The importance of calcium and alkalinity is well recognised. However the
reasons why they are important, what levels are desirable and how to achieve
such levels often is not.
What is alkalinity?
Alkalinity is simply the water's ability to maintain its pH (from dropping and to an extent rising). In other words it is a mesaure of the water's buffer capacity.
What is pH?
pH is just a measure of the number of free hydrogen ions (protons) in the water. The lower the pH the higher the number of free protons. The actual number does not have a unit (like grams or litres), it is the power of ten (or log base 10 if you prefer) of the concentration of protons divided by the volume of the sample.
What is an ion?
An ion is an atom or group of atoms that have either gained (therefore they are negatively charged since atoms always have no net charge), or lost one or more electrons. A negatively charged ion is called an anion and a positvely charged ion is called a cation.
What chemicals go to make up alkalinity?
A number of negatively charged ions go through an equilbrium equation. The mains ones we talk about are carbonate and hydrogen carbonate (also known as bicarbonate) and hydroxide, others such as borate and sulphate are present but are far less important.
How do the alkalinity ions help maintain pH?
First thing to know is that acids provide hydrogen ions. Since pH is a measure of the free protons then any acids that enter the tank water will tend to lower the pH.
The alkalinity ions can actually take a number of different forms, and can bind or release protons to themselves based upon the relative amount of protons is the water.
Huh?
Ok, let's take an example.
We have lots of caronbate ions in the water. Along comes an acid (let's make it a simple mineral acid, hydochloric acid) and would rather like to get rid of its hyrogen ion (proton). In our case hydrochloric acid really does not like being HCl, in a good solvent like water, it would much rather be its two ions, H+ and Cl-.
For the sake of this example we will pretend there is only one hydrogen ion (proton) and one carbonate ion. Since we suddenly have this extra proton in the water, the carbonate ion, CO3, now becomes a bicarbonate ion, HCO3-.
Therefore the free proton is now bound up and so the pH is not forced downwards.
Now lets say there is two protons but still the one carbonate ion. The bicarbonate ion is formed and then carbonic acid (H2CO3) is formed. But given a third free proton, one proton will still remain free and so the pH will go down. One other thing to note is that these ions are all whizzing around in the water, its only by chance that they meet each other.
For this you can start to see the importance of the alkalinity buffer. We need sufficient buffer not only to catch free protons but also sufficient numbers to increase the chance of the proton and the buffer ion bumping into each other.