Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) is used as a primary standard in volumetric analysis because it is not deliquescent and its crystals can be weighed accurately. On the other hand, sodium dichromate ($\text{Na}_2\text{Cr}_2\text{O}_7$) is deliquescent (absorbs moisture from the air) and cannot be used as a primary standard. Therefore, the statement '$\text{Na}_2\text{Cr}_2\text{O}_7$ is preferred over $\text{K}_2\text{Cr}_2\text{O}_7$ in volumetric analysis' is incorrect.

The other statements are true:

• Acidified $\text{K}_2\text{Cr}_2\text{O}_7$ oxidises $\text{H}_2\text{S}$ to elemental sulphur, which appears milky.
• In an acidic medium, the dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$) is stable and imparts an orange colour to the solution.
• On increasing the pH beyond 7 (making the medium basic), the orange dichromate ion converts into the yellow chromate ion ($\text{CrO}_4^{2-}$).