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An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids whose acidity is associated with their carboxyl group -COOH. Sulfonic acids, containing the group OSO₃H, are relatively stronger acids. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: -OH, -SH, enol group, and the phenol group. In biological systems organic compounds containing only these groups are not generally referred to as organic acids.

Generally, organic acids are weak acids and do not dissociate completely in water, whereas the strong mineral acids do. Lower molecular weight organic acids such as formic and acetic acids are miscible in water, but higher molecular weight organic acids such as benzoic acid are insoluble in molecular (neutral) form.

On the other hand, most organic acids are very soluble in organic solvents. p-toluenesulfonic acid is a comparatively strong acid used in organic chemistry often because it is able to dissolve in the organic reaction solvent.

Exceptions to these solubility characteristics exist in the presence of other substituents which affect the polarity of the compound.

Simple organic acids like formic or acetic acid are used for oil and gas well stimulation treatments.^{[citation needed]} These organic acids are much less reactive with metals than are strong mineral acids like HCl or mixtures of HCl and HF. For this reason, organic acids are used at high temperatures or when long contact times between acid and pipe are needed.

The conjugate bases of organic acids such as citrate and acetate are often used in biologically-compatible buffer solutions.

Biological systems create many and more complex organic acids such as L-lactic, citric and D-glucuronic acids that contain hydroxyl or carboxyl groups. Human blood and urine contain these plus organic acid degradation products of amino acids, neurotransmitters and intestinal bacterial action on food components. Examples of these categories are alpha-ketoisocaproic, vanilmandelic and D-lactic acids, derived from catabolism of L-leucine and epinephrine (adrenaline) by human tissues and catabolism of dietary carbohydrate by intestinal bacteria, respectively.

The general structure of a few weak organic acids. From left to right: phenol, enol, alcohol, thiol. The acidic hydrogen in each molecule is colored red.

The general structure of a few strong organic acids. From left to right: carboxylic acid, sulfonic acid. The acidic hydrogen in each molecule is colored red.

CH₃COOH (Acetic acid)

HCOOH (Formic acid)

C₆H₈O₇ (Citric acid)

C₂H₂O₄ (Oxalic acid)

• Acid-base extraction