The heats of combustion of phenol and the three cresols
Precise measurements of the thermodynamic properties of phenol and some of its homologues are being made in this Laboratory. As part of this programme, the heats of combustion of phenol and the three cresols (methylphenols) have been accurately determined and are here reported; the heats of combustion of the six xylenols (dimethylphenols) will be reported elsewhere. The combustion calorimetry of the phenols involves the difficulty that these compounds are somewhat hygroscopic and are prone to oxidation by air or oxygen. Addi tionally, phenol and i ts lower homologues, although most are solids at 25°, have vapour pressures at 25° which are too great to permit their being placed unenclosed within a combustion bomb, when work of high precision is attempted. These problems were all overcome in the present work by sealing pellets of the solid phenols inside polyethylene bags, which were then placed inside the crucible of the combustion bomb. Experiments on m-cresol, a viscous liquid at 25°, were also conducted with samples sealed in polyethylene bags, in order to make the conditions of measurement closely comparable with those adopted for the solid phenols. The polyethylene bags were reetangular (25 mm X 30 mm; average mass 0·09 g), and were made from polyethylenefilm 0·05 mm thick. To make the edges of the bag, two surfaces of the film were sealed together by means of a low-temperature soldering iron having f1. wheel-and-axle bit. Pellets of a solid phenol were prepared in a " dry-box " and were placed inside a weighed open-ended polythene bag; in the case of m-cresol, the liquid was pipetted into a weighed bag. The bag was then sealed, and the whole weighed. The bag and contents were placed inside the platinum crucible of the bomh1• 2, and platinum fuse wire, connected to the bomb's firing circuit, was placed in contact with the top surface of the bag. With this arrangement, passage of a current through the fuse wire always ignited the bag and its contents, no kindling material other than the bag being required. The general technique used in the combustion experiments has been described13• The energy equivalent of the calorimeter was determined electrically, and all temperature measurements were made with a platinum resistance thermometer. The energy of combustion, flU~, of the polyethylenefilm was measured as -46427 ± 9 Jfg. The samples ofthe phenols used were prepared in this Laboratory; sample purities were determined by a freezing point method and exceeded 99·9 moles per cent in every instance. Mean values for the standard heats of combustion, flH~, of phenol and the cresols are summarized in Table 1. The uncertainties