Thermal behaviour of nitric acid dihydrate in 1:2 HNO3 /H2 O solutions.

The liquid-solid phase transitions of 1:2 HNO[SUB3]/H[SUB2]O solutions were investigated in bulk samples. Two forms of nitric acid dihydrate (NAD) (denoted further NAD(I) and NAD(II) can precipitate. After cooling down to 200 K, NAD(I) crystallizes and melts congruently at 238 K. On cooling down to 173 K, NAD(I) first crystallizes and a polymorphic form NAD(II) appears on heating at about 210 K, which is governed by a nucleation process at temperatures below 200 K. NAD(II) then transforms into nitric acid trihydrate at 235 K via a peritectic reaction. The existence of the two forms is confirmed by Raman studies. Our infrared experiments on bulk samples are in good agreement with those reported in the literature leading to the conclusion that the NAD(I) studied here is also observed in polar-stratospheric-cloud (PSC) particles. In stratospheric conditions, NAD(II) probably cannot freeze in the PSC particles. [ABSTRACT FROM AUTHOR]