I notice that most of the liquid zinc assay products are made with zinc sulfate heptahydrate, which from what I gather breaks down better in water. Yet there are also some that are made with zinc sulfate monohydrate. The heptahydrate form is far harder to come by in a pure food grade powder compared to the monohydrate.
http://www.newmediaexplorer.org/chris/2003/11/16/do_your_own_zinc_level_testupdate.htm
Any idea if there is a notable difference in using one form over the other?
The difference would be that if you made a 0.1% solution, as is suggested in your link, then there would be different amounts of zinc present, depending on whether you used the mono or heptahydrate.
If as the link suggests the exact amount of zinc in the solution is important, you would need to adjust the amount of solid so that you are adding the same amount of zinc, regardless of form.
To do this you need to understand the concept of mole and molar mass. This is a way of expressing concentration in terms of the number of molecules present; a mole of any substance contains the same number of molecules. One mole of a substance (in grams) is equal to the molecular mass of the substance determined from the sum of the atomic weight of its constituents.
You can look up molar mass of the substances you are interested in. It is 179.47 for zinc monohydrate and 287.53 for zinc heptahydrate.
So an equal weight of the monohydrate would contain a little more zinc than the heptohydrate; therefore you need to use less than 0.1 g to make up the equivalent of 0.1% heptahydrate.
You use the molar masses to calculate how much monohydrate - ie 0.1 g multiplied by 179.47 divided by 287.54 - ie 0.06 g monohydrate added per 100 ml solution.
