of ammonia can be obtained according to the most reliable absorbance read. When temperature goes extremely high or low, it is advised to control the room temperature. Thus, this method can ensure the efficiency of operation as well as the accuracy of measuring data.