Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 ANALYSIS 71radical scavengers, indicating reaction between these products and the scavengersor their production by the scavengers in processes other than radical reactions2’.This is particularly important when NO is used.4. Analysis4.1 INTRODUCTIONThis section is divided into four main sub-sections. The first three deal withequally important methods of analysis. The last is devoted to other methods thatare not of general application. However, this does not mean to say that such methodshave necessarily a minor role to play in the kinetic study of reactions. Anobvious example is the use of the calorimetric method for determining the extentof a heterogeneous component of a gas-phase process.4.2 CHEMICAL METHODS4.2.1 GeneralChemical methods involving derivative formation have now been largely supersededby physical methods of analysis, particularly gas chromatography. However,under some circumstances it may be impossible to use the latter techniques.Alternative chemical methods of product analysis are described by Melville andGowenl~ck~~~, and are particularly necessary in the field of combustion205. Forinstance, formaldehyde may be estimated by the chromotropic acid method206, andglyoxalic acid and glyoxal by colorimetric meth~ds~~’~~~~.Hydrogen is difficult to analyse by gas chromatography; it may be done byGSC using nitrogen as the carrier gas, but with low sensitivity. However, H, can beselectively absorbed on activated silica gel at liquid oxygen temperatures from a“liquid nitrogen” fraction containing CO and CH4, followed by later desorptionat elevated temperatures and a PVT determination. Carbon dioxide is very convenientlyadsorbed and estimated by passing through soda-lime.It is sometimes necessary to remove halogens and halogen acids from a gaseousreaction mixture prior to quenching in aliquid nitrogen trap; acetone, for example,polymerises in the presence of hydrogen halideszo9. Various solids absorb thesegases very efficiently2’ ’. Sometimes, selective absorption may be achieved, suchthat halogen may be separated from halogen halide. Quantitative estimation maybe subsequently made by potentiometric titration techniques.Any substance that has paramagnetic properties will catalyse the ortho-parahydrogen conversion. This therefore represents a method for the detection of freeradicals and atoms. Virtually pure para-H, is metastable up to 500” C. The con-References pp. 104-111