JEE Main · 2025 · 03 Apr 2025 · Shift 2 · Practical and Analytical Chemistry

A primary standard must be very pure, stable in air and non-hygroscopic, with a fixed, known composition. $\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ is deliquescent and takes up moisture, so it cannot be weighed accurately (which is why ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ is preferred); $\mathrm{NaOH}$ is hygroscopic and also absorbs atmospheric $\mathrm{C}{\mathrm{O}}_2$; and $\mathrm{FeS}{\mathrm{O}}_4$ is readily air-oxidised, so its $\mathrm{F}{\mathrm{e}}^{2+}$ content is not definite. Oxalic acid and sodium tetraborate (borax) satisfy all these requirements. The compounds that should not be used as primary standards are $\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$, $\mathrm{NaOH}$ and $\mathrm{FeS}{\mathrm{O}}_4$.