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Water in the Laboratory - A Tutorial



Overview of Purification Techniques


The major water purification technologies are described below. Each technology has its benefits and limitations. Some are able to remove a large fraction of several contaminants, while others excel at removing one specific type of impurity down to very low levels. As a result, in order to remove all contaminants to the levels required for critical applications, it is necessary to use a combination of technologies.


 

     
 

Benefits

  • Removes a broad range of contaminants and therefore useful as a first purification step.
  • Reusable.

Limitations

  • Contaminants are carried to some extent into the condensate.
  • Requires careful maintenance to ensure purity.
  • Consumes large amounts of tap water (for cooling) and electrical energy (for heating).
  • Not environment-friendly.
 
     

Distillation

Distillation is probably the oldest method of water purification. Water is first heated to the boiling point. The water vapor rises to a condenser where cooling water lowers the temperature so the vapor is condensed, collected and stored.



Many contaminants remain behind in the boiling vessel. However, the process has several limitations:

  • Inorganic contaminants are able to migrate along the thin water film that forms on the inner walls of the still. This explains why ions can be found in the distillate, whose resistivity is therefore usually between 0.5 and 1 MΩ•cm @ 25 °C (i.e., about 500 ppb total ionic contamination in water). Contaminants are also extracted from the glass or metal boiling pot used to heat the water (silica, sodium, tin, copper).

  • Organics with boiling points lower than 100 °C will automatically be transferred to the distillate, and even organics with a boiling point superior to 100 °C can dissolve in the water vapor and also pass into the distillate. In addition, new organochlorine compounds may be generated during the distillation process, which provides the energy required for chlorine in the tap water (added for sanitization purposes) to react with the natural organic substances also present in this water. This explains why the TOC level of distilled water is typically around 100 ppb.

  • Distillation is a slow process that requires storage of water for long periods. During this time, recontamination occurs from the ambient air (inorganic and organic volatile substances, bacteria, particulates and algae) and the container (organics from plastic tanks or ions from glass reservoirs).

  • Distillation requires large amounts of energy and water, and therefore is expensive to operate. In addition, a still requires regular cleaning of the boiling pot with HCl, a brush and sand paper to remove the contaminants accumulated during the process.

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