Editing Urea (section)

===Historical preparation===
Urea was first noticed by [[Herman Boerhaave]] in the early 18th century from evaporates of urine. In 1773, [[Hilaire Rouelle]] obtained crystals containing urea from human urine by evaporating it and treating it with alcohol in successive filtrations.<ref>Rouelle (1773) [https://books.google.com/books?id=q1ATAAAAQAAJ&pg=PA451 "Observations sur l'urine humaine, & sur celle de vache & de cheval, comparées ensemble"] (Observations on human urine and on that of the cow and horse, compared to each other), ''Journal de Médecine, de Chirurgie et de Pharmacie'', '''40''' : 451–468. Rouelle describes the procedure he used to separate urea from urine on pages 454–455.</ref> This method was aided by [[Carl Wilhelm Scheele]]'s discovery that urine treated by concentrated [[nitric acid]] precipitated crystals. [[Antoine François, comte de Fourcroy]] and [[Louis Nicolas Vauquelin]] discovered in 1799 that the nitrated crystals were identical to Rouelle's substance and invented the term "urea."<ref>Fourcroy and Vauquelin (1799) [https://books.google.com/books?id=LsrBxKHCiAwC&pg=PA48 "Extrait d’un premier mémoire des cit. Fourcroy et Vaulquelin, pour servir à l’histoire naturelle, chimique et médicale de l’urine humaine, contenant quelques faits nouveaux sur son analyse et son altération spontanée"] (Extract of a first memoir by citizens Fourcroy and Vauquelin, for use in the natural, chemical, and medical history of human urine, containing some new facts of its analysis and its spontaneous alteration), ''Annales de Chimie'', '''31''' : 48–71. On page 69, urea is named "urée".</ref><ref>Fourcroy and Vauqeulin (1800) [https://books.google.com/books?id=0zhOAAAAcAAJ&pg=PA80 "Deuxième mémoire: Pour servir à l’histoire naturelle, chimique et médicale de l’urine humaine, dans lequel on s’occupe spécialement des propriétés de la matière particulière qui le caractérise,"] (Second memoir: For use in the natural, chemical and medical history of human urine, in which one deals specifically with the properties of the particular material that characterizes it), ''Annales de Chimie'', '''32''' : 80–112; 113–162. On page 91, urea is again named "urée".</ref> [[Berzelius]] made further improvements to its purification<ref>{{cite book| last = Rosenfeld | first = Louis | name-list-style = vanc |title=Four Centuries of Clinical Chemistry|url=https://books.google.com/books?id=KPX6Yvax9jkC&pg=PA41|date=1999|publisher=CRC Press|isbn=978-90-5699-645-1|pages=41–}}</ref> and finally [[William Prout]], in 1817, succeeded in obtaining and determining the chemical composition of the pure substance.<ref>{{cite journal| last = Prout | first = William | name-list-style = vanc |year=1817|url=https://books.google.com/books?id=3-kaAQAAMAAJ&pg=PA526 |title=Observations on the nature of some of the proximate principles of the urine; with a few remarks upon the means of preventing those diseases, connected with a morbid state of that fluid|journal=Medico-Chirurgical Transactions|volume=8|pages=526–549|pmc=2128986| pmid = 20895332 | doi = 10.1177/095952871700800123 }}</ref> In the evolved procedure, urea was precipitated as [[urea nitrate]] by adding strong nitric acid to urine. To purify the resulting crystals, they were dissolved in boiling water with charcoal and filtered. After cooling, pure crystals of urea nitrate form. To reconstitute the urea from the nitrate, the crystals are dissolved in warm water, and [[barium carbonate]] added. The water is then evaporated and anhydrous alcohol added to extract the urea. This solution is drained off and evaporated, leaving pure urea.