The pH scale is defined in terms of hydrogen ion activity in aqueous solution and is not therefore strictly applicable to pH measurement in non-aqueous systems. If measurements are to be performed in non-aqueous liquids, there are several problems which may be encountered. The low ionic strength and low conductivity of some non-aqueous solvents may result in high noise or drift. In addition, the glass membrane of the pH electrode may become dehydrated from immersion in the non-aqueous solvent and thus lose its ability to respond to H+ ions.
The ideal way to minimize these effects is to use a separate reference and measuring electrode with an auxiliary salt bridge containing a non-aqueous electrolyte filling solution. If this is not available a standard pH electrode may be used providing the following steps are performed:
- Select an electrode with a low resistance membrane and a free flowing diaphragm such as the Schott Blueline 13.
- Use an electrode containing a non-aqueous filling solution (such as saturated Lithium Chloride in Ethanol for non-polar solvents or Lithium Chloride in Glacial Acetic Acid for polar solvents).
- Soak the electrode in the non-aqueous solvent for ten minutes after standardisation and before use in the sample.
- Between successive measurements, the electrode should be rinsed with the non-aqueous solvent used to dissolve the sample.
If the reading begins to drift after considerable time in a non-aqueous solvent, the pH glass bulb must be rehydrated by immersion in an aqueous buffer.
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