Alkaline error can occur when testing pH in high pH solutions; you can get falsely low readings.
Alkine error happens when you test in solutions with a high pH. The hydrogen ions that typically reside or interact with the hydrated layer on the pH sensing bulb end up being substituted with alkali ions (ie. basic ions). This then causes the pH readout from your meter to be lower than your solution's true pH. Luckily, if you have a new pH electrode with a modern glass formulation, alkali error tends to only start occurring when you're testing in solutions with a pH of 12 or higher.
Hanna developed a high-temperature (HT) glass that minimizes alkali error in highly basic, alkaline solutions.
Examples of Alkaline Error in Sodium Hydroxide Solution
| Alkaline Error in 0.1 M Solution | ||||
| pH | GP (General Purpose Glass) | HT (High Temperature Glass) | LT (Low Temperature Glass) | HF (Hydrofluoric Acid - Safe Glass) |
| 10.0 | ||||
| 10.5 | 0.06 | |||
| 11.0 | 0.15 | |||
| 11.5 | 0.05 | 0.22 | ||
| 12.0 | 0.01 | 0.18 | 0.30 | |
| 12.5 | 0.11 | 0.05 | 0.28 | --- |
| 13.0 | 0.23 | 0.11 | 0.35 | --- |
| 13.5 | 0.35 | 0.16 | 0.45 | --- |
| 14.0 | 0.48 | 0.20 | 0.54 | --- |
| Alkaline Error in 1.0 M Solution | ||||
| pH | GP (General Purpose Glass) | HT (High Temperature Glass) | LT (Low Temperature Glass) | HF (Hydrofluoric Acid - Safe Glass) |
| 10.0 | 0.01 | 0.25 | ||
| 10.5 | 0.14 | 0.25 | ||
| 11.0 | 0.02 | 0.30 | 0.48 | |
| 11.5 | 0.11 | 0.01 | 0.46 | 0.71 |
| 12.0 | 0.21 | 0.06 | 0.62 | --- |
| 12.5 | 0.32 | 0.11 | 0.79 | --- |
| 13.0 | 0.43 | 0.15 | --- | --- |
| 13.5 | 0.45 | 0.21 | --- | --- |
| 14.0 | 0.65 | 0.27 | --- | --- |