[paulE]

Hello

I'm trying to determine if a sample of mineral has Zn.Just to  learn and practice, i took a Zn((NO)₃)₂ solution, added saturated H₂S solution and waited to see a strong ZnS precipitate. But nothing happened.
But when I tried adding to the nitrate solution saturated Na₂S in a buffered (pH 5 aprox) solution,i could notice the fine white"coloidal" cloud of ZnS i suppose...
So i then tried adding directly the saturated Na₂S in the Zn nitrate solution. A quite white and thick precipitate formed...but I think is  more like Zn hidroxide than ZnS. I measured the pH of the  saturated Na₂S and is very high so it seems plausible for it to be hidroxide instead of ZnS.
So now i'm not sure if the cloud in buffered solution could also be hidroxide. My books say ZnS starts to precipitate at pH 3 or  so.I can't understand this behaviour:
-Why I do not get precipitation of ZnS with saturated H₂S solution (buffered or not)?
-Why then it works with Na2S? (Asuming it is no hidroxide)

Thank you

[Hunter2]

Zink hydroxide is acidic in solution, so with H2S not much happen, As you already wrote in a buffer ZnS will precipitate. I assume it will happen also with H2S, if you get the pH to neutral or slightly alcaline.