The usable pH range is determined depending on the type and the structure of the ion selective electrode.
In general, this range is smaller when the concentration of the ions to be measured is lower.
Also, depending on the ion selective electrodes, the components of the ion selective membrane may be dissolved by the pH or the electrode potential may change due to the impact of the pH.
Furthermore, the impact of pH may cause a decrease in the slop of the ion selective electrode, or parallel shifting of the calibration curve.
To prevent these effects, set the pH to an appropriate value in advance and keep it constant.
The following table shows the impact of pH in 100 mg/L standard solution against each ion sensor cartridges.
Confirm the impact of pH and take measure to avoid it because the impact of interfering ions varies depending on the composition of the sample solution.
| Ion sensor cartridge | pH range | Adjust to acid side | Adjust to alkalaine side |
|---|---|---|---|
| NO3- | pH4 ～ 7 (100 mg/L NO3-) | H2SO4 | NaOH |
| K+ | pH5 ～ 11 (100 mg/L K+) | HCl | NaOH |
| Ca2+ | pH5 ～ 10 (100 mg/L Ca2+) | HCl | NaOH |
| Na+ | pH5 ～ 10 (100 mg/L Na+) | HCl | Ca(OH)2 |
| NH4+ | pH5 ～ 8 (100 mg/L NH4+) | HCl | Ca(OH)2 |
One simple method to set the pH of the sample solution to an appropriate value for measurement is to prepare a pH buffer solution (not containing interfering ions) that will not interfere with the ion selective electrode in advance, and add a small amount of it to the sample solution.