Elemental analysis
a) In the absence of N or S: The filtrate is exposed to dilute nitric acid and silver nitrate.
Formation of a white precipitate suggests the presence of chlorine.
b) In the presence of N and/or S :
The filtrate is exposed to dilute sulphoric acid then boiled to 1/3 initial volume and cooled.
Formation of a white precipitate after the addition of dilute nitric acid and silver nitrate
suggests the presence of chlorine.
Equations:
NaCN+AgNO 3 --> AgCN white ppt
Na 2 S+AgNO 3 gives Ag 2 S black ppt
In the presence of N or S these two precipitates may interfere with the white
colour of the result of the chlorine test. Therefore dilute sulphoric acid is added
because in the presence of N or S: Na 2 S+ dilute H 2 SO4 gives H 2 S gas NaCN+ dilulte
H 2 SO 4 gives HCN gas
There is no interference with the white colour expected from the chlorine test in solid.