Dipping a test strip into a solution and matching it to a color chart seems easy enough, but what you may not realize is there are outside factors that can affect the results. There are five basic errors in pH measurement that can occur when measuring the pH of a solution, leading to a false reading. Let’s take a look.
Acid Base Error
Our pH test strips are made with indicators, which help determine the pH of a solution visually. Indicators are acids and bases, so when they’re added to un-buffered or very weakly buffered solutions, they can affect the pH.
The amount of indicator in a test strip is usually in the range of micrograms, so the effect in even a 1-2ml volume is almost non-existent. However, if you’re 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 color value to appear. However, if the two solutions contain different concentrations of salts, the color 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 than 0.2N, pH values are fairly accurate.
The zwitterion characteristic exhibited by amino acids, which is then carried through a protein chain, affects our ability to measure the pH of protein solutions. 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 affects the ionization constant for weak acids and weak bases, therefore, it also affects the concentration of hydrogen ions present in a solution. Ipso facto, temperature can affect pH readings.
You should make sure temperatures are consistent when taking pH measurements. This is especially true when using pH test strips, since pH meters can be adjusted for temperature differences.
The Alcohol Error
Differences in the composition of a solvent will affect its acid-base equilibrium. These same types of changes will occur with a pH indicator as well. The same color reaction in an alcohol-based solution and in an aqueous buffered solution may not yield identical pH values. Normally, at room temperature this error may alter results by up to 0.5 pH unit.
All of these errors can result in inaccurate pH readings, however, in most instances, you won’t come across them. If you are having troubles with our pH test strips and you think they aren’t working, check out our knowledge base for frequently asked questions. You might just find the answer you’re looking for.