Carbonate hardness/pH.... what is it?

The carbonic acid-bicarbonate (ion) buffer system

pH and carbonate hardness in aquariums - and in general when it comes to water chemistry -  can be seen as linked due to their overlapping relationship. Often when reading blogs, books, magazines, or talking to other hobbyists, people interchange terms like carbonate hardness and pH. The relationship is not that simple especially if chemistry isn't your favourite subjects (like most of us) but, I will have a go at explaining anyway.

What is pH ? It is the concentration of Hydrogen (H+) ions in a solution. The scale is logarithmic ( i.e. when you get a pH reading of 8 or 10 - you actually have a much lower concentration of free H+ ions then if you have a pH of 3.... sounds backwards.... that's science). The pH of tap water is around 7 which is considered neutral.

What is a Buffer ? In aquariums and other solutions they are substances which act as regulators for H+ ions i.e. they accept them or release them depending on their availability in the solution and also the concentration of the buffer. This causes them to buffer the pH at a certain value hence the name.

What are acids? they are substances which release hydrogen (H+) ions

What are Bases? they are substances which take up hydrogen (H+) ions

So how are they all linked? Consider a saltwater aquarium with a pH of 8.3 (High pH = basic). Carbonic acid is formed in a reversible reactions via the following reaction:

CO2 + H2O  <-->  H2CO3 (carbonic acid)  <-->  H + HCO3 (hydrogen and bicarbonate ions)

When the aquarium experiences an increase in hydrogen atoms (H+) the bicarbonate (or base) combines with the H+ to form carbonic acid H2CO3 (acid). The carbonic acid is relatively unstable and eventually turns into H2O and CO2. If there is a increase in hydroxide (-OH) ions in the aquarium the carbonic acid actually releases bicarbonate and water e.g.

OH (Hydroxide) + H2CO3 (carbonic acid)  <--> H2O (water) + HCO3 (bicarbonate ion)

So basically these two reactions keeps your pH stable.

So your aquarium - hopefully a buffered one - maintains a stable pH by using buffers which release or take up hydrogen (H+) and hydroxide (-OH) ions via carbonic acid or bicarbonate. When you measure the kH with your aquarium test kit your a detecting the available bicarbonate molecules. From my experience kH tends to be more important in saltwater aquariums because, the animals which live in these ecosystems require a higher pH and are very sensitive to changes in pH. Many of the commonly available freshwater species (from fish stores) can survive in water from the tap providing it has be been de-chlorinated i.e. they are happy with the neutral pH.

In the aquarium hobby it is quite common for people to add tap water directly into their freshwater or saltwater aquariums. This  tap water contains a neutral pH with very little buffering capacity i.e.  one of the reasons why it is dangerous to add it to saltwater aquariums.

Hope this info was helpful and not too confusing.....