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I'm given $\ce{CoCl2*6H2O}$ as well as $\ce{Na3PO4*12 H2O}$

The chemical formula for Cobalt Violet is $\ce{Co3(PO4)2}$. I haven't done stoichiometry in a long time but I think that's creatable, right?

Some website says that I need $\ce{NaH2PO4·10H2O}$, instead of the Sodium phosphate dodecahydrate which is what I have.

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My best guess that pH control is important - at basic pH (and $\ce{Na3PO4}$ is pretty basic) you will precipitate cobalt hydroxide as well. To prevent that, the recipe calls for $\ce{NaH2PO4}$ to provides a lower pH. Cobalt phosphate is so insoluble that it will precipitate even in somewhat acidic medium.

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    $\begingroup$ Right, I hadn't considered that. It shouldn't be too difficult to make it work with the other salts and some pH adjustment though. $\endgroup$
    – Michael DM Dryden
    Commented Oct 11, 2014 at 23:52
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The dihydrogen phosphate anion is actually a weak acid and will dissociate to make both $\ce{HPO4^{2-}}$ and $\ce{PO4^{3-}}$ when dissolved in solution. If it works with the dihydrogen phosphate salt, it should work with the hydrogen phosphate and phosphate salts, too. (After all, it's the phosphate anion that precipitates out with the cobalt, if your formula is correct.)

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