Revision as of 20:02, 3 March 2010

High alkalinity in their brewing water keeps many home brewers from being able to brew lighter beers. To overcome this obstacle home brewers would use reverse osmosis water to dilute their brewing water or even build their brewing water from scratch through the addition of salts. Another option is the use of acids to neutralize some or all of the water's alkalinity. If most or all of the water's alkalinity is the result of temporary hardness (dissolved calcium and magnesium carbonate) another options exists which is commonly overlooked: alkalinity precipitation using lime. Due to its low cost this method of brewing water treatment is widely used in the brewing industry but home brewers appear to have largely stayed away from it due a lack of understanding of the chemistry behind it and missing support in brewing water calculators.

Based on A.J. deLange's work [deLange] and through pointers in technical brewing litarature [Narziss&Back,2009] I was able to get it to work in my brewery and add support for the necessary calculations to the water calculation spreadsheet (Kaiser_water_calculator.xls). The following article attempts to shed light on this process and provide a better understanding of the chemistryu behind it which hopefully allows other brewers to get this to work with their water in their brewery.

A unintuitive concept

Most brewing books mention that the alkalinity of brewing water can be lowered by the addition of a lime (calcium hydroxide) which itself is a strong alkali. This always confused me and it took a while until the chemistry behind this process became clear.

To better understand the chemical process described here the reader should have read An Overview of pH as well as About chalk and the carbonate system.

Calcium hydroxide is a strong base and as such it completely dissociates into calcium and hydroxide when it dissolves in water:

Ca(OH)₂ -> Ca²⁺ + 2 OH^-

The resulting hydroxide ions want to raise the pH of the solution and in pure water will be able to that very effectively. Hoever, the water we are treating with lime contain bicarbonate and carbonic acids with act as a pH buffer and stand in the way of a dramatic pH rise. While we have experienced alkalinity only as a hinderence for lowering the pH of the mash a pH buffer works both ways. I.e. it keeps the pH from falling as much as as it keeps it from rising.

Since the ratio of carbonic acid, bicarbonate and carbonate stands in close relation to the pH of the solution (Figure 1) the hydroxide ions from the lime need to react with the carbonic acid (H₂CO₃), carbon dioxide (CO₂ that hasn't formed carbonic acid yet) and bicarbonate (HCO^-₃) to increase the concentration of carbonate (CO^2-₃) through the following reactions

CO₂ + OH^- -> HCO^-₃

H₂CO₃ + 2OH^- -> 2HCO^-₃ + 2H₂O

References

[deLange]

[Narziss&Back, 2009]