Solving Mass Balance Issues

A refractive index (RI) detector is probably the most widely used in GPC. The main advantage of an RI detector is that any polymer solution will generate a response. This detector has several disadvantages  

Two specialized detectors have been developed with specific applications for SEC. The first is the laser light-scattering detector, which has been around for almost 20 years but now has new electronics and computer data acquisition capabilities. Substitution of bulky He-Ne gas lasers with small, inexpensive diode lasers has greatly reduced the size and cost of laser light-scattering detectors, and the development of reference flow viscometers has provided similar size and cost advantages for viscometer detectors. 

Each detector measures a different and complementary variable. The lightscattering detector gives a response that is proportional to molecular weight and concentration. Likewise, the viscometer detector response is proportional to the intrinsic viscosity and concentration. 

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Additional detection techniques that can be employed to help solve mass balance issues with RP-HPLC are MS [30], chemiluminescent nitrogen-specihc detector [31], evaporative light-scattering detector, ELSD [32], and corona charged aerosol detection [CAD] [33],... 

We have to solve the PFTR mass-balance equation to find Cj (z), but the first issue is exactly what form we should use for r or rj in the mass-balance equations. This is a major topic of this chapter. 

Thus we see that environmental modeling involves solving transient mass-balance equations with appropriate flow patterns and kinetics to predict the concentrations of various species versus time for specific emission patterns. The reaction chemistry and flow patterns of these systems are sufficiently complex that we must use approximate methods and use several models to try to bound the possible range of observed responses. For example, the chemical reactions consist of many homogeneous and catalytic reactions, photoassisted reactions, and adsorption and desorption on surfaces of hquids and sohds. Is global warming real [Minnesotans hope so.] How much of smog and ozone depletion are manmade [There is considerable debate on this issue.]... 

But the main reason to leave reactive systems for this chapter is because it includes some additional characteristics. Everything that we have learned about approaching and solving material balance problems is useful and necessary. But for reactive systems we need to discuss the issue of material balance in a reactor. An important feature is the advantage of the use of mole instead of mass. Why As you recall from chemistry, the relative quantities of reagents and products in a chemical reaction are given by the stoichiometry. For example ... 

For substances that do not partition into a phase (e.g., ionic species into air), the Z value is zero and a division by zero issue can arise when solving the mass balance equations. This can be circumvented by using aquivalence (essentially f/H) as the equilibrium criterion or activity (concentration in water/solubility in water or equivalently fugacity/vapor pressure). Indeed, when examining fugacities, it is desirable to calculate the activity to ensure that subsaturation conditions prevail, that is, all fugacities are less than the liquid or subcooled liquid vapor pressure. 

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