Chemical Standards
Analytical chemistry pertains to determining the chemical composition of a substance and the quantity of each substance present. From the nanogram to kilogram scale, there is a method for determining what is there and how much. But the important question asked many times is"how do analytical chemists know that they are right?" The answer to this question is chemical standards.
In its simplest terms, a chemical standard is a substance for which the exact composition is known. Or, at least, known as far as possible. Detection limits for most analytical techniques are in the parts per billion (ppb) or parts per trillion level (ppt) and although this may seem a very small amount, it still means a billion molecules of an impurity would not be detected in a drop of water. This is the nature of chemistry. Chemists deal in incredibly large numbers of molecules.
This inherent limit in detection of substance is a difficulty for analytical chemists and why the use of chemical standards has been adopted. The essential idea of a chemical standard is that all chemists agree on its composition and then build from there. By comparing solutions or other compounds to the standard, their composition is determined and so on.
To illustrate this process, consider the procedure for standardizing a solution that is approximately 1 molar hydrochloric acid. The exact concentration of the hydrochloric acid is unknown, but because it will be used for titration, knowing the concentration is very helpful. To determine the exact concentration, titrating the hydrochloric acid against a solution of base, for example sodium hydroxide, would be appropriate. The titrated volume and molarity of the base can be used to determine the acid concentration. But how is the concentration of sodium hydroxide in its solution obtained? That is determined by titrating against a primary standard, potassium hydrogen phthalate.
In other words, the potassium hydrogen phthalate (a primary standard) is used to determine the concentration of the sodium hydroxide (a secondary standard) which is then used to determine the concentration of hydrochloric acid. Why go through this complicated process? To ensure that the results are accurate and valid. By reference back to aprimary standard, providing that the analytical technique is valid and carried out properly,analytical chemists can provide some degree of assurance for their results.
Potassium hydrogen phthalate is used as a primary standard because it is a solid, organicacid salt of moderate molecular weight that is of known purity. The fact that it is a solid meansthat it can be weighed on an analytical balance quite accurately -- to seven decimal places if necessary. The fact that it is the salt of an organic acid--a diacid, actually means that it is readily soluble in water or aqueous solution. The fact that it has a moderate molecular weight means not a lot is required for a titration but enough that a good weight can be obtained. And the purity of the substance has been checked and verified many times over. In addition, it is easy to obtain and reacts in a rapid, quantitative reaction with any of the simple bases, such as sodium hydroxide. All of these attributes allow it to be used as a primary standard. There are surprisingly few compounds that qualify. For acid-base reactions, sodium carbonate, sodium tetraborate, benzoic acid, potassium hydrogen phthalate, and potassium hydrogen iodate make up the complete list. Consequently, analytical chemistry invariably requires the use of secondary standards.
Secondary standards are substances that can be referred back to primary standards, such asthe sodium hydroxide in the above illustration. These are generally stable substances that will not change composition in the short term. However, long-term storage is not possible as the secondary standards will eventually "go off." For example, sodium hydroxide stored in a glass bottle or volumetric will slowly dissolve the glass, leading to a change in composition. Even storage in a nalgene bottle results in the slow absorption of water and impurities from the nalgene. Although storing sodium hydroxide this way extends its usefulness as a secondary standard, fresh solutions need to be made and calibrated against a primary chemical standard on a regular basis.
A further set of standards for chemical analysis are those provide by the National Instituteof Standards and Technology (NIST). These are chemical compounds or mixtures that have beentested at multiple laboratories, using a variety of methods, and provide the basis for the validation of new tests and techniques, along with providing quality assurance for the laboratories themselves. For example, an analytical chemist might be interested in determining the composition of a sample of dirt. Having devised a technique for doing so, he or she would request a sample of "standardized dirt" from NIST intending to test their technique against the known chemical composition of the sample. If their technique provides the same results as the analysis of the standard sample, within the accepted margins of error, then they have a valid method for measuring the composition of dirt. This is important if the analytical chemist plans to use this technique for research or for testing soil samples for commercial clients. The quality of the results is assured by their accuracy.
In the end, chemical standards are the only method that chemists have of assuring that analytical results are accurate and valid. Testing against standards is a necessary part of chemical analysis.
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