Acid error:

At low pH range - less than 0.5, the values determined by the glass electrode tend to be somewhat higher. This error is because of the activity of water that we have ignored although writing Nernst equation for the indicator electrode. We have supposed that activity of water might be taken to be unity as it is in huge excess in the solution and it behaves as a pure substance. Therefore, in highly acidic solution, the activity of the water becomes less than unity since a good amount is used in hydrating the protons. Same effects are also observed on addition of large amount of any dissolved salt and on addition of a miscible non-aqueous solvent such as ethanol. The net result in each case is the measured pH will be too high.