What Is Standard Addition?

Some scientific analytical techniques are designed to find the amount of individual substances inside a mixture. At times this is straightforward, but some samples interfere with the ability of the technique to assess the true concentration of a substance in a mixture. Standard addition is a way of finding the actual concentration of a material in a mixture by comparing it to an identical mixture with more of the material added. Through comparison of the result received with the known quantity of substance, the analyst can figure out how much of the substance is present in the first sample.

Chemicals tend to interact with each other, in different ways, depending on the characteristics of the individual materials. Generally, a chemical in a simple solution, such as sodium chloride (NaCl) in water (H₂0,) can be recognized by an analytical technique to assess the amount of the NaCl present in a sample of the solution. The NaCl concentration is not masked by the presence of other chemicals that interfere with the sensitivity of the analysis.

Some complex samples, such as stagnant water, contain a variety of different molecules. The actual concentration of the substance of interest in the sample may be masked by the presence of these other chemicals. For example, the characteristics of the target substance may be altered by interaction or bonding with other components, resulting in registered levels of the substance that are too low. As the point of assessing concentrations of individual components of a sample is accuracy, this poses a problem for chemists.

One way of solving the problem is to compare the unknown sample concentration against a known set of results for a similar sample. This involves much primary testing, however, and samples can vary widely in composition, so a standard known set of results can be impractical. Instead of using this complicated and perhaps costly method, chemists can choose to employ standard addition.

This method of analysis involves splitting the sample up into two or more portions. One remains unaltered, but the chemist adds a chemical to the other portion(s). The chemical to be added is exactly the same as the substance in the sample that the chemist wishes to know the concentration of. He or she adds a known quantity of the chemical to the second portion of the sample, and tests this and the original sample. For more accuracy, further portions, with varying concentrations of added chemical, can also be tested.

As the analyst knows how much extra chemical he or she put into the altered samples, the original sample can be compared to the portions with the standard addition. This comparison of results can tell the analyst how much of the substance was in the original sample. Generally, mathematical calculations, or a graph of the standard addition results, is used to figure out the initial concentration in the sample. Standard addition is a common analytical technique in chemistry analysis, where it is also known as "spiking the sample."