"Bleach doesn't oxidide sulphide to sulphate, rather it is oxidised to elemental sulphur. I know this from practical experience with bleach
based H2S scrubbers: you end up with a bottle full of elemental sulphur!
Also the slag from thermite reactions boosted with CaSO4 contains CaS. To get rid of this smelly substance (CaS + 2 H2O === > Ca(OH)2 + H2S!) I
treat it with thin bleach: again elemental sulphur forms.
Hypochlorite (bleach) is a mighty oxidiser but it isn't capable of oxidising sulphides to sulphates or sulphur to sulphate. If it did have
that capability then every Dick, Tom and Harry amateur would making sulphuric acid from sulphur and bleach!"
While this comment is predominately correct, is it completely? Can excess H2S create some sulphates and, if so, how?
Per Watt's Dictionary Chemistry, HClO can oxidize S to H2SO4 (but I would not expect anything but a very dilute solution). However, we must first form
the required HClO from the basic NaClO/NaOH solution by reacting with a weak acid. Perhaps, the very dilute basic NaOH in the bleach can have its pH
slightly lowered by an excess of H2S. Also, besides H2S dissolved in water, any dissolved CO2 for that matter, could now slowly liberate HClO.
NaClO + H2CO3 --> NaHCO3 + HClO
Granted, however, gaseous H2S on contact with NaClO would be primarily reduced as previously noted:
H2S (g) + NaClO + H2O --> 2 H2O + S (s) + NaCl
But, assuming some HClO is around, my conjecture as to possible reaction paths to explain Watt's statement:
2 HClO + S --> SO2 + 2 HCl
SO2 + H2O <---> H2SO3
H2SO3 + HClO --> H2SO4 + HCl
and, as such, perhaps some sulfates would indeed be formed. |
