Estimating local concentrations

To estimate local concentration of counterions in the immediate vicinity of NaPAA, we first calculated the local concentration of pendent carboxylate groups by using equation 11,... 

It is clear that the experimentally measured degree of mixing depends on the spatial resolution (compared to Lg) and on the time resolution (compared to Tg) of the probe used to estimate local concentrations in the mixture. The mixture appears as uniform if the scale of concentration fluctuations is smaller than the resolution of the probe. Generally, it is admitted that micromixing takes place in the range between molecular dimensions up to the scale where non-uniformities can be detected by usual means of ob-... 

The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentration of the reacting species in or at the micelle. Also for higher-order reactions the pseudophase model is usually adopted (Figure 5.2). However, in these systems the dependence of the rate on the concentration of surfactant does not allow direct estimation of all of the rate constants and partition coefficients involved. Generally independent assessment of at least one of the partition coefficients is required before the other relevant parameters can be accessed. 

The difficulty in accurately estimating the degree of local concentration remains one of the principal reasons susceptibility to SCC in a specific environment or circumstance is difficult to predict. Measurement of nominal stresses or levels of corrodent in the bulk environment can be quite misleading as predictors of SCC susceptibility. 

A drum contains 42 gal of toluene. If the lid of the drum is left open (lid diameter = 3 ft), determine the time required to evaporate all the toluene in the drum. The temperature is 85°F. Estimate the concentration of toluene (in ppm) near the drum if the local ventilation rate is 1000 ft3/min. The pressure is 1 atm. 

A third way to gain some knowledge about the concentrations of chemicals in the environment involves some type of modeling. Scientists have had, for example, fair success in estimating the concentrations of chemicals in the air in the vicinity of facilities that emit those chemicals. Information on the amount of chemical emitted per unit time can be inserted into various mathematical models that have been designed to represent the physical phenomena governing dispersion of the chemical from its source. Certain properties of the chemical and of the atmosphere it enters, together with data on local weather conditions, are combined in these models to yield desired estimates of chemical concentrations at various distances from the source. These models can be calibrated with actual measurement data for a few chemicals, and then used for others where measurement data are not available. 

Fate and transport modeling was nsed to estimate the concentration of the insecticide in insect tissne consnmed by birds. The details of this modeling effort, which we omit here, are rather complex and involve characteristics of the field application of the insecticide, local weather, mnltiple pathways of exposure to insects, sequestration of insecticide by mortality of insects, and integration over 0- to 20-g pools of insect tissne that wonld compose a bird s daily diet. The model of the pesticide s fate and transport made a prediction abont the concentration variable, which is characterized by the p-box shown in the lower left graph of Figure 6.14. This p-box synthesizes all of the knowledge and nncertainty captured in the modeling effort. The model predicts the distribntion fnnction for concentrations, whatever it is, snrely lies within the bonnds shown. 

The presence of oe-hydroxycarboxylic acids together with a-aminoacids could lead to an estimate of the local concentration of ammonia when these molecules were synthesised. Such an estimation method implies the assumption that the syntheses of the two classes of molecules were simultaneous and started from the same organic substrate, i.e. aldehydes25 . From aldehydes, aminoacids can be obtained by the Strecker synthesis (aldehyde, HCN, NH3 in aqueous solution), while hydroxyacids can be synthesised from the cyanhydrin synthesis (aldehyde + HCN) followed by a hydrolysis. Nevertheless, it must be emphasised that all aminoacids detected in carbonaceous chondrites cannot be obtained by the Strecker synthesis. This remark limits the interest of the previous arguments concerning the concentration of NH3 during the accretion phase. 

Here, CA and CB (upper case) denote the mean molar concentrations of reactants A and B while CA and CB (lower case) denote the local concentration fluctuations that result from turbulence. When the species are perfectly mixed, the second term on the right side containing the correlation of the concentration fluctuations, will approach zero. Otherwise, if the species are not perfectly mixed, this term will be negative and will reduce the reaction rate. Estimating this correlation term is not straightforward and numerous models are available. An excellent discussion on this subject was given by Hannon [1],... 

This single line (go = 1.935 0.005, AH 140 G) is usual for V4+ centers, which are coupled by strong spin-exchange interaction [23, 164], These V4+ ions are localized in some areas with high local concentration (V4+)s (the so called nano-phases - because of their probable size), which can be estimated from the exchange narrowing of a spectrum by the equation [23] ... 

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