Gravimetric and Combustion Analysis

Layers of rock exposed in the Grand Canyon by the erosive action of the Colorado River provide a window on a billion years Of Earth s history. /From J. Grotzinger. T. H. Jordan, F Press, and R. Siever, Understanding Earth. 5th ed. (New York  

In the 1800s, geologists understood that new layers (strata) of rock are deposited on top of older layers. Characteristic fossils in each layer helped geologists to identify strata from the same geologic era all around the world. However, the actual age of each layer was unknown. 

In 1910, Arthur Holmes, a 20-year-old geology student at Imperial College in London, realized that radioactive decay could be used to measure the age of a rock. Physicists had discovered that U decays with a half life of 4.5 billion years to eight atoms of He and suspected that the final product was Pb. Holmes conjectured that when a U-containing mineral crystallized, it should be relatively free of impurities. Once the mineral solidified, Pb would begin to accumulate. The ratio Pb/U is a clock giving the age of the mineral. 

Holmes isolated U minerals from a Devonian -age rock. He measured the U contenl by the rate of production of radioactive Rn gas, which is a decay product along the way to Pb. To measure Pb, he fused (Section 28-2) each mineral in borax, dissolved the fused mass in acid, and quantitatively precipitated milligram quantities of PbS04. The nearly constant ratio Pb/U = 0.046 g/g in 15 minerals was consistent with the hypotheses that Pb is the end product of radioactive decay and that little Pb had been present when the minerals crystallized. More importantly, the average age of the minerals was 370 million years. 

Geologic period Pb/U (g/g) Millions of years Today s accepted value 

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Interactive Analytical Chemistry CD-ROM. Developed by William J. Vin-ing. University of Massachusetts, Amherst, in conjunction with the text authors, this CD-ROM is packaged free with every copy of the book. Prompted by icons with captions in the text, students explore the corresponding Intelligent Tutors, Guided Simulations, and Media-based Exercises. This CD-ROM includes tutorials on statistics, equilibria, spectrophotometry, electroanalytical chemistry, chromatography, atomic absorption spectroscopy, and gravimetric and combustion analysis. Also included on the CD-ROM as an Adobe Acrobat PDF file is Chapter 37, Selected Methods of Analysis. Students will be able to print only those experiments that they will perform, and the printed sheets can be easily used in the laboratory. 

A historically important form of gravimetric analysis was combustion analysis, used to determine the carbon and hydrogen content of organic compounds burned in excess 02 (Figure 27-4). Instead of weighing combustion products, modem instruments use thermal conductivity, infrared absorption, or coulometiy (with electrochemically generated reagents) to measure the products. 

Figure 27-4 Gravimetric combustion analysis for carbon and hydrogen.

Traditional methods of elemental analysis depend on specific chemical reactions for given elements, either in solution using titrations (known as volumetric analysis) or precipitation of solids that can be weighed (gravimetric analysis). Although such methods are still used for specific and very accurate purposes, they have been replaced in routine work by automated instrumental methods. Combustion analysis is used to determine C, H, N, and sometimes S, by complete oxidation of the compound forming C02, H20, N2 and S02. The gases are separated and determined automatically... 

Arrhenius Preexponent A Thermo gravimetric analysis, modeling To calculate the rates of decomposition of the resin and combustion of the volatiles released by this decomposition... 

Volatilization gravimetric methods are time- and labor-intensive. Equipment needs are few except when combustion gases must be trapped or for a thermogravi-metric analysis, which requires specialized equipment. 

Standard tests consisted of proximate, ultimate, higher heating value, ash composition, ash fusibility temperatures, Hardgrove grindability, and screen analyses. Special bench scale characterization tests consisted of micro-proximate analysis and micro-ultimate analysis (C, H, N) micro-proximate and micro-ultimate analyses were performed on particulate samples collected from varying stages of combustion in the DTFS and CMHF. In addition, selected samples of SRC and chars from partial combustion or pyrolysis of the SRC were submitted for Thermo-Gravimetric analyses. 

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