« on: December 11, 2011, 09:33:45 AM »
We're pushing the limits of my water chemistry knowledge, but the Net Mash Acidity goes up a lot more per gram of MgSO4 than per gram of CaSO4 according to Bru'N water. So if I'm trying to add SO4, I get less of a pH drop from MgSO4 than CaSO4.
Per unit of SO4, though, the effect on pH should be about the same. CaSO4-2H2O is 56% sulfate by mass, and MgSO4-7H2O is 39%. So you'd need to add about 44% more MgSO4 to get the same amount of SO4. A mole of Mg also contributes more hardness than a mole of Ca (4.1 mol CaCO3 equivalent vs. 2.5), though, so to add the same amount of SO4 the equivalent hardness contribution is nearly the same.
1 g/gal CaSO4 -> 154 ppm CaCO3 hardness, 147 ppm SO4
1.44 g/gal MgSO4 -> 153 ppm CaCO3 hardness, 148 ppm SO4