Primary standards and standard solutions

Primary standards are stable chemical compounds that are available in high purity and which can be used to standardise the standard solutions used in titrations. Titrants such as sodium hydroxide or hydrochloric acid cannot be considered as primary standards since their purity is quite variable. So for instance sodium hydroxide standard solution may be standardised against potassium hydrogen phthalate, which is available in high purity. The standardised sodium hydroxide solution (secondary standard) may then be used to standardise a standard solution of hydrochloric acid. Table 3.1 lists some commonly used primary standards and their uses.

Table 3.1 Primary standards and their uses

Primary standard

Uses

Potassium hydrogen phthalate Potassium hydrogen phthalate Potassium iodate

Anhydrous sodium carbonate Zinc metal

Standardisation of sodium hydroxide solution Standardisation of acetous perchloric acid Standardisation of sodium thiosulphate solution through generation of iodine Standardisation of hydrochloric acid Standardisation of EDTA solution

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