pH Sensor Cleaning

Industrial pH sensors are often continuously immersed into process solutions that tend to contaminate the glass membrane of the measurement electrode or the liquid junction of the reference electrode or both. The result is one, or a combination of the following:

1. Zero Point Shift
2. Reduced Slope
3. Long Response Time

Ideally the pH sensor should be clean at all times in order to guarantee an ideal pH measurement. Periodic cleaning of the pH probe should be part of any regular plant maintenance schedule. Since the cleaning frequency depends solely on the goals of the application, a cleaning cycle time has to be established individually for each pH measurement. The cleaning interval could vary between hours and weeks.

The pH cleaning procedure also depends on the kind of contamination. The following procedures have been found practical and effective:

• Initially try to remove any visible coating from the membrane or the diaphragm by rinsing the electrode with a mild detergent.
• Hard scale-like calcium deposits may be removed by soaking the electrode for several minutes in a solution of 0.1Ml HCl acid.
• To remove oil and fat deposits the use of a strong household solvent is recommended. If this is unsuccessful a brief rinsing in ethyl alcohol is
  advisable.
• Applications containing proteins tend to contaminate the glass membrane and liquid junction of the sensor. Soaking the electrode assembly forseveral hours in a solution of 1% pepsin in 0.1 mol/l HCl is normally a remedy. Pepsin is an enzyme that is effective at softening protein deposits.
• Inorganic coatings such as hydrocarbons can be removed using commercially available glass cleaning solutions.
• Process liquids containing sulfides will react with the silver chloride used in the reference element and present in most reference electrolytes.
  This reaction will lead to the contamination of the liquid junction with silver sulfide deposits which discolor the porous ceramic material. To
  remove the silver sulfide precipitation the sensor has to be soaked in a thiourea/HCl solution until the diaphragm is totally bleached.
• Hard scale-type coatings may be removed with hydrogen peroxide or sodium hypo-chlorite.
• Coatings that are acid or alkaline soluble may be removed by either rinsing the electrodes in 0.1M HCl or in 0.1M NaOH for 5 to 10 minutes.

It is very important that after each above cleaning process the sensor should be soaked in 3M KCl electrolyte for approximately 12 hours, or preferably a day for rehydration. Since the cleaning solution may penetrate the diaphragm during cleaning the diffusion potential can be affected. After the cleaning and soaking process, it is absolutely necessary to perform a recalibration before a new pH measurement takes place.

During the cleaning process the electrode should only be rinsed or soaked. Under no circumstances should a sensor be cleaned mechanically, i.e. with a knife, screwdriver or any other sharp tool as this could destroy the electrode. Rubbing with a cloth must also be avoided as this will introduce static electricity into the glass shaft of the electrode and prolong the response time considerably. Please consider the Hamilton pH Cleaning Solution Set (Ref 238290) and Hamilton Storage Solution (Ref 238931) as part of this process.