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I am writing an assignment on 'Preparing and Using Standard solutions for Use in Analysis in Titrations and Spectroscopy' and one part is asking about primary and secondary standard solutions.

I know the different between them and have stated what they are and some examples, but one question I have to answer is 'Explain why Primary standard substances are not always used in standardization, and why Secondary standard substances are used instead'.

I have looked at many sources online and they all just tell me the differences between primary and secondary, was wondering if anyone could shine some light on why a secondary standard solution would be used to standardize a solution instead of a primary?

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If you are doing pH titrations, and your primary standard is an acid - say - potassium hydrogen phthalate, you can't use that to standardize an acid directly. Instead, you would make a secondary standard of - say - sodium hydroxide, titrating against your primary standard. Then, you are ready to standardize the acid you want to use in your titration.

Secondary standards are also used if the primary standard is expensive. Hydrochloric acid solutions are cheap and stable, but you can't make them by weighing in a solid. So instead, you make an expensive primary standard and a lot of cheap hydrochloric acid of unknown concentrations, and determine the titer of the hydrochloric acid. Then, you can use it as a secondary standard.

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