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pH MAINTENANCE
pH Electrode Maintenance
In addition to selecting the appropriate electrode for your application, the best way to produce the most reliable and reproducible measurements while ensuring the longest lifetime for your pH electrode is to make sure proper maintenance and care is given to the sensor.

Preparation
Before measuring with your pH electrode you must perform a few simple maintenance steps.  The pH electrode is delivered with a protective watering cap.  Prior to measuring and calibrating the pH electrode remove this cap.  It contains an aqueous solution to keep the pH electrode from drying out.  In most cases a 3 mol/l KCl solution (L 300, PN#285138554) is used.  For specific cases please check the technical data of your pH electrode.  You are also welcome to contact us for further assistance.

Measuring
To begin taking measurements with your pH electrodes a few maintenance steps need to followed.  Open the refill opening on refillable pH electrodes before taking measurements.  Immerse the pH electrode into the solution to be measured at least up to the diaphragm.  When using refillable pH electrodes please pay attention that the electrolyte fill level is 2 inches (5 cm) above the level of the medium to be measured (see refilling).  Rinse the pH electrode with distilled water between measurements, however do not wipe off.  Carefully dab off excess drops from the pH electrode.  After finishing your measurements close the pH electrode's refill opening and clean the electrode with deionized water.  Store the pH electrode as appropriate.

Refilling maintenance
pH electrode maintenance - refill

The refill port on the refillable pH electrodes.

Any electrolyte missing from the electrolyte chamber of the pH electrode's reference system must be refilled regularly, in order to guarantee that the fill level of the electrolyte solution is at least 2 inches (5 cm) above the level of the medium being measured).  BlueLine electrodes can be refilled simply by pumping electrolyte solution into them with a small dispensing bottle.  If your pH electrode is rarely used you should completely renew the electrolyte solution.  If crystals have formed in the electrolyte area they can be dissolved by warming the pH electrode in a water bath, 113 °F (45 °C).   Once the crystals have been dissolved the electrolyte solution should be rinsed a couple of times with fresh electrolyte solution then filled to the correct level.

Calibration maintenance
All pH electrodes should have calibration maintenance performed regularly.  If the pH electrode is being used under extreme conditions or at the limit of their measuring range they should be calibrated more frequently and when necessary adjusted to guarantee the appropriate accuracy.  Your pH electrode measurements can only be as precise as the accuracy of the last calibration.  To determine if your pH electrode has changed since the last setting, the electrode should be calibrated using standard pH buffer solution.  If any measurement errors are discovered with your pH electrode the pH meter must be adjusted to compensate for the deviation, i.e. the slope and zero point on the pH meter should be adjusted.  Only use fresh buffer solutions and make sure that the pH electrode and pH buffer temperatures have stabilized.  Note: When using pH electrodes without an integrated temperature sensor, the pH meter must be set to the actual temperature of the buffer solution.   If you use pH buffer solutions from a bottle, ensure that the bottle is reclosed immediately after you pour out the solution.  Always dispose of the used pH buffer solution after calibrating your pH electrode.  For a more accurate calibration we recommend using our certified pH buffer ampoules, which are sterilized in superheated steam according to DIN 19 266.  These pH buffer ampoules are traceable to PTB and NIST standards.

Storage maintenance
The pH electrodes should be stored between 32 and 104 °F (0 and 40 °C) in aqueous solution.  A combination pH electrode should never be stored dry or in distilled water.  When not used the pH electrode should always be stored in the watering cap that is shipped with it.  The pH electrode's watering cap should be filled with the following solutions depending on the type of electrode it is:
  • Combination or reference electrode: In case of pH electrodes with liquid electrolyte you can use the electrolyte which was used to fill the electrode's electrolyte chamber or a specific storage solution such as L 911, PN# 285138590.  For low maintenance electrodes with gel electrolyte you can usually use KCl 3 mol/L (L 300, PN#285138554).
  • Single glass electrodes: Should be stored in deionized water.
Depending on the storage conditions (temperature and humidity) the aqueous solution in the pH electrode's cap can run dry.  If this happens place the pH electrode in a container of storage solution for 24 hours.  After reconditioning the pH electrode it must be checked to determine if it is still measuring correctly.

Cleaning maintenance for a combination or half-cell pH electrode
Contamination on the pH glass membrane surface or the diaphragm can shorten the life-time of the electrode tremendously and cause measuring errors.  Also, any particles or dirt inside the reference electrode should be removed as quickly as possible.  It is always better to clean the electrode with chemicals instead of mechanically.  Mechanical cleaning methods have a high possibility of destroying the electrode.  Before moving to more detailed cleaning steps, the pH electrode's glass membrane should be cleaned with an ethanol tinctured cloth.  If the pH electrode continues to have errors during calibration, the cleaning process should be continued using the following steps.  Depending on the type of contamination, submerge the reference electrode or combination pH electrode into the cleaning solution past the diaphragm.
  • Inorganic coatings and deposits can be removed by placing the pH electrode in solutions such as HCl 0.1 mol/l or NaOH 0.1 mol/l at room temperature for several minutes.  If the substances aren't dissolved the after the first attempt gently increase the solution's temperature to a maximum of 122°F (50°C).  If this does not remove the substance increase the concentration slightly and repeat the process.
  • Organic coatings and deposits can be removed by rinsing the pH electrode with suitable organic solvents (eg. ethanol or acetone, etc).  The electrode can be wiped off with a soft, moist cloth.  Make sure that the solvent used does not damage pH electrode's shaft.
  • Proteins can be removed by placing the pH electrode in a hydrochloric pepsin solution (cleaning solution L 510 PN# 285138538) for approximately one hour.
  • Fats can be removed by using a tenside solution.
  • Sulfides on a ceramic diaphragm can be removed by placing the pH electrode in a thiocarbamide/HCl solution (7.5% in HCl 0.1 mol/l) until color is removed.  Blocked ceramic diaphragms can be made functional through careful rubbing with sandpaper or using a diamond file.
  • Platinum diaphragms must not be cleaned mechanically.  A chemical cleaning (i.e. with diluted hydrochloric acid) will rinse and reopen the diaphragm.
  • After cleaning, rinse off the pH electrode with distilled water, do not rub it dry, and condition it for at least one hour in electrolyte solution.  Don't forget to recalibrate the measuring equipment before performing further tests.
Note: Ensure that any cleaning agent which has leaked into the electrode does not come into contact with the reference system; if necessary rinse out the reference electrode with electrolyte solution.  Also, take care not to scratch the pH electrode's glass membrane.

Reference electrodes can be cleaned by replacing the current electrolyte and washing the electrode with fresh electrolyte.  If this doesn't remove the dirt or particles repeat this step once again.  If the reference electrode continues to be dirty the electrolyte can be heated, 113 °F (45 °C) and the first step can be attempted once more.  We do not recommended cleaning the inside of the reference electrode with chemicals because the reference system may be damaged irreparably.

pH electrode aging
Please note that in spite of the best maintenance program every pH electrode undergoes a natural aging process.  The response time increases and the slope decreases with the age of the pH electrode.  The following factors may shorten the pH electrode's lifetime considerably:
  • Incorrect storage
  • Special measuring conditions (e.g. Extreme pH values, hydrofluoric acid, hot phosphoric acid, and strong alkaline solutions)
  • High temperatures
  • Rapid changes in temperature
Some ways to increase the lifetime of your pH electrode are listed below:
  • Calibrate the pH electrode at suitable intervals in order to find out if it still works well.
  • Always clean the pH electrode directly after the measurement with deionized water and store it in the recommended way.
  • Check the complete pH electrode (surface/diaphragm/reference) for precipitations.  If necessary clean the pH electrode in the recommended way.
  • Observe that measurements of aggressive and/or hot media can shorten the pH electrode's life-time.  When a pH electrode is used for measuring aggressive media it is important to rinse the electrode after the measurement and to follow the storage procedures.
  • In case you use a refillable pH electrode always take care that the refilling hole is opened during measurement and calibration.  It should be closed during storage.
  • Only single glass pH electrodes can be stored in deionized water.  The life-time of reference and combination pH electrodes is reduced tremendously when stored in deionized water.
  • Never store a pH electrode dry.
  • Never use pH electrodes as solution stirrers.
  • Never try to clean pH electrodes mechanically.
The warranty does not cover damage caused by measuring conditions and mechanical damage.

For information on pH electrode measurements please click here.
For general information on calibrating a pH measurement electrode please click here.
For information on different pH measurement electrode calibration methods and the difference between pH solutions please click here.
For specific information on pH glass electrode types please click here.
For specific information on pH reference systems and diaphragms please click here.
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