Laboratory preparation of pure water can be generally done by distillation, ion exchange and electrodialysis. The advantage of distillation is low cost of equipment and simple operation. The disadvantage is that it can only remove non volatile impurities in water and has high energy consumption. The water obtained by ion exchange method is called deionized water, and the deionized effect is good, but it can not remove non ionic impurities in water, which is often contained in trace organic matter.
Under the action of the DC electric field, the electrodialysis method uses the anion and cation exchange membrane to remove the ion type impurity in the selective infiltration of the anion and cation in the raw water, and the electrodialysis method can not remove the nonionic impurity. In laboratory decoration, water should be selected according to need. The purity of water should not be pursued blindly.
一级水用于有严格要求的分析试验，包括对颗粒有要求的试验。如高压液相色谱分析用水。一级水可用二级水经过石英设备蒸馏或离子交换混合床处理后，再经过0.2μm微孔滤膜过滤来制取。 二级水用于无机痕量分析等试验，如原子吸收光谱分析用水。二级水可用多次蒸馏或离子交换等方法制取。 三级水用于一般化学分析试验。三级水可用蒸馏或离子交换等方法制取。 由于在一级水、二级水的纯度下，难于测定其真实的pH值，因此，对pH值范围不做规定。
Class I water is used for rigorous analytical tests, including required tests on particles. For example, high pressure liquid chromatography is used to analyze water. The first grade water can be treated by two level water through quartz equipment distillation or ion exchange mixing bed, and then filtered by 0.2 micron m microporous filter membrane. Grade two water is used for inorganic trace analysis and other tests, such as atomic absorption spectrometric analysis. The two stage water can be prepared by multiple distillation or ion exchange. Three level water is used for general chemical analysis test. The three stage water can be prepared by distillation or ion exchange. Because of the difficulty of determining the true pH value under the purity of first class water and two stage water, the range of pH value is not specified.
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