What is the formula for total hardness (TH) in water treatment?

Total hardness (TH) in water treatment is primarily defined as the concentration of divalent cations in water, which are responsible for the hardness properties of water. The two most significant contributors to total hardness are calcium ions (Ca²⁺) and magnesium ions (Mg²⁺).

The formula for total hardness reflects this contribution, with total hardness calculated as the sum of the concentrations of these two ions. Calcium and magnesium are prevalent in many natural water sources and their presence leads to scaling and other issues in water systems, making it essential to understand their combined effect on water quality.

Hardness is often measured in terms of milligrams per liter (mg/L) of calcium carbonate (CaCO₃) equivalent, emphasizing the practical impacts of these ions on the water's characteristics. Therefore, the formula indicating that total hardness is quantified as the sum of calcium and magnesium is fundamentally correct for water treatment applications.

The other choices involve ions that do not contribute to the total hardness in the same significant way or combine ions that do not represent hardness accurately. For instance, sodium (Na⁺) and potassium (K⁺) are typically associated with total dissolved solids (TDS) rather than hardness, while bicarbonate (HCO