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« on: December 30, 2010, 10:32:59 AM »
The problem with chalk additions in some water programs is that they are calculating the amount of carbonate added to water with the chalk addition. Since chalk is calcium carbonate, it seems perfectly logical that the carbonate number is the correct one. Unfortunately, carbonate chemistry is a little more complicated than that.
First of all, in the typical drinking water pH range, carbonate (CO3) will mostly not exist in the water. It naturally converts to the bicarbonate (HCO3) form in the typical drinking water pH range. Many of you will recognize that the carbonate species exist in different ionic forms depending upon the pH of the system. It exists as carbonic acid (H2CO3) at low pH, as bicarbonate (HCO3) at middle pH, and as carbonate (CO3) at high pH. For the most part, our brewing is in the middle pH range and bicarbonate is prevalent.
The most important reason that we need to convert the carbonate concentration to its equivalent bicarbonate concentration is that the formula that we're using to calculate alkalinity assumes that everything is in the bicarbonate form. So we have to convert our alkalinity producers to their equivalent bicarbonate concentration.
Since the milliequivalent concentration of the carbonate species will not change when they transform to its various forms, we can calculate what the equivalent amount of each species (in mg/L) is as it transforms and find that numerical conversion value.
Some important chemistry information:
milliequivalents per liter is equal to the ionic concentration divided by the ion's equivalent weight.
The ion's equivalent weight is equal to the ion's molecular weight divided by the ion's charge.
For Carbonate, the eq wt = 60 mg/mole divided by its charge (-2), or 30 mg/mole
For Bicarbonate, the eq wt = 61 mg/mole divided by its charge (-1), or 61 mg/mole
I'm going to add another alkalinity ion for something else I'll present later. The equivalent weight of hydroxide (OH) is 17 mg/mole divided by it charge (-1), or 17 mg/mole.
Since the milliequivalents per liter do not change when we convert from one form of carbonate ion to another, we can calculate what that numerical conversion from carbonate to bicarbonate is. That conversion is simply the ratio of the equivalent weights of the ions. In the case of carbonate and bicarbonate, that ratio is 61/30 or 2.033333333. To convert a calculated concentration of carbonate ion to its actual concentration of bicarbonate ion at the typical drinking water pH range would be to multiply the carbonate concentration by 2.033.
So for a typical 1 gram per gallon chalk addition, the calcium concentration would be 105.7 ppm. But instead of the 158.4 ppm carbonate concentration, the bicarbonate concentration is actually 322.3 ppm (158.4 x 2.033). Note that the mEq/L are equal: 158.4/30 = 5.28 and 322.3/61 = 5.28.
The real problem with chalk is that it just isn't that soluble in water. There are entire book chapters written on the subject of calcium carbonate solubility since it is critical to life and critical to potable water supply engineers like myself.
At standard temperature and pressure (STP), the solubility of chalk is about 47 mg/L, which is not that much. That equates to less than 0.2 grams of chalk in each gallon of water. Those of you that use chalk know that it just doesn't seem to dissolve in water. You can bubble air through the water to get it to dissolve faster, but if you're working with air at atmospheric pressure, then you're only going to get that 47 mg/L into the water. That amount of chalk provides about 55 ppm HCO3 or about 45 ppm alkalinity, which may not be enough for the typical brown or black beer mash.
Work by Troester and DeLange have confirmed that chalk solubility in the mash isn't much higher. Apparently, the acids present in the mash are pretty weak and cannot provide the protons needed to dissolve the chalk. It takes extra effort in the form of adding CO2 to the water to get the chalk to dissolve in water.
I have done tests with water and chalk added at a rate of 2 grams per gallon and have easily dissolved it when I added CO2 to the headspace of the soda bottle and pressurized to over 15 psi with a carbonator cap. This improves the solubility by over 10 times, but that may not really be practical if your dealing with water needed for a 14 barrel mash.
To add alkalinity to mashing water we can also add baking soda (NaHCO3), but then we have to worry about a practical limit for sodium (150ppm, but it should really be kept below 100 ppm).
So, we need another option to add alkalinity to their mashing water.
Pickling Lime (aka Slaked Lime) is calcium hydroxide (Ca(OH)2). It is very soluble in water and does not face the solubility problems that chalk has. But I haven't seen anyone discussing how it should be added.
We need to go through the same milliequivalent/liter game that we went through with the carbonate/bicarbonate transformation. The ratio of equivalent weights between bicarbonate and hydroxide is 61/17 = 3.588.
Therefore, the concentration of calcium added when 1 gram of pickling lime is added to 1 gallon of water is 142.8 ppm and the concentration of hydroxide is 121.1 ppm. Converting that hydroxide concentration to its equivalent bicarbonate concentration is: 121.1 (ppm OH) x 3.588 = 434.7 ppm.
As you might expect with a strong base like pickling lime, it has pretty high alkalinity producing potential. When its added in the small amounts needed to control mash pH, it doesn't really convert into bicarbonate in the mash. It just consumes any acid it comes in contact with, converting those OH ions directly into H2O when an acid (H) is encountered. Since Alkalinity is defined as the measure of the capacity of a water to neutralize strong acid, it doesn't matter that the alkalinity is from carbonate, bicarbonate, or hydroxide. But since our brewing chemistry analyses are based alkalinity calculated from bicarbonate content, it is important to perform the conversion of hydroxide to its equivalent bicarbonate concentration.
Since the issue of errors in some water calculation programs was the genesis of this discussion, I should end with its discussion. Those programs assume the carbonate concentration can be treated as a reduced concentration of bicarbonate. Considering the limited solubility of chalk, its not a bad assumption. Unfortunately, the severely limited solubility of chalk make even that assumption to optimistic unless the brewer is going to dissolve the necessary quantity of chalk under CO2 pressurization. In addition, if the brewer does actually use CO2 to dissolve the chalk in the water, then the alkalinity calculated for the chalk addition would definitely be wrong with those water calculation programs. The real solution to adding alkalinity (without too much sodium) is to get brewers up to speed with using pickling lime for adding mash alkalinity and forget about chalk.
I trust this information will be helpful.
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