A primary standard is an element of high purity that is used to measure an indefinite chemical concentration of a known reactant chemical. It is used to perform titration or to calibrate standard solutions.
A primary standard has the distinct qualities of low hygroscopicity, low reactivity with surrounding environment, a predictable reactivity and a large molar mass, which makes it ideal for determining accurate and reliable chemical concentration assessments.
The high purity of a primary standard enables the precise calculation of concentration, as any impurity in the preparation can modify the stoichiometric point of the reaction and distort the results. The stoichiometric point of the reaction can also be altered if the primary standard chemical attracts or absorbs moisture from the air or reacts with it. The large molar mass allows the primary standard to be easily and correctly weighed on a balance with little error. This enables accurate calculation of molar concentration of solutions that are prepared in volumetric flasks.
A color-changing compound is used to indicate the stoichiometric point of the reaction between the reactant chemical and the primary standard in titration. The molar equivalents of the reactant chemical and the primary standard are then determined using the equation of the reaction.
Some examples of primary standards include sodium borate, sodium carbonate, sodium oxalate, potassium bromate, potassium iodate, potassium hydrogen iodate and pure salts and metals such as copper, zinc and sodium.