Chemical Concentrations

Knowledge of the principles underlying the fate and transport of chemicals in the environment allows problems ranging from local to global scales to be defined and analyzed. This first chapter presents fundamental concepts that apply universally to any environmental medium. The subsequent three chapters focus on surface waters, the subsurface environment, and the atmosphere, respectively see Fig. 1-1 for a diagram of some of the interrelationships among these media. In each chapter, each medium is discussed in terms of its basic physical, chemical, and biological attributes then the fate and the transport of introduced chemicals are considered. 

Perhaps the single most important parameter in environmental fate and transport studies is chemical concentration (C). The concentration of a chemical is a measure of the amount of that chemical in a specific volume or mass of air, water, soil, or other material. Not only is concentration a key quantity in fate 

Most laboratory analysis methods measure concentration. The choice of units for concentration depends in part on the medium and in part on the process that is being measured or described. In water, a common expression of concentration is mass of chemical per unit volume of water. Many naturally occurring chemicals in water are present at levels of a few milligrams per liter (mg/liter). The fundamental dimensions associated with such a measurement are [M/L3]. The letters M, L, and T in square brackets refer to the fundamental dimensions of mass, length, and time, which are discussed further in the Appendix. For clarity in this book, specific units, such as (cm/hr) or (g/m3), either are free-standing or are indicated in parentheses, not in square brackets. 

Another common unit of concentration in water is molarity. Recall that a mole of a chemical substance is composed of 6.02 X fO23 atoms or molecules of that substance. Molarity refers to the number of moles per liter of solution and is denoted M, with neither parentheses nor square brackets around it in this book. 

A related unit, normality (N), refers to the number of equivalents of a chemical per liter of water. An equivalent is the amount of a chemical that either possesses, or is capable of transferring in a given reaction, 1 mol of electronic charge. If a chemical has two electronic charge units per molecule, 1 mol of the chemical constitutes two equivalents [e.g., a mole of sulfate (SO2-) is equal to two equivalents, and a one molar (1 M) solution of sodium sulfate (Na2S04) is two normal (2 N)]. 

Most liquid- and gas-phase UV-vis spectroscopic measurements rely on the well-known Beer s Law1 (Equation 6.1) to relate the amount of light absorbed by a sample to the quantity of some chemical species that is present in that sample. 

While spectra are sometimes recorded in units of transmittance (T) or percent transmittance (%T), these units are not suitable for determining chemical concentration. The reason for this comes from the relationship between absorbance and transmittance1 as shown in Equation 6.2. 

It can easily be seen from Equations 6.1 and 6.2 that only absorbance is linear with concentration. 

Most liquid- and gas-phase UV-vis spectroscopic measurements rely on the well-known Lambert-Beer Law  

Spectra of solid samples are usually recorded in the units of reflectance (R) or percent reflectance (%/ ), which is analogous to percent transmittance in that reflectance equals the ratio of the reflected radiation to the incident radiation. With diffuse reflectance, the reflected signal is attenuated by two phenomena absorption (coefficient k) and scattering (coefficient s). Lollowing the Kubelka-Munk theory, these two coefficients are related to the reflectance of an infinitely thick sample, by 

if the scattering coefficient remains constant during a measurement, the absorption may be deduced from the reflectance. 

The concept of isosbestic points can prove helpful in various ways  

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Most chemically reacting systems tliat we encounter are not tliennodynamically controlled since reactions are often carried out under non-equilibrium conditions where flows of matter or energy prevent tire system from relaxing to equilibrium. Almost all biochemical reactions in living systems are of tliis type as are industrial processes carried out in open chemical reactors. In addition, tire transient dynamics of closed systems may occur on long time scales and resemble tire sustained behaviour of systems in non-equilibrium conditions. A reacting system may behave in unusual ways tliere may be more tlian one stable steady state, tire system may oscillate, sometimes witli a complicated pattern of oscillations, or even show chaotic variations of chemical concentrations. 

Analogous considerations apply to spatially distributed reacting media where diffusion is tire only mechanism for mixing chemical species. Under equilibrium conditions any inhomogeneity in tire system will be removed by diffusion and tire system will relax to a state where chemical concentrations are unifonn tliroughout tire medium. However, under non-equilibrium conditions chemical patterns can fonn. These patterns may be regular, stationary variations of high and low chemical concentrations in space or may take tire fonn of time-dependent stmctures where chemical concentrations vary in botli space and time witli complex or chaotic fonns. 

It is convenient to analyse tliese rate equations from a dynamical systems point of view similar to tliat used in classical mechanics where one follows tire trajectories of particles in phase space. For tire chemical rate law (C3.6.2) tire phase space , conventionally denoted by F, is -dimensional and tire chemical concentrations, CpC2,- are taken as ortliogonal coordinates of F, ratlier tlian tire particle positions and velocities used as tire coordinates in mechanics. In analogy to classical mechanical systems, as tire concentrations evolve in time tliey will trace out a trajectory in F. Since tire velocity functions in tire system of ODEs (C3.6.2) do not depend explicitly on time, a given initial condition in F will always produce tire same trajectory. The vector R of velocity functions in (C3.6.2) defines a phase-space (or trajectory) flow and in it is often convenient to tliink of tliese ODEs as describing tire motion of a fluid in F with velocity field/ (c p). 

Because of tire underlying dissipative nature of tire chemical systems tliat tire ODEs (C3.6.2) represent, tliey have anotlier important property any volume in F will shrink as it evolves. For a given set of initial chemical concentrations tire time evolution under tire chemical rate law will approach arbitrarily close to some final set of points in... 

Attractors can be simple time-independent states (points in F), limit cycles (simple closed loops in F) corresponding to oscillatory variations of tire chemical concentrations with a single amplitude, or chaotic states (complicated trajectories in F) corresponding to aperiodic variations of tire chemical concentrations. To illustrate... 

For tliis model tire parameter set p consists of tire rate constants and tire constant pool chemical concentrations l A, 1 (Most chemical rate laws are constmcted phenomenologically and often have cubic or otlier nonlinearities and irreversible steps. Such rate laws are reductions of tire full underlying reaction mechanism.)... 

Many competitive programs to perfect a metallic anode for chlorine arose. In one, Dow Chemical concentrated on a coating based on cobalt oxide rather than precious metal oxides. This technology was patented (9,10) and developed to the semicommercial state, but the operating characteristics of the cobalt oxide coatings proved inferior to those of the platinum-group metal oxide. 

Mineral Dressing. Physical and chemical concentration of raw ore into a product from which a metal can be recovered at a profit. 

Although the size separation/classification methods are adequate in some cases to produce a final saleable mineral product, in a vast majority of cases these produce Httle separation of valuable minerals from gangue. Minerals can be separated from one another based on both physical and chemical properties (Fig. 8). Physical properties utilized in concentration include specific gravity, magnetic susceptibility, electrical conductivity, color, surface reflectance, and radioactivity level. Among the chemical properties, those of particle surfaces have been exploited in physico-chemical concentration methods such as flotation and flocculation. The main objective of concentration is to separate the valuable minerals into a small, concentrated mass which can be treated further to produce final mineral products. In some cases, these methods also produce a saleable product, especially in the case of industrial minerals. 

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

Cahbration is an important focus in analytical chemistry. It is the process that relates instmment responses to chemical concentrations. It consists of two basic steps estimation of the cahbration model parameters, and then prediction for new samples of unknown concentration. Cahbration refers to the step of the analytical process in Figure 2 where measurements are related to concentrations of chemical species or other chemical information. 

Optimization lefeis to the step in the analytical process (Fig. 2) where some sort of treatment is performed on samples to generate taw data which can be in the form of voltages, currents, or other analytical signals. These data have yet to be caUbrated in terms of chemical concentrations. 

Concentration cell corrosion is caused by chemical concentration differences between areas contacting metal surfaces. An electrochemical cell is established by virtue of the difference between chemical compositions in the water on, and adjacent to, corroding areas. 

Another way to evaluate risks is to calculate the sensitivity of the total risk estimates to changes in assumptions, frequencies, or consequences. Risk analysts tend to be conservative in their assumptions and calculations, and the cumulative effect of this conservatism may be a substantial overestimation of risk. For example, always assuming that short-term exposure to chemical concentrations above some threshold limit value will cause serious injury may severely skew the calculated risks of health effects. If you do not understand the sensitivity of the risk results to this conservative assumption, you may misallocate your loss prevention resources or misinform your company or the public about the actual risk. 

Depth profiling by SALI provides quantitative information through interfaces and for extremely thin films, in the form of reliable chemical concentrations. 

Electrochemistry plays an important role in the large domain of. sensors, especially for gas analysis, that turn the chemical concentration of a gas component into an electrical signal. The longest-established sensors of this kind depend on superionic conductors, notably stabilised zirconia. The most important is probably the oxygen sensor used for analysing automobile exhaust gases (Figure 11.10). The space on one side of a solid-oxide electrolyte is filled with the gas to be analysed, the other side... 

Detector tube kits generally include a hand pump that draws a known volume of air through a chemically treated tube intended to react with certain contaminants. The length of color stain resulting in the tube correlates to chemical concentration. 

For listed chemicals with the qualifier "solution, such as ammonium nitrate, at concentrations of 1 percent (or 0.1 percent in the case of a carcinogen) or greater, the chemical concentrations must be factored into threshold and release calculations because threshold and release amounts relate to the amount of chemical in solution not the amount of solution. 

Mass balance (C) should only be indicated it it is directly used to calculate the mass (weight) of chemical released. Monitoring data should be indicated as the basis of estimate only if the chemical concentration is measured in the wastestream being released into the environment. Monitoring data should flfll be indicated, for example, if the monitoring data relates to a concentration of the toxic chemical in other process streams within the facility. 

All data available at your facility must be utilized to calculate treatment efficiency and influent chemical concentration. You areDfll required to collect any new dataforthe purposes of this reporting requirement. If data are lacking, estimates must be made using best engineering judgment or other methods. 

Of concern is that there is very little information often available concerning the effects of common waste waters on evaporation rates. As noted, the evaporation rate of a solution will decrease as the solids and chemical concentrations increase. However, the overall effects on evaporation rates of dissolved constituents as well as color changes and other factors of wastewater are largely unknown. 

SLAB calculates chemical concentrations at positions downwind and heights above the ground. Tlic plume may be denser-than-air, neutrally-buoyant, or less dense than air. Thermodynamics effeci.s are accounted for, including latent heat exchanges due to the condensation or evaporation ot liquids, Time averaged results may be calculated. SLAB is the easiest of the publicly-available dense gas models to set up and mn. It has been extensively validated against large-scale field data. 

Robinson, N. P, Kyle, H., Webbeg S. E., and Widdicombe, J. G. (1989). Electrolyte and other chemical concentrations in tracheal airway surface liquid and mucus. /. Appl. Physiol. 66, 2129-2135. 

In this step, the assessor qiuuitifies tlie magnitude, frequency and duration of exposure for each patliway identified in Step 2. Tliis step is most often conducted in two stages estimation of exposure concentrations and calculation of intakes. The later estimation is considered in Step 4. In tliis part of step 3. the exposure assessor determines the concentration of chemicals tliat will be contacted over the exposure period. E.xposure concentrations are estimated using monitoring data and/or chemical transport and environmental fate models. Modeling may be used to estimate future chemical concentrations in media tliat are currently contaminated or tliat may become contaminated, and current concentrations in media and/or at locations for which tliere are no monitoring data. The bulk of the material in tliis chapter is concerned witli tliis step. 

Will the toxicant be distributed to a larger area, and if so what will be its form (physical and chemical), concentration, and duration of residence tluoughout the area of distribution This description should include the concentrations at various locations and times tliroughout its residence, and it should include air and waterborne materials as well as those taken up by biological materials such as pliuits luid animals. 

The primary question is the rate at which the mobile guest species can be added to, or deleted from, the host microstructure. In many situations the critical problem is the transport within a particular phase under the influence of gradients in chemical composition, rather than kinetic phenomena at the electrolyte/electrode interface. In this case, the governing parameter is the chemical diffusion coefficient of the mobile species, which relates to transport in a chemical concentration gradient. 

Apart from bottom blowdown valves (main BD valve), other types of valves are often employed. Blowdown valves also can be used to control high water levels, drain the boiler for cleaning or inspection purposes, and maintain chemical concentrations and water chemistry below maximum permitted levels. 

Economizer corrosion rates are enhanced by higher heat-transfer rates excessive heat flux may create localized nucleate boiling zones where gouging, as a result of chemical concentration effects, can occur. Air heaters are also located in the exit gas system. They do a job similar to that of economizers except that they preheat combustion air. 

A chemical formed from a reaction of two or more elements. Chemical concentration ... 

Fig. 7-3 Coordinate system showing the formation downwind from a source of Gaussian distributions of chemical concentrations in the horizontal and vertical. Ellipses denote the loci of two standard deviations.

Functions of Standards. Fluorescent standards can be used for three basic functions calibration, standardization, and measurement method assessment. In calibration, the standard is used to check or calibrate Instrument characteristics and perturbations on true spectra. For standardization, standards are used to determine the function that relates chemical concentration to Instrument response. This latter use has been expanded from pure materials to quite complex standards that are carried through the total chemical measurement process (10). These more complex standards are now used to assess the precision and accuracy of measurement procedures. 

If the sample and standard have essentially the same matrices (e.g., air particulates or river sediments), one can go through the total measurement process with both the sample and the standard in order to (a) check the accuracy of the measurement process used (compare the concentration values obtained for the standard with the certified values) and (b) obtain some confidence about the accuracy of the concentration measurements on the unknown sample since both have gone through the same chemical measurement process (except sample collection). It is not recommended, however, that pure standards be used to standardize the total chemical measurement process for natural matrix type samples chemical concentrations in the natural matrices could be seriously misread, especially since the pure PAH probably would be totally extracted in a given solvent, whereas the PAH in the matrix material probably would not be. All the parameters and matrix effects. Including extraction efficiencies, are carefully checked in the certification process leading to SRM s. 

Devices called sensors, which are sensitive to physical influences other than electricity and light, like pressure, temperature, chemical concentrations, or magnetic fields, can convert non-electric signals into electrical ones (see, e.g., the review of Janata [108] for chemical sensors). 

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