Using Epson Salt with water already in it? (Magnesium Sulfate Heptahydrate)
My pharmacy sells two types of Epson Salt:
Epson Salt (MgSO4)
Epson Salt Heptahydrate (MgSO4-7H20)
The second one is basically Epson Salt with water added to it. Why? From Wikipedia:
"Since the anhydrous form is hygroscopic (readily absorbs water from the air) and is therefore difficult to weigh accurately, the hydrate is often preferred when preparing solutions, for example in medical preparations."
When spreadsheets and programs here refer to using Epson Salt, I imagine they mean the "pure" version right?
Doesn't the 'pure' version over time turn in the secone one with water meaning that that the weight wil be different if buy the first one and use it over many years?
In the end does it really matter which I use?
I doubt it. First you will never need to add magnesium in large amounts and for many beers not at all. Second, my guess is that almost any Epsom Salt you buy at the drugstore is of the hydrated form. Otherwise after a short time on the shelf at the store or your home the stuff would become a big, damp glob.
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.
Using the molecular weight for the hydrate only includes the water that is part of the crystal structure of the salt when it is exposed to H2O in the air. Magnesium sulfate will take on a bunch of extra water beyond that (adsorption). Keep the lid on the container to avoid this.
Epsom salt is the heptahydrate, and will not pick up any more water. The anhydrous form MgSO4 is used as a drying agent because it rapidly pick up and hold onto water. It is used in organic chemistry synthesis to remove water from solvents, for example.
If you are using the anhydrous form of the salt, as others have said, you will have problems keeping it dry and weighing it out. Stick with the Epsom salt variety
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