In the detection of heavy metals in soil, samples can be digested by electric heating plate or microwave. What's the difference between them?

Details needing attention in the process of soil wet digestion Yan Pingchuan 1 Foreword Due to the complexity of soil structure, it is necessary to digest soil components, especially the content of heavy metal elements. At present, there are three commonly used digestion methods: wet, dry ashing and microwave digestion. The national standard (GB/T17138 ~17141997) (hereinafter referred to as the national standard method) is mainly used for the determination of copper, lead and zinc in soil. In the following, I put forward my personal views on the details that easily affect the detection quality and operation safety in the process of wet soil digestion by national standard method, hoping to exchange experiences with my peers. 2 Soil composition and target soil for adding acid are generally composed of inorganic substances (small-grained rocks that have weathered into sand, silt and clay), organic substances (decomposed plants, animal remains and fertilizers are collectively referred to as humus), water and air. Soil samples used for testing usually contain only inorganic and organic components. The digestion method adopted by the national standard is to destroy the complex soil structure with various acids at high temperature, and finally make a clear, transparent aqueous solution suitable for instrument detection. Firstly, aqua regia (the ratio of hydrochloric acid to nitric acid is 3: 1) and hydrofluoric acid are added to destroy the lattice structure of the soil, so that the embedded heavy metal elements are precipitated. The main function of hydrofluoric acid is to destroy the lattice structure of silicon oxide and produce chemical reaction: SiO2(s)+4HF(aq)→SiF4(g)+2H2O(l). SiF4 produced by the reaction is toxic, corrosive and irritating, which can cause human body burns. Then perchloric acid is added to destroy the organic components in the soil, so that heavy metal elements originally enriched in soil humus of animals and plants can be separated out. 3 The national standard method for controlling the acid adding time mentioned that "after wetting with water, add 10ml hydrochloric acid, and heat it on the electric heating plate in the fume hood at low temperature to make the sample decompose initially. When it evaporates to about 3ml, take it out and let it cool slightly, then add 5ml nitric acid, 5ml hydrofluoric acid and 3ml perchloric acid, cover it, heat it on the electric heating plate at a medium temperature of about 1h, and then open the cover to continue heating to remove silicon. Wash the crucible cover and inner wall with water, and add 1 ml nitric acid solution to heat and dissolve the residue. Transfer the solution to a 25ml volumetric flask, cool it, fix the volume, shake it evenly, and wait for the test. " Some technical documents mentioned that aqua regia was used for digestion, that is, aqua regia was prepared by hydrochloric acid and nitric acid in proportion and then added at one time; Or hydrochloric acid is added first and then nitric acid is added in proportion. The author thinks that the latter method has better digestion effect by controlling the time of adding acid. The author thinks that it is not appropriate to add hydrofluoric acid and perchloric acid after nitric acid in the national standard method. First, three acids will react with each other: HNO 3+HCLO 4->; NO2+ (nitro cation) +H2O+ClO _ 4-, nitric acid in perchloric acid to get protons, alkaline, will reduce the role of nitric acid digestion. Secondly, perchloric acid reacts with low-valent nitrogen oxides contained in nitric acid, which reduces the digestion effect of perchloric acid. In experiments, we often add perchloric acid, because nitric acid can't get rid of it, which produces a lot of brown smoke, that is, NO2 gas. Therefore, the best time to add perchloric acid should be about 10~20 minutes after nitric acid is added, and the nitric acid smoke is completely volatilized. Finally, nitric acid solution is added and heated to dissolve a large number of inorganic salt crystals produced after soil digestion. The author thinks that the heating time should be longer, let the solution boil to volatilize nitric acid, and then constant volume. The national standard method for controlling the amount of acid added mentions that soil samples are weighed in PTFE crucible and wetted with water. The purpose of uniform wetting with water is to wash the soil on the crucible wall to the bottom of the crucible, which not only prevents the soil from being "washed" when nitric acid is added, but also prevents the sample with uneven wetting after adding acid from being "boiled" when it is unevenly heated. On the premise of ensuring that the soil is completely wet, the less water added, the better. Adding more water is equivalent to diluting the acid, which can not guarantee the digestion effect and will prolong the digestion time.