What is the Langelier index used to determine the point of stability of?

The Langelier index is primarily used to evaluate the equilibrium of calcium carbonate in water. This metric allows for the determination of whether the water is in a state of saturation, undersaturation, or oversaturation regarding calcium carbonate. When water is saturated, it holds the potential to deposit calcium carbonate, which can lead to scaling and other issues in pipes and fixtures. Conversely, if the water is undersaturated, it can corrode pipes and equipment, leading to leaching of metals and other materials.

Understanding the Langelier index is crucial in maintaining a stable water supply system, as it helps water treatment operators adjust the composition of the water, ensuring it does not cause scaling or corrosion in the distribution system. The index is calculated using factors such as pH, temperature, total dissolved solids, and calcium hardness, all of which directly relate to the solubility of calcium carbonate. This makes the option concerning calcium carbonate the correct choice, as it is the specific focus of the Langelier index in assessing water stability.