Dithionite ions ( Hydrosulfite ions ) undergo both acid and alkaline hydrolysis to thiosulfate and bisulfite, and sulfite and sulfide, respectively
2 S2O4- - + H2O - - - > S2O3- - + 2 HSO3-
3 Na2S2O4 + 6 NaOH - - - > 5 Na2SO3 + Na2S + 3 H2O
" Sodium hydrosulfite Test Solution " contains Copper sulfate or Complexed Copper sulfate in a buffered solution having slightly alkali pH .
Copper ions will combined with Sulfide ions ( from alkaline hydrolysis of Hydrosulfite ions ) to yield Copper sulfide ( Black Solid )
S- - + Cu++ - - - > CuS ( Black Solid )
Na2S + CuSO4 - - - > Na2SO4 + CuS ( Black Solid )
3 Na2S2O4 + 6 NaOH + CuSO4 - - - > 5 Na2SO3 + Na2SO4 + 3 H2O + CuS ( Black Solid )