Principles Related to Practical Chemistry

A primary standard is a highly pure, stable compound with a known and exact composition that can be accurately weighed and dissolved to prepare a solution of known concentration.

• NaOH is hygroscopic, so it cannot be used as a primary standard.

• ${\mathrm{FeSO}}_4·6{\mathrm{H}}_2\mathrm{O}$ is unstable and easily oxidised, so it cannot be used.

• ${\mathrm{Na}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ is hygroscopic, so it cannot be used.

A primary standard is a highly pure, stable compound with a known and exact composition that can be accurately weighed and dissolved to prepare a solution of known concentration.

• NaOH is hygroscopic, so it cannot be used as a primary standard.

• ${\mathrm{FeSO}}_4·6{\mathrm{H}}_2\mathrm{O}$ is unstable and easily oxidised, so it cannot be used.

• is hygroscopic, so it cannot be used.