Chemical precipitation

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Formation of a separable solid substance from a solution. This is achieved by converting the substance into an insoluble form by chemical reaction or by changing the composition of the solvent to reduce the solubility of the substance in it.

Chemical precipitation is used to remove dissolved metals from solutions, such as process wastewaters containing toxic metals. Soluble metal ions are converted to insoluble form by chemical reaction with a precipitating reagent. The solid particles formed by this reaction are removed from solution by settling and/or filtration.

The effectiveness of a chemical precipitation process is dependent on several factors which include:

  • type and concentration of metal ions present in solution
  • precipitant used
  • reaction conditions (pH, temperature, etc.)
  • presence of other constituents that may inhibit the precipitation reaction.

Metal ions such as zinc can be precipitated out of solution by addition of sodium hydroxide:

Zn SO4 (aq) + 2NaOH (aq) → Zn(OH)2 (s) + Na2SO4 (aq)

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