Acid Base Error

Because indicators are themselves acids or bases of differing strengths, when they are added to unbuffered or very weakly buffered solutions, they will effect the pH of the solution. That being said, however the quantity of indicator in a test strip is usually in the range of micrograms, thus the effect in even a 1-2ml volume is almost non-existent. If however, one is using a liquid system indicator, the effect may be pronounced enough to cause an erroneous reading by as much as one pH unit.

The Salt Error

When two different solutions containing inorganic material, having equivalent pH values are tested with an indicator, we would expect the same pH colour value to appear. However, if the two solutions contain different concentrations of salts, the colour value may not be the same. It was discovered that even a "Neutral Salt" will bring about a significant pH change. Sorensen, Kolthoff, and Michaelis, all developed elaborate tables that provided us with corrections for these salt errors, however under normal conditions, where salt values are less that 0.2N we may expect pH values as being close to accurate.

"Protein" Effects

The zwitterion characteristic exhibited by amino acids, and hence carried through a protein chain may have a profound effect on our ability to measure the pH of protein solutions with a degree of certainty. In some cases it may be impossible to measure their pH at all, and in fact many of the indicators are themselves used for accurate determinations of proteins.

Temperature Effects

Since temperature effects the ionisation constant for weak acids and weak bases, then it must effect the concentration of hydrogen ion present in solution as well. This of course will reflect upon the pH value obtained during any given measurements. It is therefore wise to ensure that temperatures are consistent when taking pH measurements. This is especially true when using pH strips, since we can adjust pH meters for temperature differences.

The Alcohol Error

Differences in the composition of a solvent will effect its acid-base equilibrium. These same types of changes will occur with the indicator as well. This results the same colour reaction in an alcoholic solution and in an aqueous buffered solution may not yield identical pH values. Normally at room temperature (23"C) this effect may be between 0-0.5 pH unit.