Which results are used to develop the Langelier saturation index?

• A. Alkalinity, pH, CaCO3 saturation and CO2
• B. Alkalinity, pH, CaCO3 saturation and CO2
• C. Alkalinity, pH, temperature, CaCO3 and total dissolved solids
• D. Alkalinity, pH, temperature, CaCO3 and MgCO3 saturation and CO2

Answer: (C)

Langelier Saturation Index estimates whether water will form scale by comparing the current pH to the pH at which calcium carbonate would be at saturation given the water’s carbonate chemistry. To do this well, you need five inputs: alkalinity (which buffers pH and sets the amount of carbonate species available), pH (the actual state of the carbonate system), temperature (CaCO3 solubility changes with temperature), calcium concentration expressed as CaCO3 hardness (the calcium that combines with carbonate to form CaCO3), and total dissolved solids (which approximate ionic strength and influence activity coefficients in the carbonate system). With these, you derive the saturation pH and determine if the water is undersaturated (tends to dissolve CaCO3), at equilibrium, or supersaturated (tends to form scale). The correct set uses alkalinity, pH, temperature, CaCO3 (calcium hardness), and total dissolved solids, rather than adding separate CO2 or magnesium saturation terms.