Ionic strength adjustors for HORIBA ion-selective electrode measurements. An ISA — or TISAB in the case of fluoride — is added in a fixed proportion to both your standards and your samples so that every solution the electrode sees has the same background ionic strength.

Why this reagent is not optional

An ion-selective electrode does not respond to concentration. It responds to ion activity, which is concentration multiplied by an activity coefficient that falls as the total dissolved salt content of the solution rises. Calibrate in clean standards and then measure a sample containing other dissolved salts, and the same concentration of your target ion will produce a different reading.

Adding ISA swamps that variability. Because both standards and samples are dominated by the same high, known ionic strength, activity coefficients become effectively identical and the electrode's response tracks concentration directly. Omitting the adjustor is the single most common reason ISE results come back consistently low while looking entirely plausible.

TISAB does more than adjust ionic strength

For fluoride, use 500-F-TISAB. Total Ionic Strength Adjustment Buffer performs three jobs at once: it fixes ionic strength, it buffers pH into the window where fluoride exists as free F⁻ rather than as HF or interfering hydroxide, and it complexes aluminum and iron, which otherwise bind fluoride and hide it from the electrode. Fluoride measurements made without TISAB are unreliable — particularly in drinking water compliance work.

Available adjustors

• 500-NH3-ISA — ammonia
• 500-NH4-ISA — ammonium
• 500-CA-ISA — calcium
• 500-CL-ISA — chloride
• 500-F-TISAB — fluoride (total ionic strength adjustment buffer)
• 500-NO3-ISA — nitrate
• 500-K-ISA — potassium
• 500-NA-ISA — sodium

Each bottle is 500 mL. Use the adjustor matched to the ion you are measuring.

Good practice

• Add the same ratio of adjustor to every standard and every sample without exception
• Add the adjustor before measuring, and allow the reading to stabilize
• Do not carry over adjustor between solutions — rinse the electrode between measurements
• Prepare only what you will use; do not store adjusted samples for later re-measurement

Related items

Ion standard solutions, ion-selective electrode kits, replacement ISE cartridges and membranes and ISE filling solutions are all stocked — see the Horiba Parts & Accessories collection.

Browse all HORIBA instruments.