Spectrophotometric characterization of some analgesics and antipyretics.
The dissociation constants of protonated 1,3,7-trimethylxanthine (caffeine), 1,2-dihydro-1,5-dimethyl-4-(1-methylethyl)-2-phenyl-3H-pyrazol-3-one (propyphenazone), 3,3-diethyl-2,4-dioxotetrahydropyridine (pyrithyldione), N-(4-ethoxyphenyl)acetamide (phenacetin) and N-(4-hydroxyphenyl)-acetamide (paracetamol) have been determined by spectrophotometric measurements made over the range of hydrogen-ion concentration from H(0)-8 to PH 16. The values obtained were: caffeine, pK(1)-0.11 +/- 0.05; propyphenazone, pK(1) 0.91 +/- 0.02; pyrithyldione, pK(1) -3.75 +/- 0.05, pK(2) 11.56 +/- 0.05; phenacetin, pK(1) -1.35 +/- 0.03, pK(2) approximately 15-16; paracetamol, pK(1) -1.71 +/- 0.04, pK(2) 9.68 +/- 0.06. In basic solution caffeine was unstable and at pH 14 decomposed with a half-life of 54.6 min.
Djokić
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Pharmaceutical and Chemical Industry "Galenika", 29 November 111, 11000 Belgrade, Yugoslavia.
Dumanović
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Marković
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Muk
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