Acetic acid

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Acetic acid
GAA.JPG
Nearly glacial acetic acid
Acetic acid.png
Structure of acetic acid
Names
IUPAC name
Acetic acid
Systematic IUPAC name
Ethanoic acid
Properties
CH3COOH
Molar mass 60.053
Appearance Colorless liquid
Odor Pungent, vinegar-like
Density 1.049
Melting point 16 to 17 °C (61 to 63 °F; 289 to 290 K)
Boiling point 118 to 119 °C (244 to 246 °F; 391 to 392 K)
Miscible
Acidity (pKa) 4.76
Viscosity 1.22
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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.

Properties

Physical

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.

Chemical

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 superbases (e.g., organolithium reagents), it can be doubly deprotonated to give LiCH2CO2Li. Reduction of acetic acid gives ethanol.

When heated above 440 °C (824 °F), acetic acid decomposes to produce methane and carbon dioxide, or ketene and water:

CH3COOH → CH4 + CO2
CH3COOH → CH2CO + H2O

Acetic acid is mildly corrosive to metals including magnesium, iron and zinc, forming hydrogen gas and metal acetates.

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

Because aluminium forms a passivating acid-resistant film of aluminium oxide, aluminium tanks are used to transport acetic acid.

Availability

Stop bath.jpg

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.

Preparation

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.

Projects

Handling

Safety

Diluted acetic acid, aka vinegar, is irritant to nose and mouth, while glacial acetic acid is corrosive. Otherwise it is not particularly toxic.

Storage

Storage for glacial or concentrated acetic acid should be done in closed bottles, away from any rust susceptible metals, best in an acid cabinet.

Disposal

Acetic acid can be neutralized with a base or a carbonate, such as calcium carbonate.

References

Relevant Sciencemadness threads