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H. Bothe Institut für Physikalische Chemie Technische Universität Braunschweig 3300 Braunschweig Germany

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H. K. Cammenga Institut für Physikalische Chemie Technische Universität Braunschweig 3300 Braunschweig Germany

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Caffeine has been found to display a low-temperatureβ- and a high-temperatureα-modification. By quantitative DTA the following data were determined: transformation temperature 141±2°; enthalpy of transition 4.03±0.1 kJ·mole−1; enthalpy of fusion 21.6±0.5 kJ·mole−1; molar heat capacity
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\begin{array}{*{20}c} {{\vartheta \mathord{\left/ {\vphantom {\vartheta {^\circ C}}} \right. \kern-\nulldelimiterspace} {^\circ C}}} & {100(\beta )} & {100(\alpha )} & {150(\alpha )} & {100(\alpha )} \\ {{{C^\circ _\mathfrak{p} } \mathord{\left/ {\vphantom {{C^\circ _\mathfrak{p} } {J \cdot K^{ - 1} \cdot mole^{ - 1} }}} \right. \kern-\nulldelimiterspace} {J \cdot K^{ - 1} \cdot mole^{ - 1} }}} & {271 \pm 9} & {287 \pm 10} & {309 \pm 11} & {338 \pm 10} \\ \end{array}$$ \end{document}
in good accord with drop-calorimetric data. For the constants of the equation log (p/Pa)=−A/T+B, static vapour pressure measurements on liquid and solidα-caffeine, and effusion measurements on solidβ-caffeine yielded:
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\begin{array}{*{20}c} {A = 3918 \pm 37; 5223 \pm 28; 5781 \pm 35K^{ - 1} } \\ {B = 11.143 \pm 0.072; 13.697 \pm 0.057; 15.031 \pm 0.113} \\ \end{array}$$ \end{document}
. The evaporation coefficient ofβ-caffeine is 0.17±0.03.
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Journal of Thermal Analysis and Calorimetry
Language English
Size A4
Year of
Foundation
1969
Volumes
per Year
1
Issues
per Year
24
Founder Akadémiai Kiadó
Founder's
Address
H-1117 Budapest, Hungary 1516 Budapest, PO Box 245.
Publisher Akadémiai Kiadó
Springer Nature Switzerland AG
Publisher's
Address
H-1117 Budapest, Hungary 1516 Budapest, PO Box 245.
CH-6330 Cham, Switzerland Gewerbestrasse 11.
Responsible
Publisher
Chief Executive Officer, Akadémiai Kiadó
ISSN 1388-6150 (Print)
ISSN 1588-2926 (Online)

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