Murexide is a purple dye first
reported by Scheele (Karl Wilhelm, 1742-1786) in 1776. He obtained
uric acid from human calculi (calculus vesicae), hence his name for it:
lithic acid. Murexide is the purple product formed when uric acid is treated
successively with nitric acid and ammonia.
||William Prout (1785-1850), yes, he of Prout's hypothesis, studied it in 1818 [Philosophical Transactions of the Royal Society 1818, 108, 420; Annales of Philosophy 1818, XIV, 363]. He was also medical doctor and in 1821 published a book 'An Inquiry into the nature and treatment of Gravel, Calculus, and other diseases connected with a deranged operation of the urinary organs.' Rather than human calculi, he used boa constrictor excrement, which can contain 90% of ammonium acid urate. More about boas here and a humorous look at the problems associated with them here.||
Prout found that oxidation
of uric acid with nitric acid gave a colourless product (alloxan) which
he called erythric acid. The addition of ammonia gave a purple product
which he showed was an ammonium salt of another acid which he called purpuric
acid. The scarcity of materials hindered the widespread appeal of this
chemistry, but Prout suggested that ammonium purpurate may be useful as
a purple dye for wool.
The purple color formed is used as a test for uric acid
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