How to Use Carbonated Water and Chalk to Raise Mash pH

Many homebrewers who use reverse osmosis (RO) water eventually encounter a problem when brewing dark beers: mash pH can fall too low. While roasted malts and dark crystal malts contribute wonderful colour and flavour, they also add significant acidity. In some recipes, particularly porters, stouts and dark milds, this can push mash pH below the ideal range.

One solution is to increase alkalinity by adding calcium carbonate, more commonly known as chalk. Unfortunately, chalk has a reputation for being ineffective because it does not dissolve readily in water. Simply stirring chalk into the mash often leaves much of it undissolved, making its impact difficult to predict.

A useful workaround is to dissolve the chalk in carbonated water before brew day.

The science is straightforward. Carbon dioxide dissolved in water forms carbonic acid, which allows calcium carbonate to dissolve much more easily. Once dissolved, the chalk contributes bicarbonate ions that can help neutralise malt acidity and raise mash pH. This is useful for 100% RO brewers to help them combat issues arounf buffering the higher acidity in dark, roasted and crystal malts. Other ideas are also not to add the dark malts until mash out or sparging and treat the mash pH the same as for pale beers. However, you may still not be in teh correct pre-boil pH ball park if the dark malts lower the mash pH below the ideal range of 5-5.2 pH.

How Much Chalk Do I Need to Dissolve in Carbonated Water to Raise the Alkalinity?

A useful rule of thumb is that 1 gram of dissolved chalk in 10 litres of water contributes approximately 16 ppm calcium and 48 ppm bicarbonate, making it easy to estimate the effect on your mash before reaching for a water calculator.

To use the carbonated water and chalk method:

1. Add the required amount of chalk to a small volume of carbonated water.
   • This can be shop-bought sparkling water or water carbonated using a SodaStream, corny keg, or carbonation cap.
2. Shake the mixture thoroughly and allow time for the chalk to dissolve.
   • The resulting liquid can then be added directly to the mash liquor.

This technique is particularly useful when brewing with 100% RO water, where there is little or no natural alkalinity available to balance highly acidic grain bills.

Example Additions to 100% RO for a Stout Recipe

The following additions are a ball park figure for 100% RO water and a 5 gallon (19l) Irish Stout recipe containing 8-12% dark and crystal malts.

Irish Stout from 100% RO Water (5 US gallons)

For every 5 gallons (19 L) of finished beer:

• Add 4 g Calcium Chloride
• Add 3 g Gypsum
• Add 2 g Dissolved Chalk (dissolved in carbonated water)

This will place most Irish stout recipes in the correct calcium range (55–65 ppm) while providing enough alkalinity to prevent mash pH from dropping too low.

Why Not Just Use Baking Soda?

Unlike baking soda, dissolved chalk increases alkalinity without adding sodium, making it a good choice for brewers who want to maintain a traditional flavour profile.

Regular pH Readings

As with any water adjustment, a reliable pH meter is invaluable. Measure the mash pH about 10 to 15 minutes after dough-in and make further adjustments only if required.

Process Round Up

For brewers producing dark beers with very soft water, dissolving chalk in carbonated water is a simple and effective way to improve mash pH control and achieve a smoother, more balanced finished beer.