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Electrode Care

Introduction .

Glass Electrode.

Reference Electrode.

• The glass or pH electrode and reference electrode of a pH Meter, whether existing as separate units or as a compound unit are the most likely cause of inaccurate readings and slow response.
  
  
• The electrodes are expensive components of the system and require care and maintenance if they are to give a number of years of reliable service.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

Care and maintainance


• The glass or pH electrode consists of a special glass bulb at its end and is subject to contamination and abrasive damage that can interfere with the accuracy and response of pH measurements.
  
  
• Allowing the glass bulb to dry out can permanently damage the glass electrode. To prevent the glass electrode from drying out it should be stored, long term, in a concentrated solution of potassium chloride (4M KCl),
  
  The storage in a concentrated KCl solution matching the internal solution of the electrode also prevents the dilution of the internal solution through osmosis.
  
  Never store the glass electrode in deionised water for any length of time as this will result in ions being leached from the special glass bulb/membrane causing permanent damage.
  
  Short term storage, in between sample measurements, is best in a pH buffer solution closest to the pH of your sample solution.
  
  
• Liquids with a high concentration of solids increase the risk of contaminating the glass bulb and reducing the accuracy and response of the electrode.
  
  To prevent permanent contamination, don't leave the electrode in the sample solution longer than necessary and rinse off immediately with distilled water, dry and return to the storage solution.
  
  
• To reduce the risk of scratching the glass bulb, always rinse bulb by squirting distilled water onto the bulb from a squeeze bottle and then dabbing the bottom of the bulb with a tissue to remove excess water.
  
  Wiping will risk permanent, abrasive damage of the bulb and may also introduce interfering static electricity charges to the bulb.
  
  
• The life of the glass electrode is limited and may fail to respond satisfactorily with time. This is refered to as natural electrode aging. When this occurs the electrode will need to be replaced.
  
  
Restoration


• If contamination of the glass bulb is suspected, restoration can be attempted by following any of the following instructions.
  
  
  • soak the pH electrode in 0.1 M HCl or 0.1 M HNO3 solution for 20 minutes
    
    
  • A more aggressive cleaning solution of 1 part bleach to 10 parts water can be used for 10 minutes, if the acid solutions are not completely successful. This should remove stuborn organic deposits
    
    
  • Most detergents or methanol (methylated spirits) should remove oily, greasy deposits.
    
    
  • 1% pepsin in 0.1m HCl for 5 minutes will remove protein build up.
    
    
  • For removal of protein deposits soak the pH electrode in 1% pepsin in 0.1m HCl for 5 minutes
    
    
  • Following any of the above cleaning solutions remember to rinse the electrode in distilled water and replace the internal filling solution of the reference electrode, (concentrated or 4M KCl), of a compound electrode.
    
    
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• The reference electrode is usually fitted with a glass frit (see other means), forming part of the electrical circuit.
  
  
• Frequent measurements of samples with a high solids concentration, especially solids of a gummy nature, can see the contamination and clogging of the frit's pores,
  
  
• This alters the electrical potential of the liquid/liquid interface, interfering with accurate pH measurements. ,
  
  
• Rinsing of the electrode with distilled water immediately following sample measurement reduces contamination of the frit.
  
  
• Note: the best type of electrode assembly to maintain is of the compound electrode type that employs a liquid sleeve junction that is able to be dismantled and cleaned.
  
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