The analysis of organic compounds for carbon and hydrogen continues to be among those in greatest demand in many petroleum and petrochemicals research laboratories. Increasing demands for carbon and hydrogen analyses in this laboratory necessitated a search for a faster, more accurate and more stable method than the conventional semimicro gravimetric method used for the past several years.
Lack of a temperature and humidity-controlled balance room, plus the inherent difficulties associated with weighing micro absorbers influenced our decision against the rapid microgravimetric methods, such as those of CLARK 1 and ROBERTSON et al. 2. WHITE et al. 3 reported excellent accuracy and speed using an automatic macrocombustion-gravimetric method, but the necessary 0.2-g sample is frequently not available in our laboratory. UNTERZAUCHER 4 and JOHANSSON 5 have published methods in which the water and carbon dioxide from a conventional microcombustion are determined volumetrically.
NAUGHTON AND FRODYMA 6, and SCH6NIGER 7 have described methods in which the water and carbon dioxide from a combustion are measured manometrically. Although their methods do not offer any substantial improvements in accuracy over the gravimetric procedure, they do feature the desired speed and the potential for good stability and accuracy. Each of these methods involves the use of a two-liquid manometer. Observations made in this laboratory during use of a two-liquid manometer indicate several reasons for variations in results. In order to correct for these variations, major innovations were made in the manometer system. The two-liquid manometer was abandoned, and an entirely new mercury manometer designed which is immersed in a bath maintained at a constant elevated temperature.
In the method finally developed, combustion of the sample is accomplished in a modified Pregl apparatus which utilizes an automatically controlled gas sample heater. Water and carbon dioxide are collected in dry ice and liquid nitrogen traps, respectively, and selectively vaporized into an evacuated manometer immersed in a constant temperature bath. The observed pressure of each component is referred to a calibration curve to determine the amounts of carbon and hydrogen present. Nitrogen oxides are removed from the combustion gases by adsorption on activated manganese dioxide in an external scrubber.