# Reaction of long-term stored zinc acetate dihydrate

I was investigating solubility of zinc acetate dihydrate for various organic solvents, and for this I took out zinc acetate dihydrate in my lab containor. When I opened it very big aggregation was formed and some powders looked like transparent crystal. In various papers that synthesize zinc oxide nanopowder the researchers often use zinc acetate dihydrate as precursor and 2-methoxyethanol as its solvent. So I tried to dissolve it in 2-methoxyethanol. As I dissolve it and stirred for about 1 hours at room temperature, the solution turned somewhat cloudy. Ethanol did not dissolve it, and it turned opaque white powder as I raise temperature to about 60 ℃. When I report this problem to my senior he said that is strange and let me use recently bought zinc acetate dihydrate. 2-methoxyethanol successfully dissolved it, and the solution did not turn cloudy. I think that the old zinc acetate dihydrate reacted with air or water during long-term storage, but I can't find the information for it. Anyone who knows this?

• add a drop or two of acetic acid. This often helps. Zince acetate, as a salt of a weak base and a weak acid is prone to hydrolysis. Mar 7 at 9:39

My guess is the old lot of zinc acetate dihydrate may have partially lost its crystal water (reportedly loses water at 100 °C). Or it may gain water in humid regions . This could affect the dissolution in ways:

1. Assuming it is still dihydrate, more (or less) of zinc acetate is used than thought.
2. Missing (or extra) crystal water = less (more) of solvent, which is less (more) polar.

Have you checked if the fresh lot acts as expected?

Theoretically, you could determine mean zinc content (e.g. AAS or EDTA complexometry) and eventual adding of extra water if it was lost.

My advice is - as the purpose is to determine solubilities in solvents - forget about using the old lot. It is too poorly defined substance, as the water content change may not be even across all crystals sizes.

For investigation of matter behavior, you need to keep experiments as much controlled as possible. Using old lots brings unknown parameters.

Another possibility coming on mind is slow hydrolysis by crystal and aerial water and evaporation of acetic acid, leaving basic zinc acetate and/or zinc oxide/hydroxide. Like:

\begin{align} \ce{Zn(CH3COO)2 . 2 H2O(s) &-> Zn(OH)2(s) + 2 CH3COOH(g)}\\ \ce{Zn(CH3COO)2 . 2 H2O(s) &-> ZnO(s) + 2 CH3COOH(g) + H2O(g)} \end{align}

Heating $$\ce{Zn(CH3CO2)2}$$ in a vacuum results in a loss of acetic anhydride, leaving a residue of basic zinc acetate, with the formula $$\ce{Zn4O(CH3CO2)6}$$.
$$\ce{4 Zn(CH3COO)2 . 2 H2O(s) -> Zn4O(CH3COO)4(s) + 2 (CH3CO)2O(g) + 8 H2O(g)}$$