ACIDS

Like the other classes of organic compounds, the organic acids may be arranged in homologous series. One of the most important of these series is the fatty-acid series, the name having been given to it because the derivatives of certain of its members are constituents of the fats. Some of the most important members of the series are given in the following table. They are all monobasic, and this fact is expressed in the formulas by separating the replaceable hydrogen atom from the rest of the molecule:

H·CHO₂	formic acid, a liquid boiling at 100°.
H·C₂H₃O	acetic acid, a liquid boiling at 118°.
H·C₃H₅O₂	propionic acid, a liquid boiling at 140°.
H·C₄H₇O₂	butyric acid, a liquid boiling at 163°.
H·C₁₆H₃₁O₂	palmitic acid, a solid melting at 62°.
H·C₁₈H₃₅O₂	stearic acid, a solid melting at 69°.

Formic acid (H·CHO₂). The name "formic" is derived from the Latin formica, signifying ant. This name was given to the acid because it was formerly obtained from a certain kind of ants. It is a colorless liquid and occurs in many plants such as the stinging nettles. The inflammation caused by the sting of the bee is due to formic acid.

Acetic acid (H·C₂H₃O₂). Acetic acid is the acid present in vinegar, the sour taste being due to it. It can be prepared by either of the following methods.

1. Acetic fermentation. This consists in the change of alcohol into acetic acid through the agency of a minute organism commonly called mother of vinegar. The change is represented by the following equation:

C₂H₅OH + 2O = HC₂H₃O₂ + H₂O.

The various kinds of vinegars are all made by this process. In the manufacture of cider vinegar the sugar present in the cider first undergoes alcoholic fermentation; the resulting alcohol then undergoes acetic fermentation. The amount of acetic acid present in vinegars varies from 3 to 6%.

2. From the distillation of wood. The liquid obtained by heating wood in the absence of air contains a large amount of acetic acid, and this can be separated readily in a pure state. This is the most economical method for the preparation of the concentrated acid.

Acetic acid is a colorless liquid and has a strong pungent odor. Many of its salts are well-known compounds. Lead acetate (Pb(C₂H₃O₂)₂) is the ordinary sugar of lead. Sodium acetate (NaC₂H₃O₂) is a white solid largely used in making chemical analyses. Copper acetate (Cu(C₂H₃O₂)₂) is a blue solid. When copper is acted upon by acetic acid in the presence of air a green basic acetate of copper is formed. This is commonly known as verdigris. All acetates are soluble in water.

Butyric acid (H·C₄H₇O₂). Derivatives of butyric acid are present in butter and impart to it its characteristic flavor.

Palmitic and stearic acids. Ordinary fats consist principally of derivatives of palmitic and stearic acids. When the fats are heated with sodium hydroxide the sodium salts of these acids are formed. If hydrochloric acid is added to a solution of the sodium salts, the free palmitic and stearic acids are precipitated. They are white solids, insoluble in water. Stearic acid is often used in making candles.

Acids belonging to other series. In addition to members of the fatty-acid series, mention may be made of the following well-known acids.

Oxalic acid (H₂C₂O₄). This is a white solid which occurs in nature in many plants, such as the sorrels. Its ammonium salt ((NH₄)₂C₂O₄) is used as a reagent for the detection of calcium. When added to a solution of a calcium compound the white, insoluble calcium oxalate (CaC₂O₄) precipitates.

Tartaric acid (H₂·C₄H₄O₆). This compound occurs either in a free state or in the form of its salts in many fruits. The potassium acid salt (KHC₄H₄O₆) occurs in the juice of grapes. When the juice ferments in the manufacture of wine, this salt, being insoluble in alcohol, separates out on the sides of the cask and in this form is known as argol. This is more or less colored by the coloring matter of the grape. When purified it forms a white solid and is sold under the name of cream of tartar. The following are also well-known salts of tartaric acid: potassium sodium tartrate (Rochelle salt) (KNaC₄H₄O₆), potassium antimonyl tartrate (tartar emetic) (KSbOC₄H₄O₆).

Cream of tartar baking powders. The so-called cream of tartar baking powders consist of a mixture of cream of tartar, bicarbonate of soda, and some starch or flour. When water is added to this mixture the cream of tartar slowly acts upon the soda present liberating carbon dioxide in accordance with the following equation:

KHC₄H₄O₆ + NaHCO₃ = KNaC₄H₄O₆ + H₂O + CO₂.

The carbon dioxide evolved escapes through the dough, thus making it light and porous.

Citric acid (H₃·C₆H₅O₇). This acid occurs in many fruits, especially in lemons. It is a white solid, soluble in water, and is often used as a substitute for lemons in making lemonade.

Lactic acid (H·C₃H₅O₃). This is a liquid which is formed in the souring of milk.

Oleic acid (H·C₁₈H₃₃O₂). The derivatives of this acid constitute the principal part of many oils and liquid fats. The acid itself is an oily liquid.