First of all, it is important to realize that temporary hardness has little to do with metal ions such as calcium. Strictly speaking, hardness refers to the concentration of calcium and magnesium ions in water. We have traditionally thought only of total and calcium hardness, but there are two other ways to consider water hardness. Permanent hardness is that which cannot be removed by boiling.
The Concentration of Ions
Temporary hardness is that which can be removed by boiling. When temporary hardness precipitates, most is attributable to calcium carbonate. Boiling such water causes the bicarbonate ion to become a carbonate ion, which then binds with calcium. Calcium carbonate is almost insoluble, so it precipitates, thus removing both calcium and carbonate from the water. Because carbonate is alkaline in character, the alkalinity of the water decreases at the same time.
Determining the concentration of temporary hardness is complicated because its concentration is a function of the concentration of alkaline ions in relation to their reaction with calcium.
How Does Alkalinity Fit Into the Equation?
Alkalinity exists in three forms, carbonate (CO32-), bicarbonate (HCO3-) and hydroxide (OH-). We rarely encounter hydroxide alkalinity in cooling tower applications since it exists only when added or when pH exceeds 10. Carbonate and bicarbonate alkalinity freely change from one form to the other depending on the pH of the water. Carbonate ions are scarce in low-pH water. Raise the pH of that water and some of the bicarbonate ions become carbonate ions.
Bicarbonate & Calcium Ions
One important difference is that bicarbonate ions are considerably more soluble than carbonates. Calcium has only limited solubility with carbonate, but it is considerably more soluble with bicarbonate. If the pH is less than 10, it is safe to assume that the hydroxide alkalinity is zero. Under this condition, the concentration of carbonate alkalinity equals two times the phenolphthalein alkalinity, or C = 2P. Additionally, the concentration of bicarbonate alkalinity equals the total alkalinity minus two times the phenolphthalein alkalinity, or B = M – 2P. It also is helpful to understand that total alkalinity equals bicarbonate alkalinity plus carbonate alkalinity, or M = B + C.
What Did That Mean?
The concentration of temporary hardness depends on the ratio of bicarbonate to carbonate alkalinity. As cooling tower water cycles up, the alkalinity and pH both increase. As this happens, the ratio of carbonate to bicarbonate increases. This causes the solubility of calcium to decrease accordingly. For every number of cycles of concentration, a specific ratio of carbonate to bicarbonate ions exists. This ratio depends on pH and alkalinity, not hardness. The concentration of temporary hardness can be defined as the concentration in parts per million of carbonate alkalinity that precipitates in order to return the water to the equilibrium at which no more calcium precipitates.
We Don’t Boil Tower Water, So How Does it Apply to Us?
Boiling is not the only action that can remove temporary hardness. Boiling simply converts very soluble bicarbonate ions to less soluble carbonate ions that then precipitate as calcium carbonate. Chardon’s PowerPure technology removes temporary hardness in its reaction chamber by precipitating calcium as calcium carbonate. Removing the carbonate in the form of calcium carbonate leaves bicarbonate ions which are quite soluble.
This permits a significantly higher concentration of calcium to exist in the solution because the carbonates are removed. Remember that the ratio of temporary to permanent hardness depends entirely on the ratio of carbonate to bicarbonate alkalinity. In other words, temporary hardness is removed when calcium carbonate forms in the reaction chamber. The removal of temporary hardness as calcium carbonate shifts the Saturation Index to a point where the remaining, or permanent hardness remains soluble.
Matt Welsh is the Vice President and Water Consultant at Chardon Labs. He helps consult a wide range of customers utilizing various methods of water treatment, from chemical to chemical-free approaches, large and small applications, and across a wide range of geographical influences. With 20 years of water treatment experience, including a wide range of troubleshooting and service in potable water and non-potable HVAC and industrial applications, he is an expert in water treatment chemistry for cooling towers, boilers, and closed-loop systems.