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Zinc citrate dihydrate vs trihydrate

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Bubblefab:
Hi, wondering if someone can explain the differences between zinc citrate, zinc citrate dihydrate and zinc citrate trihydrate. Parricularly in terms of their differing solubility in water.

wildfyr:
They are different amounts of water already in the crystal structure. A simple google search of "hydrates" could have gotten you that answer. What exactly are you trying to learn about with respect to solubility? Each one probably has similar, but not necessarily identical solubility in water. I couldn't find the answer with a cursory search of the literature.

Bubblefab:
Yes sorry I have gathered that now. My situation is that I'm trying to dissolve zinc citrate in water as an additive to a cosmetic (just experimenting). I purchased zinc citrate, and it doesn't dissolve fully even at 0.1g per 100ml. I've been told it's because it's not zinc citrate trihydrate. But it seems as though the dihydrate and trihydrate should have a similar slight  solubility 2.6g in 1L? Just wondering what I'm doing wrong. And wondering what form the original "zinc citrate" is.

Borek:
Technically that puts you in a violation of our forum rules, as we don't help in self medication and preparation of cosmetics.

But let's stick with the chemistry. Hydrates contain - in their crystal structure - water molecules. When the hydrate is dissolved in water, these molecules combine with the solvent. Solubility is a property of a main component, in your case amount of zinc citrate that dissolves will be identical no matter what kind of the hydrate it is, but the more water in the crystal the more solid will be required. How much more? Pretty easy to calculate from molar masses and known formulas.

Assuming the number you listed (2.6 g of zinc citrate per 1L) is a correct one, 0.1 g per 100 mL should dissolve without problems (no matter which hydrate we are talking about, there is a large margin). If it doesn't dissolve it can mean it is contaminated with something insoluble.

Bubblefab:
Thank you, that's explained the chemistry really well  :)

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