Caswell No. 484; Etching acid; HFA; Fluorohydric acid; Fluoric acid; Hydrofluoric acid; Aqueous hydrogen fluoride; EPA Pesticide Code: 45601
Catalyst, reactive solvent; etching glass and enamel; production of refrigerants, herbicides, plastics, and electrical components
Hydrofluoric acid (HF) is an aqueous solution of hydrogen fluoride gas.
At room temperature, it appears as colorless transparent to light yellow smoke liquid with pungent odor. It has a specific gravity of 0.98 which is slightly lighter than water. It has a boiling point of 19.4 °C, being highly volatile. It can release white smoke once placed in air. The aqueous solution containing less than 60% hydrogen fluoride appears as a colorless clear fuming liquid. Industrial products are usually aqueous solution containing 40 to 45% HF.
It has pungent odor. It can react with sulfur trioxide or chlorosulfonic acid to generate fluoride sulfonic acid, and can react with halogenated aromatic hydrocarbons, alcohols, olefins, hydrocarbons to generate fluorine-containing organic compound. When being dissolved in water, it can produce highly corrosive acid, being medium-intensity acid. It is extremely smelly, being very toxic and is prone to cause ulceration when get touch with the skin with a severe extent being larger than any acids.
If inhaled of its vapor, it can have fatal effects, thus strict attention should paid during usage.
Hydrofluoric acid can also react with general metals, metal oxides, and hydroxide, generating a variety of metal fluoride salts, but the effect is not as dramatic as hydrochloric acid. Gold, platinum, lead, paraffin and some plastics (polyethylene, etc.) does not react with it and thus being able to be used as containers.
It can erode glass and silicate to produce gaseous silicon tetrafluoride. The reaction is as follows: SiO2 + 4HF → H2O + SiF4 ↑, glass is a silicon compound so the hydrofluoric acid can’t be put into glass containers.Hydrofluoric acid can form acid salt, hydrofluoric acid is a monobasic acid, but can produce a series of acid salts such as NaHF2, KHF2, NH4HF2, which are the other three kinds of halogen acid.
The weak acidity of hydrofluoric acid; because of the strong binding capability of hydrogen atom with fluorine atom, hydrofluoric acid can’t be completely dissociated in the water. In the hydrohalic acid, only hydrofluoric acid is weak acid (its ionization constant is 3.5 × 10 ^-4; its apparent ionization degree is about 10% at a concentration of 0.1mo1.L^-1, therefore, HF can reluctantly taken as a kind of strong acid. The ionization degree at high concentration is higher than that at low concentration. This property is different from other kinds of general weak electrolytes.