Primary and Secondary Standard solutions in chemistry (titration & colorimetry)

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Primary standards, such as potassium hydrogen phthalate (KHP) (C8H5KO4), display unique characteristics such as high purity, maintain stability during storage within a long period of time whether in solid or liquid form, large molar mass as calculated to be 204 (RMM), low reactivity with the surrounding air, high stoichiometry and low hygroscopicity (property of absorbing water from its surroundings), which makes them ideal in making precise assessments of the unknown concentration of a known chemical.

Secondary standards such as sodium hydroxide (NaOH) do not have the properties listed above, therefore it is low in purity, it is highly liable in absorbing the water molecules (H2O) from the atmosphere, it has high reactivity, it’s concentration changes over time, has lower molar mass known to be 40 (RMM). They are used in standardisations by comparing against primary standards.

Primary standards are high in purity; whereas secondary standards have a low purity. Primary standards such as potassium hydrogen phthalate (C8H5KO4) remain stable when stored and the concentration does not alter over time whether it is in solid form or liquid form; however, secondary standards, for e.g. sodium hydroxide (NaOH), does not remain stable and the concentration changes rapidly over time. The primary standard, potassium hydrogen phthalate for example, has a higher molar mass (204 RMM) than the secondary standard, sodium hydroxide (40 RMM) for e.g. Primary standards’ reactivity with the surrounding is low in oppose to secondary standards where it is high and react with the water molecules in the atmosphere.

The function of the both standards is to provide as a reference to be used when standardising a solution. Initially, a primary standard is used to standardise a secondary standard.

Titration is the quantitative technique used to identify the concentration of an unknown solution by using a solution of which its concentration is known. The known solution, named titrant, is added into the analyte (unknown solution) from a burette until the reaction between the two is identified as complete by the colour change of the indicator.

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Colorimetry is also used to identify the concentration of an unknown sample, however it is typically used for the identification of concentration of coloured solutions. The colorimeter enables to pass different wavelengths of light through the sample, hence measures the amount of light absorbed by the sample.

Titration Errors

  • Using diluted titrant and analyte: Once the burette is rinsed with distilled water, if it is not rinsed with the titrant, which was the sodium hydroxide (NaOH); it will become slightly diluted. This error will lead onto introducing a little more of sodium hydroxide with the vinegar, ...

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