3 - The Quantitative Analysis of Carbon Compounds and the Determination of Physical Constants and Molecular Weights

Abstract

The simultaneous determination of carbon and hydrogen is the most frequently demanded procedure in an organic analysis. It is always carried out by burning a weighed sample and measuring the carbon dioxide and water produced. Determination of carbon by a digestion process is most often applied to biological samples and explosives, and is also valuable for isotopic determinations; of course hydrogen can not be determined in this way. Nitrogen in organic compounds is usually determined by the Dumas or Kjeldahl method, while oxygen is determined by Unterzaucher's method. The “oxygen flask” method of combustion is now almost universally employed for determinations of halogens and sulfur. The classical method for phosphorus involves wet combustion with mixed acids in a Kjeldahl flask, followed by gravimetric determination as ammonium phosphomolybdate. The oxygen flask method is not suitable for the determination of arsenic, which tends to alloy with the platinum sample holder. The defect can be avoided by using special platinum or silica. During the last ten years, organic functional group analysis has been developed extensively. Determination of physical constants, such as boiling and melting points, and molecular weight by ebullioscopic, cryoscopic, vaporimetric, and isopiestic methods, as well as chromatography in its various forms are now indispensable to the organic chemists.