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In chemistry, a primary standard is a reagent which is very pure, representative of the number of moles the substance contains and easily weighed. A reagent is a chemical that is used to cause a chemical reaction with another substance. Often, reagents are used to test for the presence or quantity of specific chemicals in a solution.
Properties of Primary Standards
Primary standards are typically used in titration to determine an unknown concentration and in other analytical chemistry techniques. Titration is a process in which small amounts of a reagent are added to a solution until a chemical reaction occurs. The reaction provides confirmation that the solution is at a specific concentration. Primary standards are often used to make standard solutions (a solution with a precisely known concentration).
A good primary standard meets the following criteria:
- high level of purity
- low reactivity (high stability)
- high equivalent weight (to reduce error from mass measurements)
- not likely to absorb moisture from the air (hygroscopic) to reduce changes in mass in humid versus dry environments
- inexpensive and readily available
In practice, few chemicals used as primary standards meet all of these criteria, although it's critical that a standard is of high purity. Also, a compound which may be a good primary standard for one purpose may not be the best choice for another analysis.
Examples of Primary Standards and Their Uses
It may seem odd that a reagent is needed to establish the concentration of a chemical in solution. In theory, it should be possible to simply divide the mass of the chemical by the volume of the solution. But in practice, this isn't always possible.
For example, sodium hydroxide (NaOH) tends to absorb moisture and carbon dioxide from the atmosphere, thus changing its concentration. A 1-gram sample of NaOH may not actually contain 1 gram of NaOH because additional water and carbon dioxide may have diluted the solution. To check the concentration of NaOH, a chemist must titrate a primary standard (in this case a solution of potassium hydrogen phthalate (KHP). KHP does not absorb water or carbon dioxide, and it can provide visual confirmation that a 1 gram solution of NaOH really contains 1 gram.
There are many examples of primary standards; a few of the most common include:
- Sodium chloride (NaCl) is used as a primary standard for silver nitrate (AgNO3) reactions.
- Zinc powder may be used to standardize EDTA solutions after it has been dissolved in hydrochloric acid or sulfuric acid.
- Potassium hydrogen phthalate or KHP may be used to standardize perchloric acid and an aqueous base in an acetic acid solution.
Secondary Standard Definition
A related term is "secondary standard". A secondary standard is a chemical that has been standardized against a primary standard for use in a specific analysis. Secondary standards are commonly used to calibrate analytical methods. NaOH, once its concentration has been validated through the use of a primary standard, is often used as a secondary standard.