The mixture of sodium nitrate and conc. H2SO4 should not be heated very strongly above 200°C because
it has following disadvantages.
(1) Sodium sulphate is formed at high temperature. Which is hard substance like some and it is very difficult to remove from the distilating flask.
(2) Glass apparatus may ***** above 200°C
(3) A part of citric acid vapors decompose to give NO2 which imparts a yellow ***** to nitric acid.
