Water

Water is essential to all life on Earth. Beer is over 80 percent water. Needless to say we are glad we have it, but what choices do we have to make about the water that goes into our beer? Most water is rarely completely pure, but often has a large number of things disolved in it.

There are a handful of ions relevant to the brewing process. There are ions that affect the mash chemistry either by buffering or altering the pH. Some ions are necessary cofactors for mash enzyme activity.

Table 1: Some notable ions in brewing water and how each ion effects mash chemistry, fermentation, and on the flavor of finished beer. Some less important effects were omitted for clarity.
Ion	Mash	Fermentation	Finished Beer
Calcium	Lowers pH. Needed for enzymatic activity	Essential nutrient for yeast metabolism
Magnesium	Lowers pH	Essential nutrient for yeast cell division	Enhances flavor at low concentrations
Sodium			Salty flavor creates perception of roundness
Carbonates	Buffers the acidity from dark grains
Chloride	Very weakly basic
Sulfate	Weakly basic		Enhances hop bitterness

Table 2: Various salts can be used to modify the ion concentrations in water used for brewing. This table shows the contribution in parts per million (ppm) of 1 gram of each salt disolved into 1 gallon of water. It should be noted that chalk has very low solubility at neutral pH and more easily dissolves in the mash solution which is slightly acidic. Values for kosher salt and chalk from Noonan [1], Values for baking soda from Palmer [2], all other values from morebeer.com [3].
Salt	Formula	Ca2+	Mg2+	Na+	HCO3–	Cl–	SO42–
Baking Soda	NaHCO3			75	191
Burton Salts	(mixture)	44	11		27		105
Calcium Chloride	CaCl2	72				128
Chalk	CaCO4	105			158
Epsom Salt	MgSO4		26				103
Gypsum	CaSO4	62					148
Kosher Salt	NaCl			110		170

References

^Noonan, Gregory. 1986. Brewing Lager Beer. Boulder: Brewer's Publications.
^Palmer, John. "15.4 Using Salts for Brewing Water Adjustment." How to Brew. Retrieved 2012-12-19. http://www.howtobrew.com/section3/chapter15-4.html
^Morebeer.com. Retrieved 2012-12-19. http://morebeer.com/search?search=salt