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ION SELECTIVE ELECTRODE (ISE) MAINTENANCE
Before Measuring with your ISE
To ensure that your Ion Selective Electrode (ISE) is ready to use for ISE measurements a few initial maintenance steps should be followed.

  1. Place the ISE in deionized water for approximately ten (10) minutes.  If crystals have formed in the electrolyte area they can be dissolved by warming the ISE in a water bath, 113 °F (45 °C).
  2. Condition the combination ISE or half-cell ISE respectively for approximately five (5) minutes in standard solution.  In order to achieve the fastest response time, the concentration of the standard solution should be slightly below the measuring range expected.
  3. Remove any air bubbles in the ISE's bridge electrolyte solution by slightly knocking against the shaft.
  4. Calibrate the ISE according to the operating manual of the ISE meter and the analysis specification.
Warning for CN 60 electrode: When dealing with acidic solutions that contain hydrocyanic acid, there is a poison danger due to the release of hydrocyanic acid.  You must add MZ/NH3/CN (PN# 150130), an alkaline reagent, to each sample to increase the pH value of the sample to > 10.  This will prevent the hydrocyanic acid from being released.  Only perform cyanide measurements in acidic media under a functioning fume hood.

ISE Calibration
Part of regular ISE maintenance is the need to calibrate the electrode on a regular basis.  Calibrate the ISE according to the data given in the operating manual of the measuring instrument and the analysis specification.

When operating ISE electrodes ensure that:
  • The ISE fill opening for the bridge electrolyte is open
  • The ISE's inner junctions are covered with bridge electrolyte
  • No air bubbles are in the ISE's bridge electrolyte
  • The ISE's depth of immersion is within the optimum range
Minimum depth of immersion: The ISE's diaphragm must be covered.
Maximum depth of immersion: Approximately 7/16 inch (1 cm) below the fluid level of the ISE's bridge electrolyte.

Note: Using a magnetic stirrer may cause interferences with the ISE.  If the measured values strongly fluctuate we recommend turning off the magnetic stirrer while measuring.

ISE Maintenance
Regular maintenance of your ISE is essential to maintaining a long life span
  • For combination electrodes: replenish used reference electrolyte solution.
  • Dirt and contamination on the sensor and diaphragm lead to measurement inaccuracies.  They can be removed using the following methods:
    • Coatings and deposits can be removed with diluted mineral acids (e.g. diluted hydrochloric acid)
    • Organic contaminations with the aid of suitable solvents
    • Grease with tensid solution or alcohol
    • Proteins with hydrochloric pepsin solution (cleaning solution L 510 PN# 285138538)
    • After cleaning, rinse off the ISEs with distilled water, do not rub dry.
  • Platinum diaphragms must not be treated by any mechanical treatment.  Rinsing (e.g. vacuuming) may be followed by chemical cleaning (e.g. with diluted hydrochloric acid).
  • The glass membrane can be cleaned by rubbing off with a fluff-free cloth soaked in ethanol.
  • When the ISE Ion selective electrode begins responding slower or measurements start drifting perform the following activities:
    • Polish the ISE's membrane surface using an S/FK polishing strip (PN# 180130) .  To do so, moisten a polishing strip with deionized water, and with circular movements polish the ISE's membrane surface for approximately 30 seconds.
    • Subsequently place the ISE into diluted standard solution for approximately 5 minutes.
Note: for K 60 and NO 60 electrodes if drifting and response times are increasing place the electrode into distilled water for one hour then store it in diluted standard solution for several hours.

ISE Storage
Some amount of maintenance should also be done prior to storing the ISE.

Combination electrodes
  • Between two ISE measurements, place the combination electrode into diluted standard solution.
  • Overnight up to one week: Push the closing ring upwards to close the refilling opening.  Then rinse the combination electrode with deionized water, blot it dry using a clean paper towel and store it in a dry place.
  • For more than a week: Remove the electrolyte solution from the reference electrode and rinse the combination electrode with deionized water, blot it dry using a clean paper towel and put on the protection cap.  Store the combination electrode in a dry place.
Halfcell electrodes
  • Between two measurements, place the electrode into diluted standard solution.
  • For storing overnight or longer, rinse the electrode with deionized water, blot it dry using a clean paper towel and put on the protection cap (closing cap of the glass tube).  Store the electrode in a dry place.

Na+ electrodes
ISEs should be stored at a temperature between 32°F and 104°F (0°C and 40°C).  Depending on the storage conditions (temperature and air humidity) the irrigation fluid in the ISE's cap may dry out prematurely.  pNa ISEs stored dry must be irrigated for minimum 24 hour in sodium chloride solution 0.1 mol/l before measurement.

ISE Aging
Please note that in spite of the best maintenance program every ISE electrode undergoes a natural aging process.  The response time increases and the slope decreases with the age of the electrode.  The following factors shorten the lifetime considerably:
  • Incorrect storage
  • Special measuring conditions (e.g. organic solutions, frequent measuring with high concentrations of interfering ions (See specific ISE manual))
  • High temperatures
  • Rapid changes in temperature
The warranty does not cover damage caused by measuring conditions and mechanical damage.

For more information on ISE products please click here.
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