Acetic acid (or ethanoic acid) is an organic compound with the chemical formula CH3COOH. It is a colorless liquid that when undiluted is called glacial acetic acid.
Vinegar is roughly 4%-8% acetic acid by volume, and its characteristic smell and taste is due to acetic acid. Although it is classified as a weak acid, concentrated acetic acid is corrosive and can attack the skin.
Acetic acid is a transparent, colorless liquid at room temperature with a slightly higher viscosity than water solidifying at about 10 °C and boiling at 117 to 118° C. It has a pungent odor and sour, acidic taste. Acetic acid is a polar protic solvent, similar to ethanol and water, but unlike water, acetic acid can dissolve not only polar compounds such as inorganic salts and sugars, but also non-polar compounds such as oils and elements such as sulfur and iodine. It readily mixes with other polar and non-polar solvents such as water, chloroform, and hexane. With higher alkanes (starting with octane), acetic acid is not completely miscible anymore, and its miscibility continues to decline with longer n-alkanes.
This dissolving property and miscibility of acetic acid makes it a widely used industrial chemical, for example, as a solvent in the production of dimethyl terephthalate.
Acetic acid undergoes the typical chemical reactions of a carboxylic acid. Upon treatment with a standard base, it converts to metal acetate and water:
- CH3COOH + MOH → CH3COOM + H2
With strong bases (e.g., organolithium reagents), it can be doubly deprotonated to give LiCH2CO2Li. Reduction of acetic acid gives ethanol.
- CH3COOH → CH4 + CO2
- CH3COOH → CH2CO + H2O
- Mg + 2 CH3COOH → Mg(CH3COO)2 + H2
- Fe + 2 CH3COOH → Mg(CH3COO)2 + H2
- Zn + 2 CH3COOH → Mg(CH3COO)2 + H2
When mixed with a sufficient oxidizing agent, usually hydrogen peroxide, acetic acid can be reacted directly with less reactive metals (such as copper) to yield an acetate, though more hydrogen peroxide will need to be added to continue the process. Copper acetate can be made this way. Acetates can also be prepared from acetic acid and an appropriate carbonate or hydoxide as in the popular reaction with baking soda:
- NaHCO3 + CH3COOH → CH3COONa + CO2 + H2O
While distilled vinegar is the most readily available form of acetic acid, due to its low concentration and the large amount of space it requires, other sources are needed for efficient use as a reagent. A good source of food grade acetic acid is vinegar essence which has a concentration between 20-80% acetic acid, depending on the brand and can be found in many stores, although in some countries it has become hard to find in recent years.
Acetic acid is also used in photographic stop baths, which consists of high concentrations of buffered acetic acid, often with a pH indicator included. These can be removed through distillation. Glacial acetic acid can be purchased from online suppliers for modest prices as well. It is also sold by some beekeeper stores.
Distilled vinegar is not distilled acetic acid. In the production of distilled vinegar, the starting ingredients are distilled, NOT the final product. There are significant amounts of organic contaminants in the product, which can be observed to decompose on heating. Sodium acetate made with baking soda and distilled vinegar and boiled to dryness tends to be brown, not clear white.
One of the most common syntheses for acetic acid discussed involves the acidification of an excess of an acetate salt to yield acetic acid. This somewhat impractical method requires the addition of concentrated sulfuric acid to anhydrous calcium acetate or sodium acetate followed by a second distillation over an anhydrous hygroscopic salt to remove water. The acetate salt in this method is easily obtained by the combination of household vinegar with a base and subsequent crystallization of the reacted solution, but this is a very tedious process for producing larger amounts of the acid.
Diluted acetic acid, aka vinegar, is irritant to nose and mouth, while glacial acetic acid is corrosive. Otherwise it is not particularly toxic.
Storage for glacial or concentrated acetic acid should be done in closed bottles, away from any rust susceptible metals, best in an acid cabinet.
Acetic acid can be neutralized with a base or a carbonate, such as calcium carbonate.
- Chemical Reagents Their Purity and Tests By E. Merck