At home water testing
Water composition is important for brewing and many brewers either send their water to a lab for analysis or build brewing water from scratch using very soft (e.g. reverse osmosis water) and salts. It is, however, also possible to test brewing water at home. The precision and amount of detail of such a water test does not match that of a professional analysis, but it is sufficient to estimate the residual alkalinity of the brewing water with an acceptable accuracy. At home water testing also allows regular testing of a water source in order to detect seasonal changes which may warrant a more precise professional analysis.
To test our water, we brewers can use water test kits available for aquarium owners. The kind of test kit we need is one that tests GH and KH. GH stands for General Hardness and measures the total hardness of the water. In water speak the total hardness is the amount of calcium and magnesium ions present. It is commonly measured as either dH (German Hardness) or "ppm as CaCO3". Both these units are equivalent measures. That means that they don't express the weight of the calcium and the magnesium ions but their number per unit of volume. With knowledge of their atomic weight and an guess on the calcium to magnesium ratio commonly found in water one can estimate the calcium and magnesium content of the analyzed water [DeLange].
KH stands for Karbonathärte (German for carbonate hardness) which is the alkalinity of the water. Like total hardness or GH it is measured as either German Hardness (dH) or "ppm as CaCO3". The conversion of 1 dH = 17.8 CaCO3 is true for both total hardness and alkalinity.
Two types test kits are available: titration based and strips. Titration based test kits contain test tubes and chemicals that are added until there is a color reaction in the water. Test strips are submersed in the water and the color reaction on their test pads is then compared against a color scale. While more difficult to use I prefer titration based tests since they provide more precise results are are easier to "read" than strips.
For the exact instructions on how to use the test kit read the instruction that come with it. In particular how to convert the amount of titrant, that has been added, to a hardness value in dH or "ppm as CaCO3". But in general they all work the same:
As mentioned earlier the process by which these tests work is commonly known as titration. During a titration a chemical, called titrant, is added to a sample of known volume. This titrant reacts with the compound to be tested until all of that compound in the sample has been consumed. An indicator shows when this point has been reached. Based on the amount of titrant that has been added the unknown amount of the compound in the tested solution can be calculated.
In the case of an alkalinity test (KH test) a strong acid is added which consumes the carbonates and bicarbonates. Those are the ions that establish the water's alkalinity. This reaction produces carbonic acids and more importantly lowers the pH. Once the pH as been lowered to about 4.3 virtually all carbonates and bicarbonates have been converted, at which point the indicator solution, which is nothing more than a pH indicator changes color from blue to yellow/green. The concentration of the test solution is designed such that each drop contains enough acid to neutralize 1 dH or 17.8 ppm as CaCO3 in 5 ml water. As a result the precision of the test can be increased by increasing the sample size to 10 ml and assuming that each drop stands for 0.5 dH or 8.9 ppm as CaCO3.
A similar reaction takes place during testing for GH.
Estimating Residual Alkalinity
Just knowing alkalinity and total hardness of the water doesn't help for brewing. Brewers are interested in residual alkalinity which is determined by the water's alkalinity, its calcium content and to a lesser extend its magnesium content. Total hardness, however, lumps calcium and magnesium content together.
When looking at various water reports one will find that while the calcium to magnesium ratio is not constant on average about 30% of the total hardness is contributed by magnesium. This seems to be true for for most waters and Figure 5 illustrates that with data from various water profiles which brewers posted on-line (homebrewtalk.com).
Using that assumption residual alkalinity can be estimated from GH and KH as
RA = KH - GH / 4
This formula works with KH and GH given as either dH or "ppm as CaCO3".
Using the test results in a spread sheet
If you don't want to calculate residual alkalinity by hand or want to estimate mash pH, you can use the water calculator. This is an Excel spread sheet that, among other features, allows entering KH and GH results from a simple water test. If you don't own Microsoft Excel check out OpenOffice which is a free office suite that supports excel spreadsheets. In fact, I develop all my spreadsheets with that software.
Step 1: enter the GH and KH test restults.
If not entered as "ppm as CaCO3" the measurement is converted to that unit. With the assumption that 30% of the total hardness are contributed by Magnesium the calcium and magnesium content of the water are calculated. No assumption needs to be made for calculating alkalinity. For the water used in the above example (GH = 9 dH; KH = 7 dH) the residual alkalinity is 86 ppm as CaCO3.
If no other values are given in the field for a more detailed water report, the results from the GH & KH test are carried over and will be used by the rest of the spreadsheet. Note that no data is available for Sulfate or Chloride content.
Step 2: After strike water volume, grist weight and beer color have been specified an mash pH estimate is made based on these parameters and the residual alkalinity of the water. "Roasted %" refers specifies the percentage of specialty malts which are roasted malts (e.g. Roasted Barley, chocolate, Carafa, black patent and others). In this example the beer color was 15 SRM and half the specialty malts were roasted malts. The mash pH is likely to be around 5.5 which is a proper pH for mashing and no water treatment is necessary.
If water treatment is necessary these techniques are supported by the spreadsheet: blending, salt and acid addition.
My acknowledgement goes to A.J. deLange who pointed me to the idea of home water testing in his article Understanding Alkalinity and Hardness.