1. General Discussion
Calcium carbonate (CaCO3) saturation indices are commonly used to evaluate a water’s scale-forming and scale-dissolving tendencies to help prevent CaCO3 scaling in piping and equipment (e.g., industrial heat exchangers or domestic water heaters). The indices can be applicable in reducing corrosion in iron, steel, and cement piping, but not in controlling lead and copper release.
Water oversaturated with CaCO3 tends to precipitate CaCO3. Water undersaturated with CaCO3 tends to dissolve it. Saturated water (i.e., water in equilibrium with CaCO3) tends neither to precipitate nor dissolve CaCO3. Saturation is the dividing line between where precipitation tends to occur (likely) or not occur (not likely). Dissolved total alkalinity (Section 2320), total calcium (Section 3500-Ca), pH (Section 4500-H+), and temperature (Section 2550) must be measured to calculate the CaCO3 saturation indices described here. Ionic strength also must be calculated or estimated from total dissolved solids (TDS) (Section 2540 C) or conductivity (Section 2510) measurements.
Measure pH at the system’s water temperature using a temperature-compensated pH meter. If pH is measured at a different temperature in the laboratory, for example, correct the measured pH.1–7 When measuring pH, minimize CO2 exchange between sample and atmosphere. Ideally, seal the sample from the atmosphere during measurements.8 At a minimum, avoid vigorously stirring unsealed samples.
There are two general categories of CaCO3 saturation indices:
• indices that determine whether a water has a tendency to precipitate or to dissolve CaCO3, and
• indices that estimate the quantity of CaCO3 that can be precipitated or dissolved.