Hydrochloric acid and sulfuric acid can be added to the influent water to reduce the pH. Sulfuric acid is more commonly used than hydrochloric acid because it is cheap, does not emit smoke and corrodes surrounding metal components, and the membrane has a higher removal rate of sulfate ions than chloride ions.

Industrial grade sulfuric acid without other additives is suitable for reverse osmosis, and commercial sulfuric acid has two concentration specifications: 20% and 93%. 93% sulfuric acid is also known as 66 Baume sulfuric acid. Be careful when diluting 93% sulfuric acid. When diluting to 66%, heating can raise the temperature of the solution to 138 ℃. It is necessary to slowly add acid to the water under stirring to avoid local heating and boiling of the aqueous solution.  

Hydrochloric acid is mainly used when scaling of calcium sulfate or strontium sulfate may occur. The use of sulfuric acid will increase the concentration of sulfate ions in reverse osmosis influent water, directly leading to an increase in the tendency of calcium sulfate scaling.

Industrial grade hydrochloric acid (without additives) is very convenient to purchase, and the general content of commercial hydrochloric acid is 30-37%. The primary purpose of reducing pH is to reduce the tendency of calcium carbonate scaling in RO concentrated water, that is, to reduce the Languerre index (LSI).

LSI is the saturation of calcium carbonate in low salinity brackish water, indicating the possibility of calcium carbonate scaling or corrosion. In reverse osmosis water chemistry, LSI is an important indicator for determining whether calcium carbonate scaling occurs. When the LSI is negative, water will corrode metal pipes, but it will not form calcium carbonate scaling.

If the LSI is positive, the water is not corrosive, but calcium carbonate scaling occurs. The LSI subtracts the actual pH of water from the pH saturated with calcium carbonate. The solubility of calcium carbonate decreases with increasing temperature (scale in a kettle is formed in this way), and decreases with increasing pH, calcium ion concentration, or alkalinity.  

The LSI value can be lowered by injecting acid solution (usually sulfuric acid or hydrochloric acid) into the reverse osmosis influent, i.e. lowering the pH value. The recommended LSI value for reverse osmosis concentrated water is 0.2 (indicating that the concentration is 0.2 pH units below the saturated calcium carbonate concentration). Polymer scale inhibitors can also be used to prevent calcium carbonate precipitation. Some scale inhibitor suppliers claim that their products can achieve an LSI of up to+2.5 for reverse osmosis concentrated water (a more conservative design is a LSI of+1.8).