(Summary description)Sodium hydroxide, a food additive, is widely used in food processing industry. An important index to measure its quality performance is mercury content. At present, its determination is carried out according to the analytical method - cold atomic absorption spectrophotometry stipulated in GB 5175-2000 national standard. In the process of implementing the standard in recent years, it is found that the analytical method has many interference factors, the sample determination is difficult to be parallel, and there is a large systematic error. In addition, the actual mercury content of our products is low, and the measured value is generally about 1 x 10%. In view of the above problems, we have made a lot of tests and explorations, analyzed the influencing factors one by one, and formulated corresponding measures to improve the accuracy and repeatability of mercury content determination.

1 analysis principle

The mercury in the sample is oxidized into divalent mercury ion by oxidant, the excess oxidant is reduced by hydroxylamine hydrochloride, the divalent mercury ion is reduced into mercury by stannous chloride, and the mercury vapor is carried through the measuring cell with air or nitrogen as carrier. Use atomic absorption spectrophotometer or ultraviolet absorption mercury meter to measure its absorbance at the wavelength of 253.7 nm.

2 measurement steps

Weigh 2G of sample (accurate to 0.01g), transfer it to a 50ml beaker containing 20ml of water, add 4ml490g / L sulfuric acid and 0.5ml4og/l potassium permanganate solution, cover the watch glass, heat and boil for several seconds and cool to room temperature. Add 1 OOG / L hydroxylamine hydrochloride solution drop by drop to fade the purple red. Connect the whole mercury measuring device and adjust the instrument to the best working state. Transfer all the solution into the washing bottle, accurately dilute it with water to the 50ml mark, add 2ml 1oog / L stannous chloride solution, quickly cover the bottle stopper and measure the absorption value. The difference of absorption value is obtained by subtracting the absorption value of blank test from the measured absorption value. The mass fraction of mercury in the sample is calculated according to the corresponding mercury mass found on the mercury standard curve.

3 existing problems

The data measured by mercury analyzer fluctuates greatly; The mercury standard curve is curved and deviates from the origin, with large systematic error, and the linear correlation coefficient r = 0.9933; Sample determination is difficult to parallel.

4 improvement measures

4.1 in view of the large fluctuation of mercury meter measurement data, the cause analysis and measures are as follows

4.1.1 the mercury measuring instrument currently used by our factory is F732-V mercury measuring instrument produced in Shanghai. It is found that the humidity of the instrument is < 80%., Temperature > 15 ℃. Once the ambient humidity is too high, the reading will fluctuate irregularly; When the ambient temperature is too low, the response value of the instrument to mercury decreases significantly, and there is almost no response below 10 ℃. Therefore, turn on the air conditioner for dehumidification in plum season and rainy season, and increase the ambient temperature in winter to ensure that the working conditions meet the requirements. You can also add a water bath heating device to preheat the sample to the solution temperature > 15 ℃.

4.1.2 whether the voltage is stable or not has a certain impact on the stability of the mercury meter display data. The power supply stability can be ensured by installing a power regulator.

The national standard stipulates that the allowable relative deviation of parallel determination of mercury content is less than 25%. From the above data, the maximum relative deviation is 23.3%, which meets the requirements.

5 Conclusion

Through the above improvements, the parallelism of the determination of mercury content has been greatly improved. At the same time, the linearity of the mercury standard curve is good and close to the origin, which reduces the systematic error in the analysis method.

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• Time of issue:2022-05-12 19:33
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