Commercially available mercury still contains a small amount of impurities and is not suitable for direct application. The purity of mercury is required to be no less than 99.999% in the production of electric light sources.
There are physical, chemical and electrolytic methods for purifying mercury, and the latter two methods are often used.
1. Use extrusion or filter pressing to pass mercury through deerskin, woolen cloth, canvas or multi-layer towels to remove solid particles in mercury.
2. Divide the mercury into small drops and pass it repeatedly through a longer alkali solution tube (the concentration of caustic soda solution is 10%) to dissolve the grease, organic matter and more active metals (such as aluminum, zinc, tin, etc.) in the mercury.
3. Divide the mercury into small drops and pass it repeatedly through a thin tube containing nitric acid solution (10% concentration) to dissolve arsenic, antimony, aluminum, cadmium, tin, etc. Similarly, pass the mercury through 80% H2SO to dissolve lead.
4. Wash the mercury with alkali and distilled water to wash away the dissolved impurities and residual acid solution.
5. Remove moisture by heating.
6. Heat to 180~200℃ in vacuum and perform vacuum distillation to remove tin and precious metal impurities insoluble in nitric acid in mercury. Distillation can be performed multiple times.
7. Allow mercury vapor to pass through a high temperature zone of 600~800℃ and perform vacuum heat treatment to remove gas. Finally, store in vacuum to avoid oxidation.