Fluorosilicic acid, also called hydrofluorosilicate, is an industrial byproduct with applications in public health, hygiene and arts and crafts. It comes from phosphorite rock used in the manufacturing of phosphate fertilizer.
Fluorosilicic acid preserves fiber strength and sets acid dyes to prevent fabric discoloration. Businesses including hotels and hospitals use fluorosilicic acid as a laundry sour, a cleaning agent added during rinsing to break down residual bleach and remove sodium bicarbonate, a chemical compound that can damage fabrics during ironing or pressing. The acid also dissolves iron and other metallic salts that cause rust stains, graying or yellow discoloration. Laundry sours containing fluorosilicic acid aren’t available for home laundries.
Tanners use fluorosilicic acid to cure and preserve animal hides and skins. It also makes fluorosilicate or fluoride salts which are used to make industrial compounds and metals and to help cool nuclear reactors. The compound is added to cement and applied to wood to preserve both materials, and it’s used in the manufacture of ceramics, glass and paints. Brewing and bottling plants use fluorosilicic acid to sterilize factory equipment. It’s also used in electroplating and glass etching, and to refine lead and acidize oil wells.
Fluorosilicic acid has been the main additive for water fluoridation in the United States since the 1950s. Combined with a related compound called hexafluorosilicate, the acid makes up fluoride mouth rinses. As of 2001, municipalities across the nation added silicofluorides to more than 9,200 water treatment systems serving more than 120 million Americans, according to the Centers for Disease Control and Prevention. Fluoride makers change fluorosilicic acid from its water-based solution into powdered derivatives called sodium fluorosilicate(F6Na2Si, CAS No. 16893-85-9) and sodium fluoride, which suppliers add to drinking water.
However, the acid’s role in water fluoridation is controversial. Scientists at Dartmouth College found in 2001 that children exposed to drinking water containing the acid have higher levels of lead in their blood. Recent studies from the U.S. Environmental Protection Agency found that too much of the additive can cause brittle bones as well. Exposure to high levels of fluorosilicic acid vapor or mist in factories can burn the eyes and cause coughing, chest pain and breathing difficulties. But the U.S. Department of Health and Human Services says the amount of fluorosilicic acid in drinking water is too minimal to cause health problems.