[gezr]

How exactly does pH affect precipitation?  Is the charge altered on one of the species or simply that there is less H+?

I am curious as to exactly why Calcium forms insoluble compounds with Naphthenic acids at a certain pH.  

[Borek]

pH - through protonation - changes concentration of the ion needed for ppt.

Oxalic acid is a weak one (pkA1 = 1.25, pKa2 = 4.27). For 0.1M oxalic acid solution at pH = 8.6 oxalic acid is dissociated 100%. In the same solution with pH = 1.3 only about 0.1% of oxalic acid is in form of Ox(2-), the rest is about 50/50 split between HOx- and H2Ox. This gives 1000 times difference so the solubility of oxalates will change 1000 times between pH 1.3 and 8.6.

Ox stands for C2O4.

[GCT]

How exactly does pH affect precipitation?  Is the charge altered on one of the species or simply that there is less H+?

I am curious as to exactly why Calcium forms insoluble compounds with Naphthenic acids at a certain pH.  

It depends, but in general it relates to the common ion effect.  That is if the compound in question, has an ionic component of H+ or even 0H- upon solvation, pH of the original solution will affect its solubility.