The addition of what chemical helps to reduce precipitation of salts on RO membranes?

The correct choice, sodium hexametaphosphate, is a polyphosphate compound that functions as a sequestering agent. It helps to stabilize calcium and magnesium ions in solution, preventing the formation of calcium carbonate and other scale-forming salts on reverse osmosis (RO) membranes. When these salts precipitate, they can cause fouling of the membranes, reducing the efficiency and lifespan of the RO system. By keeping these ions in a soluble form, sodium hexametaphosphate mitigates the risk of scaling, promoting better performance of the RO membranes.

In contrast, the other chemicals mentioned do not play the same role in controlling this precipitation effectively. Chlorine can be useful for disinfection but is harmful to RO membranes and does not help with scaling. Calcium carbonate is a scale-forming compound itself and would exacerbate the issue of precipitation rather than help prevent it. Sodium bicarbonate can influence carbonate chemistry but does not effectively prevent scaling on RO membranes like sodium hexametaphosphate does. Therefore, sodium hexametaphosphate is specifically beneficial for maintaining membrane integrity and operational efficiency in water treatment processes.