Friday, January 28, 2011

Five Gallons At A Time: Water Chemistry, Part III

The content of this page was updated on 9/7/2012.

If you've read Parts I and II, you're up to speed on the importance of mash pH and you know a few simple treatments to get it in the right ballpark with Madison municipal water. Now it's time to learn a little bit more about why calcium is good and alkalinity is bad.

Based on my own experiments, mashing Pilsner malt with distilled water will result in pH near 5.65. From there, several things will affect mash pH:

-Alkalinity in the water will raise mash pH.
-Calcium and magnesium in the water will lower mash pH.
-Adding acid to the water will lower mash pH.
-In general, darker malts will lower mash pH more than lighter malts.
-In mashes with alkaline water, thinner mashes will raise mash pH.
-In mashes with acidic water, thinner mashes will lower mash pH.

Alkalinity is the amount of strong acid required to lower the pH of a solution to a given value. For us, that value is 4.3. An alkaline solution doesn't necessarily have a high pH, but a lot of acid is needed to lower its pH. It may seem counter-intuitive, but distilled water has a small amount of alkalinity because its pH is higher than 4.3. For brewing water, alkalinity can be thought of as the number of bicarbonate ions (HCO3-) plus twice the number of carbonate ions (CO3--) present in a given volume of water. These two compounds, plus carbonic acid (H2CO3), comprise a buffer system that resists changes in water pH. If you want to treat your water with slaked lime, it'll be important to know the concentrations of the three molecules in your water supply. Otherwise, knowing the overall alkalinity will be sufficient.

In a mash, calcium ions (Ca++) react with malt phosphates to release hydrogen ions (H+). In essence, adding calcium is an indirect way of adding acidity. I suspect that magnesium ions (Mg++) behave in a similar manner, but I don't know for sure. Treating water with magnesium isn't very common because the ion is detrimental to beer flavor at fairly low concentrations.

In water reports, ion concentrations are commonly quantified in the weight-based unit ppm (parts per million) or mg/L. At the minuscule concentrations we're dealing with, the two units are interchangeable. However, because pH measures the number of hydrogen ions in a solution - i.e. molecules with one unit of ionic charge - the interactions between acidic and alkaline compounds are governed more by electric charge than molecular mass. A convenient unit to use is milliequivalents per liter (mEq/L), i.e. the number of millimoles of ionic charge contributed by a compound in a liter of water. Here's how to convert the values in your water report to mEq/L:

mEq/L = mg/L x Ionic Charge / Molecular Mass

For example, a calcium ion (Ca++) has an ionic charge of 2 (the number of + or - signs) and a molecular mass of 40.08 mg/mmol. If the calcium concentration of a given water supply is 80 mg/L, its equivalent concentration is 80 x 2 / 40.08 = 3.992 mEq/L.

With that sorted out, our water utility throws us for a loop and reports alkalinity as "mg/L as CaCO3". Furthermore, water reports that simply say "mg/L" for alkalinity often mean "mg/L as CaCO3". It's a stupid unit that equals the equivalent weight of a calcium ion plus the equivalent weight of a carbonate ion, but it can (and sometimes is) used to describe the concentrations of compounds that involve neither calcium nor carbonate. Really, it's just [mEq/L x 50]. If a given water supply has an alkalinity of 350 mg/L as CaCO3, its equivalent concentration is 350 / 50 = 7 mEq/L.

In the 1950s, a German brewing scientist named Paul Kohlbach found that the pH of 12-Plato kettle wort from a pale malt mash could be estimated by the following equation (adjusted to use mEq/L as the unit for alkalinity, calcium and magnesium):

Wort pH = pHdw + 0.084*(Alkalinity - Calcium/3.5 - Magnesium/7)

In the equation, pHdw is the pH of wort produced by a distilled water mash. I don't think Kohlbach accounted for the fact that distilled water has an alkalinity of about 0.05 mEq/L, so this is probably a better equation:

Wort pH = pHdw + 0.084*(Alkalinity - Calcium/3.5 - Magnesium/7 - 0.05)

The take-home message here is that a unit of alkalinity is 3.5 times more effective at raising pH than a unit of calcium is at lowering it, and 7 times more effective at raising pH than a unit of magnesium is at lowering it. The term [Alkalinity - Calcium/3.5 - Magnesium/7] is known as Residual Alkalinity (RA), and it quantifies the degree to which a given water supply will raise or lower the pH of wort. The same holds true for mashes, but the 0.084 multiplier will be replaced by a series of values that depend on mash thickness. We'll get to that in another post.

Returning to our water supply, here are some values from a Madison water report and their conversions to mEq/L:

Calcium (Ca++) = 80 mg/L -> 80 x 2 / 40.08 = 3.992 mEq/L
Magnesium (Mg++) = 45 mg/L -> 45 x 2 / 24.31 = 3.702 mEq/L
Chloride (Cl-) = 36 mg/L -> 36 x 1 / 35.45 = 1.016 mEq/L
Sulfate (SO4--) = 17 mg/L -> 17 x 2 / 96.06 = 0.354 mEq/L
Alkalinity = 339 mg/L as CaCO3 -> 339 / 50 = 6.78 mEq/L

Plugging these values into Kohlbach's equation allows us to calculate the residual alkalinity of the water supply:

RA = 6.78 - 3.992/3.5 - 3.702/7 = 5.111 mEq/L

Since residual alkalinity is often reported in mg/L as CaCO3, it's often nice to know the value for comparative purposes. In our example, RA = 5.111 x 50 = 256 mg/L as CaCO3. To brew a Pilsner at a water-to-grain ratio of 1.5 qt/lb, mash water with a residual alkalinity around -150 mg/L as CaCO3 would be ideal. For a stout, mash water with a residual alkalinity of 40 mg/L as CaCO3 could be appropriate. Regardless of your intended grainbill, the residual alkalinity of our water supply is astronomical. That's why Madison water is challenging to brew with. The next article in this series is here.

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