The pH of water in an aquarium is a measurement of the concentration of hydronium ions H3O+ (or just H+ for short). Neutral water has a pH of 7, and solutions with a pH lower than 7 are described as ‘acidic’ and those above pH 7 as ‘basic’ or ‘alkaline’. The pH of aquarium water is largely determined by the amount of dissolved CO2 and the amount of carbonate hardness, also known as the alkalinity*. pH can be easily measured using a calibrated pH meter but measuring the alkalinity is a little trickier.
Alkalinity can be measured by titration
Alkalinity is the capacity of a solution to resist acidification, and in a water report is usually given in terms of mg/l calcium carbonate (CaCO3) – CaCO3 serving as a universal reference material for measuring alkalinity. That doesn’t mean your water actually contains calcium carbonate. Most likely your water alkalinity comes primarily from bicarbonate and to a lesser extent carbonate, and to a smaller degree a variety of different hydroxide species. When alkalinity is measured, whatever it is that was in the water sample that made it capable of resisting acidification, it had the same degree of acidification resistance as would a pure water solution containing that many mg/l CaCO3.
To measure alkalinity in aquarium water, one method is to determine how much acid needs to be added (or titrated in) to reduce the pH of a water sample to the point where there are effectively no bicarbonate or carbonate ions left because they have all been protonated down to carbonic acid. At a pH of 4.5 or lower, this is pretty much the case.
Hanna Instruments sells an alkalinity test kit that lets you do this measurement. A drop of the pH indicator bromophenol blue is added to 5 ml of water to be tested. Above pH 4.5 the bromophenol blue is blue, converting over to yellow as the pH drops to 3.5 or below. With this kit, the carbonate hardness of the water sample measured in ppm** CaCO3 is the amount in millilitres of a dilute solution of hydrochloric acid (30 mM HCl) that had to be added to the water to make the indicator change from blue to yellow, multiplied by 300. Dividing the final ppm CaCO3 figure by 17.86 converts to the more familiar dKH reporting format.
I used the kit to measure the alkalinity in our tap water twice and came up with 288 ppm CaCO3 the first time and 282 ppm CaCO3 the second time. Reproducibility is good! Our local water report had the alkalinity listed at 290 ppm CaCO3 so pretty much right on the money. These readings would be 16.1, 15,8 and 16.2 if expressed as dKH respectively.
Bromophenol blue titration series in order of decreasing pH
The gallery above shows a sample of Shrimphaus water being titrated using the Hanna kit. To be consistent, I call the titration finished when the last trace of blueness has gone from the sample, but before the yellow colour has been maximised, generally at around a match for the ‘BPB G’ picture in the gallery. The colour change is quite rapid once it gets started; there might be a difference of 0.03 ml HCl added between each successive picture. The kit doesn’t claim any particular precision but by carefully measuring the 5 ml water sample volume I get reproducible values within 5 ppm CaCO3 of each other.
* Carbonate hardness and alkalinity are not quite the same thing. Carbonate hardness refers to HCO3– and CO32- species only, whereas alkalinity refers to anything that can accept a proton. In most situations where tap water is used in the aquarium, most of the alkalinity will be from carbonate hardness.
** Because 1 litre of water has a mass of 1,000,000 mg, measurements given in ppm (parts per million) are the same as measurements given in mg/l.