Q.: We see many references recommending dilution ratios for muriatic acid that's used for efflorescence removal, acid etching, and other surface preparation applications. The references imply that there's a standard muriatic-acid solution, but our construction-supply distributor offers muriatic acid in several concentrations. What is muriatic acid, and is there a standard concentration?

A.: In the building trades, muriatic acid has become the accepted trade name for hydrochloric acid. Commercial-grade muriatic acid is a mixture of acid and water that is commonly 31.45% acid by mass (20 Baumé).

Percent by mass is the most precise way of specifying acid strength. You can make a 2% solution by adding 1 gram of 20° Baumé acid for every 15 grams of water. But most field solutions are mixed by volume. In inch-pound units, adding 1/2 pint of 20° Baumé acid to a gallon of water produces an approximately 2% solution by mass.

As you noted, however, you can also buy muriatic acid that contains less than 31.45% acid by weight. The concentration should be printed on the container or on the Material Safety Data Sheet.

Even when you know the concentration of the muriatic acid you buy, it still may be difficult to decide how much acid to add to water for the correct dilution. Instructions we've seen in magazine articles and other publications can be confusing. One says to use a 1% muriatic-acid solution. Another says to add 1/2 pint muriatic acid (without giving the acid concentration) to 1 gallon of water, and yet another tells you to use 25% more acid than the standard ratio recommended by the manufacturer. It's probably safe to assume that when the acid concentration isn't stated, 20° Baumé is implied.

Use caution when working with muriatic acid. Use it only with adequate ventilation, and wear proper protective equipment such as goggles, acid-proof gloves, and a respirator. When diluting the acid, always pour it slowly into water. Adding water to acid can cause splattering.

Also be aware that chlorides cause corrosion of metals embedded in concrete. William G. Hime of Erlin, Hime Associates, Northbrook, Ill., doesn't recommend using muriatic acid on concrete with less than 1 inch of clear cover over rebar or on masonry walls where joint reinforcement is present.