Originally Posted by rocketman768
I sometimes see Epsom Salt as MgSO4-4H20. However, "Epsom Salt" typically means -7H20. Yes, it can mess up calculations. If the spreadsheet or whatever is assuming pure MgSO4, and you're adding a hydrate, you are undershooting the amount of Mg you're adding. The molar mass of MgSO4 is 120 g/mol. The molar mass of MgSO4-7H20 is 120 + 7*18 = 246 g/mol. So, if the brewsheet tells you to add 1 g of MgSO4, you would need to add 246/120 * 1 g = 2 g of MgSO4-7H20. Unless you can figure out what kind of Epsom Salt your brewsheet wants, you are shooting in the dark.
Here's a test you can do:
1) Start out with a water profile with 0 ppm for all the ions.
2) Add 1 g "Epsom Salt" to 19L (5 US gal) of water.
3) Compare with the following answers to figure out what "Epsom Salt" means for your sheet:
For MgSO4, it should read 10.7 ppm of Mg
For MgSO4-7H20, it should read 5.20 ppm of Mg
I'm using http://www.ezwatercalculator.com
and doing that test shows that they expect MgSO4-7H20 as the amount comes out as 5 ppm.
The only unknown is figuring out what I bought! The brand of Epsom Salt I bought only says "Epsom Salt" on it. (Pharmacy house brand). A completely different brand calls theirs "Magnesium Sulfate Heptahydrate" to make it clear.
As BigEd mentioned, it seems logical that all granular Epsom Salt sold in the stores is already hydrated and none of it seems to be one giant congealed mass.
You're right that you don't need much... my city water has an Mg ion ppm of 2 so I want to get it to at least 10 which means just adding a little bit.
In the end, it's not a critical ion (IMHO) so going slightly off target or low on this one isn't going to cause me to lose any sleep.