Acetoacetic acid

Acetoacetic acid is the organic compound with the formula CH3C(O)CH2CO2H. It is the simplest beta-keto acid group and like other members of this class is unstable.

Synthesis and properties
Acetoacetic acid is a weak acid (like most alkyl carboxylic acids) with a pKa of 3.77. It be prepared by the hydrolysis of the ethyl acetoacetate followed by acidification of the anion. In general, acetoacetic acid is generated at 0 °C and used in situ immediately. It decomposes at a moderate rate to acetone and carbon dioxide:
 * CH3C(O)CH2CO2H →  CH3C(O)CH3  +  CO2

The acid form has a half-life of 140 minutes at 37º C in water, whereas the basic form (the anion) has a half-life of 130 hours. That is, it reacts about 50 times more slowly.

Biochemistry
Acetoacetic acid is produced in the human liver under certain metabolic conditions such as during fatty acid breakdown. It is then partially converted to acetone by spontaneous decarboxylation and excreted either in urine or through respiration. The acid is also present in the metabolism of those undergoing starvation or prolonged physical exertion as part of gluconeogenesis. Acetoacetic acid is not the major ketone produced by the body, that being beta-hydroxybutyrate.

Detection
When ketone bodies are measured by way of urine concentration, acetoacetic acid, along with beta-hydroxybutyric acid (BHB), and acetone, is what is detected. This is done using dipsticks coated in nitroprusside or similar reagents. Nitroprusside changes from pink to purple in the presence of acetoacetate, the conjugate base of acetoacetic acid, and the colour change is graded by eye. The popular dipstick used to detect ketone bodies in urine "Ketostix" by Bayer, only detects acetoacetate, not BHB or acetone.