Internal filling solutions for HORIBA ion-selective electrodes. Each ISE requires its own specific electrolyte in the internal chamber — the solution is matched to the ion being measured and is not interchangeable between electrode types.

The filling solution sits behind the sensing membrane and provides the stable internal reference against which the sample is compared. As it becomes depleted or contaminated the electrode slows down, the calibration slope degrades, and readings begin to drift. Replacing the filling solution is routine maintenance, and it is usually the first thing to try when an ISE that used to work well starts calibrating poorly.

Available filling solutions

• 500-NH3-IFS — ammonia electrode filling solution
• 500-CA-IFS — calcium electrode filling solution
• 500-CL-IFS — chloride electrode filling solution
• 500-F-IFS — fluoride electrode filling solution
• 500-NO3-IFS — nitrate electrode filling solution
• 500-K-IFS — potassium electrode filling solution

Each bottle is 500 mL. Always use the filling solution specified for your electrode.

When to replace it

• The calibration slope has fallen outside the acceptable range
• Response has become slow, or readings drift instead of settling
• The solution looks discolored, cloudy, or has visible crystals or deposits
• The electrode has been in storage and is being returned to service
• Following any membrane or cartridge replacement

Good practice

• Fill to the level specified in the electrode instructions — over- or under-filling both affect response
• Exclude air bubbles from behind the membrane; they break the internal circuit
• Do not substitute a different ion's filling solution, or deionized water, for the correct electrolyte
• Recalibrate after replacing the filling solution, using standards with the matching ionic strength adjustor

Related items

Ion-selective electrode kits, replacement ISE cartridges and membranes, ion standard solutions and ionic strength adjustors are all stocked — see the Horiba Parts & Accessories collection. Filling solutions for reference and combination pH electrodes are listed separately.

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