Carbonate and bicarbonate ions are basic and can neutralize acids. Measuring carbonate alkalinity is important for maintaining water purity. Alkalinity comes from carbonate or bicarbonate ions depending on pH. Alkalinity is measured in milligrams per liter of calcium carbonate and can determine water hardness. Carbonate ions can also purify water by binding to toxic metals.
Alkalinity of carbonates measures the amount of negatively charged carbonate and bicarbonate atoms in a solution. Bicarbonate ions consist of one hydrogen atom, one 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 have a charge of -2. Both ions are basic, so they can neutralize acids when placed in an acidic solution at the right concentrations. They are often found together in their most common applications, but they are not always found together and a balance between them is not required in any given solution.
Measuring carbonate alkalinity is important in maintaining the purity and integrity of a water body, so it is especially important in environmental settings. When the pH of water is above 8.3, its alkalinity tends to come from carbonate ions, and below that, alkalinity usually comes from bicarbonate ions. If the water has high alkalinity, it can resist pH changes and remain relatively neutral, but if it has low alkalinity, its pH has the potential to drop very quickly. As the pH drops, the number of carbonate and bicarbonate ions decreases, until around pH 4.5, when all the ions are gone. The exact alkalinity of a body of water is not as important as whether it is 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 monitoring the pH of swimming pools. Alkalinity is closely related to pH, so when water test kits measure pH, they are also indirectly measuring alkalinity. In this case, there is always a balance between carbonate ions and bicarbonate ions, although there tend to be more bicarbonate ions than carbonate ions in the pool water, as the pH of the pool water often hovers quite close to neutral pH of 7.
Alkalinity is usually measured in milligrams per liter of calcium carbonate, which is a calcium ion bonded to a carbonate ion. Carbonate alkalinity also has an application in determining the hardness of water. The high alkalinity of carbonates usually results in harder water. Despite this property, carbonate ions are actually quite useful for purifying water as well. Carbonate and bicarbonate ions can bind to toxic metals in the water and cause them to precipitate out of the water as solids, acting as a natural cleaning system and giving the water some resistance to certain types of pollution.
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