In a laboratory "experiment" I added KNO2 (potassium nitrite) to [Co(H2O)6]Cl2 (hexaaquacobalt(II) Chloride). This gave a yellow precipitate which I have deduced to be "Cobalt Yellow" - [Co(NO2)6]K3.
To this was added 2M acetic acid, causing a grey precipitate. I can't find this anywhere in previous textbooks, and I was wondering if anyone could tell me what the grey precipitate was?