I was trying to dry silver nitrate powder in oven at 100C under vacuum. After couple of hours the color of silver nitrate powder turned from white to brown. does this mean, I have metallic silver instead of silver nitrate?
I. Photochemically induced reactions ― typically a slow process ― may possibly contribute to the salt aging: (i) AgNO3 + H2O ⇌ AgOH + HNO3; (ii) 2AgNO3 + H2O ⇌ Ag2O + 2HNO3; (iii) 2AgNO3 + H2O + ½O2 ⇌ 2AgO + 2HNO3. Both silver oxides may look dark brown.
II. The salt may perhaps be found contaminated by moisture and silver nitrite, at the crystals exposed surface. Colloidal silver and nitric acid could then join both previous contaminants: (iv) H2O + AgNO3 + AgNO2 ⇌ 2Ag + 2HNO3. Nitric acid could then react with silver nitrite, while generating nitrous acid : (v) AgNO2 + HNO3 ⇌ AgNO3 + HNO2. Hence, nitrous acid could also join contaminants, and possibly, react with colloidal silver to regenerate silver nitrite: (vi) Ag + 2HNO2 ⇌ AgNO2 + NO + H2O. Dissolved nitric oxide could contribute to regenerate nitrous acid: (vii) 4 NO + O2 + 2 H2O ⇌ 4 HNO2; (viii) 2NO + HNO3 + H2O ⇌ 3HNO2.
III. Accepting that equilibrium (iv) contributes to generate elemental silver, and recalling Le Chatelier's principle, it can be noticed that heating the impure salt in vacuum ― below the combined vapour pressure of (chiefly) H2O and HNO3 ― can be expected to displace that equilibrium to right, since both H2O and HNO3 should then predominantly react as vapour. Hence, heating (drying) impure silver nitrate in vacuum seemingly may possibly favour surface contamination of salt crystals by finely-divided elemental silver ― possibly nanoparticles ― comparing with heating at atmospheric pressure. That seems compatible with the mentioned relatively fast change to brown colour.
IV. Assuming that the silver nitrate powder being dried was taken from its original flask ― not clear from the enunciate ― It might be the case that it was somewhat altered by the effect of aging; either by the combined effect of exposure to humid air, excessive ambient temperature, light; and/or perhaps also by the effect of exposure to reducing organic vapours, maybe disengaged from some unnoticed laboratory source (e.g. oven). Contamination could also result from contact with solid organic matter (including bare hands), unsuitable or poorly cleaned laboratory ware (avoid finger touching). Also the purity of the ― presumably uncontaminated ― original powder should be checked. Some impurities, such as silver nitrite, may enhance aging.
V. Organic solvents used to wash or dissolve the salt may possibly reduce the silver nitrate, to either silver nitrite or elemental silver.
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