KH is a measurement of water’s chemical stability, often confused with General Hardness (GH), which measures different minerals dissolved in the water. KH is an abbreviation derived from the German term Karbonathärte, which translates directly to Carbonate Hardness. This measurement is an important indicator of the water’s acidity or alkalinity, crucial for managing water systems like aquariums or ponds.
What Carbonate Hardness Measures
Carbonate Hardness (KH) specifically measures the concentration of carbonate (\(\text{CO}_3^{2-}\)) and bicarbonate (\(\text{HCO}_3^{-}\)) ions dissolved in the water. These two ions are the primary components of total alkalinity in freshwater systems. The presence of these specific ions defines KH, distinguishing it from General Hardness (GH).
General Hardness, in contrast, measures the concentration of positively charged metal ions, primarily calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)). While these ions contribute to the overall mineral content that makes water “hard,” they do not provide the same chemical function as the carbonate and bicarbonate ions. KH is an independent measure; it is possible for water to have a high KH but a low GH.
The Role of KH in Maintaining pH Stability
The primary function of carbonate and bicarbonate ions is to act as a buffer, resisting changes in pH. Water naturally tends to become more acidic over time due to processes like the breakdown of organic waste and the dissolution of carbon dioxide. Without a buffering system, this constant introduction of acid would cause rapid, drastic drops in pH, which can be harmful to aquatic life.
The bicarbonate ion (\(\text{HCO}_3^{-}\)) works by neutralizing excess hydrogen ions (\(\text{H}^{+}\)), which increase acidity. When an acid is introduced, the bicarbonate reacts with the hydrogen ions to form carbonic acid (\(\text{H}_2\text{CO}_3\)). This chemical reaction neutralizes the acid, preventing a sudden downward swing in pH.
A high KH level means the water has a large reserve of these ions, giving it a strong capacity to neutralize acid. Conversely, a low KH suggests a weak buffer, making the water susceptible to a rapid and potentially dangerous pH crash. Monitoring KH ensures the water’s pH remains stable, even as internal processes attempt to change it.
Measuring and Interpreting KH Levels
Carbonate Hardness is typically measured using liquid titration test kits or test strips. Titration kits involve adding a reagent drop by drop to a water sample until the color changes. The results are most commonly expressed in two units: degrees of Carbonate Hardness (dKH) or parts per million (ppm).
The unit dKH, which stands for degrees of Karbonathärte, is a traditional German unit. To convert between the two common units, one degree of dKH is equivalent to approximately 17.9 ppm. A moderate KH level in a freshwater system is generally considered to be between 4 and 8 dKH, or about 70 to 140 ppm.
A reading below 4 dKH indicates soft water with a low buffering capacity, meaning the pH is highly volatile and could drop suddenly. Water with a high KH, such as above 10 dKH (180 ppm), is considered very well-buffered and strongly resists attempts to lower the pH.
Methods for Adjusting Carbonate Hardness
Adjusting KH involves adding or removing the carbonate and bicarbonate ions. To raise KH, the most common method is the controlled addition of sodium bicarbonate, often known as baking soda. Sodium bicarbonate (\(\text{NaHCO}_3\)) is effective because it directly introduces bicarbonate ions without significantly altering General Hardness. Specialized commercial buffer salts are also available.
To lower a high KH level, the most reliable approach is to remove the dissolved carbonate and bicarbonate ions entirely. This is achieved using highly purified water sources, such as reverse osmosis (RO) water or distilled water. Mixing RO water with existing tap water effectively dilutes the concentration of carbonates. Filtration media like peat moss can also be used, as it releases organic acids that consume the carbonate buffer.