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Carbonate alkalinity measures the amount of negatively charged carbonate and bicarbonate atoms in a solution. Bicarbonate ions consist of a hydrogen atom, a carbon atom, and three oxygen atoms, and have a charge of -1. Carbonate ions have the same structure as bicarbonate ions, but without the hydrogen atom, and they have a charge of -2. Both ions are basic, so they can neutralize acids if placed in an acidic solution at the right concentrations. They are often found together in their most common applications, but they are not always, and an equilibrium between them is not required in a given solution.
Measuring carbonate alkalinity is important when preserving the purity and integrity of a body of water, so it is especially important in environmental contexts. When the water's pH is above 8.3, its alkalinity tends to come from carbonate ions, and below that threshold the alkalinity usually comes from bicarbonate ions. If water has high alkalinity, it can resist changes in pH and remain relatively neutral, but if it has low alkalinity, its pH has the potential to drop very fast. As the pH gets lower, the number of carbonate and bicarbonate ions drops, until about pH 4.5 when all the ions are gone. The exact alkalinity of a body of water is not as important as being within an acceptable range, and this range can vary depending on the body of water being monitored.
A more common context in which carbonate alkalinity is measured is to monitor the pH of swimming pools. Alkalinity is closely linked to pH, so when the water testing kits measure pH, they are also indirectly measuring alkalinity. In this case, there is always an equilibrium between carbonate and bicarbonate ions, although there tend to be more bicarbonate ions than carbonate ions in pool water, since the pH of pool water often hovers quite close to the neutral pH of 7.
Alkalinity is usually measured in milligrams per liter of calcium carbonate, which is a calcium ion bound to a carbonate ion. Carbonate alkalinity also has an application in determining water hardness. Higher carbonate alkalinity usually translates to harder water. Despite this property, carbonate ions are actually also quite useful for purifying water. Carbonate and bicarbonate ions can bind to toxic metals in water and precipitate them out of the water as solids, acting as a natural cleaning system and giving the water somewhat of a resistance to certain types of pollution.