Alkalinity Reduction With Acid

Many brewers treat brewing water with acid in order, in the case of sparge water, to reduce its pH to a value less than 6.00 below which it is assumed that phenol extraction from malt husks will be minimal. In the case of mashing liquor the objective is reduction of alkalinity (buffering capacity) and hence the resistance to the pH shift of the mash into the 5.4 - 5.5 region at which the best beer results.  There are alternative ways to diminish alkalinity

  • Dilute it away with low ion content water 
  • Cause it to precipitate out of solution (as calcium carbonate) by heating the water or treating it with lime.
  • Exchange bicarbonate ions for hydroxyl ions in an ion exchanger. 


When acid reacts with bicarbonate ion (responsible for most of the alkalinity in water at nominal pH) the following reaction takes place:


Here the acid, HA, is the acid with H representing the positively charged hydrogen ion and A the negatively charged anion. For example, in the case of hydrochloric acid, HCl, would be the chloride ion.  In the case of sulfuric acid, which we would write H2SO4, the anion is the sufate ion and each is associated with a pair of hydrogen ions. Note that when we write the formula for the acid molecule that the individual ion charges are not shown. In solution the hydrogen ion(s) separate from the anion. A hydrogen ion (positive charge) is attracted to the negatively chrarged bicarbonate ion which it neutralizes forming water and carbon dioxide gas. Thus we see that for a monoprotic (one proton) acid such as hydrochloric acid, each bicarbonate ion is converted to escaping CO2 gas and is replaced by one of the acid anions. For a diprotic acid (such as sulfuric acid) two bicarbonate ions are converted and replaced. Chemists talk in terms of equivalence and would say that each equivalent of bicarbonate is replaced by an equivalent of the anion. An equivalent is Avogadro's number of ionic charges.

Which Acid to Choose

As anions of whichever acid is chosen will be left behind, it is clear that we want to use an acid that leaves behind anions that are in keeping with our goals. If we don't want sulfate ion or don't want to contribute more to a water already high in sulfate. obviously sulfuric acid would be a poor choice. If the water is already high in chloride or if we wish to avoid increasing it then hydrochloric acid would be a poor choice. If, conversely, we wish to supplement either chloride or sulfate or both one or the other or both of these acids would be a good choice. In the UK where this is often the case a company called Brupaks offers a product called CRS, which is a blend of these two acids for this purpose. In the US brewers would need to obtain food grade (FCC) acid and this is hard to do and expensive. Thefore, in the US, most home brewers use lactic acid, either in the form of the acid itself which is sold by most home brew retailers or in the form of acidulated malt (sauermalz). Lactic acid ions are quite flavorful. This, as is the case with sulfate and chloride, can be a blessing or a curse depending on whether a lactic flavor is desired in the beer.

Another popular choice in the US is phosphoric acid which is also readily available from home brew suppliers but which has the advantage of being quite flavor neutral so that quite a bit of alkalinity can be disposed of without flavor detriment.


How Much Acid Is Needed?

If a brewer has chosen the acid he is going to use, knows the alkalinity and pH of his water and the pH he wants to reduce it to it is straightforward to calculate the amount of acid which must be added. The details of the calculation are here. The calculations are not difficult but to use them one must have all the information mentioned. Let us examine the question of the target pH in a little more detail.

What Target pH Value Should be Used?

If the water is being prepared for sparging it is usually sufficient to set the pH to 6 or a little below. If the object is alkalinity reduction then the obvious target is the mash pH itself. If the water is acidified to the mash pH then no acid will be required to reduce its pH to mash pH as it is already at mash pH. Any additional acid needed to properly set mash pH  is solely that required to reduce the pH of the malts. The acidified water behaves the same way as ion free water in this regard. Though it may still have finite alkalinity that alkalinity is the acid required to move the pH (of the water only) from the pH to which it has been acidified to the pH which defines alkalinity (usually 4.3 or 4.5). In mashing, we don't go to that pH, we go to the mash pH and no lower.

Adding the Acid

It should be clear that rather than measuring the water's pH and alkalinity and calculating the amount of acid to add to reach a target pH one can arrive at the same place just by adding acid until the target is reached. This is because adding acid while monitoring pH is exactly what one does when measuring alkalinity except that one always does alkalinity measurement with strong acid (sulfuric or hydrochloric) whereas the effective 'titration' of the direct method uses the actual chosen acid which is not necessarily a strong acid.

What if you slip and add too much acid? Simply add more water until the pH rises back up to the target value.

    

Example

It has been reported that Sierra Nevada, consistent with the idea given here, treats all it's water to pH 5.5 with phosphoric acid. Chico water has average alkalinity of 76 ppm as calcium carbonate and pH which can be as low as 6.5 or as high as 7.9. Calcium content is 25 mg/L.

The chart below indicates what the effects of this may be. The curves plot the alkalinity that remains when water with alkalinity of 100 ppm as CaCO3 and pH as given on the horizontal axis is treated with acid until the pH reaches the value which corresponds to the particular curve as indicated in the legend. Solid curves with open symbols represent treatment with phosphoric acid and curves just below them in the same color represent sulfuric acid (or any other strong acid though the calculations were done for sulfuric - note that lactic acid could be considered strong in this application). Let's assume that SN measures pH of 7.2 (right in the middle of the range) on a given brew day and wants to acidify to pH 5.5 with a strong acid (sulfuric, hydrochloric, lactic or a blend of these). The 5.5 pH curves are red. The thin solid curve is the strong acid curve. Entering the chart on the bottom axis at pH 7.2 and reading up we see that were the alkalinity 100 the alkalinity remaining would be about 15 and conclude from this that the amount of alkalinity remaining for an actual alkalinity of 76 would be 15% of 76 or 11.4 ppm as CaCO3.

PhoSat

If mashing at pH 5.5 the alkalinity of the water has been satisfactorily dealt with. No additional acid will be needed to overcome the remaining alkalinity. Remember that alkalinity is measured by titrating to an end point pH of 4.5 or so. Were we to titrate from 5.5 to 4.5 we would indeed need extra acid in amount equal to 11.4/50 mEq/L.


Calculation of the Approximate Amount of Acid Needed

The curves can be used to give an estimate of the amount of acid needed to effect reduction to their labeled pH's. If alkalinity is reduced by from 100 to 15, for example, then 85 ppm as CaCO3 have been converted. Fifty ppm as CaCO3 is equivalent to 1 milliequivalent so 85/50 = 1.7 mEq of acid will have been consumed per liter of water treated. It remains to determine the normality of the acid to be used. The calculations page referred to earlier shows how to do this.

aj@wetnewf.org© A.J. deLange 2013