Chemical concentrations, expressions

Flammable Limits in Air - This is a concentration expressed as percent by volume of the chemical in air, whereby spontaneous combustion will be supported. The lowest concentration where combustion will be supported is known as the lower flammability limit (LFL) or lower explosion limit (LEL). LEL and LFL are considered interchangeable. The upper concentration limit is the UFL (Upper Flammability Limit) or UEL (Upper Explosion Limit). 

Exposure limits Guidelines for worker exposure to physical agents and hazardous chemicals, usually expressed as an allowable time of exposure or an air concentration below which health hazards are unlikely to occur among most exposed workers. 

Reactions. Reactions are expressed by fir t order equations in chemical concentration (rate constant k.h ) such that the rates of processes such as hydrolysis, oxidation, photolysis, or biolysis can be combined by adding the k terms to yield a total rate constant kT-... 

As shown in Fig. 3, CHEMGL considers 10 major well-mixed compartments air boundary layer, free troposphere, stratosphere, surface water, surface soil, vadose soil, sediment, ground water zone, plant foliage and plant route. In each compartment, several phases are included, for example, air, water and solids (organic matter, mineral matter). A volume fraction is used to express the ratio of the phase volume to the bulk compartment volume. Furthermore, each compartment is assumed to be a completely mixed box, which means all environmental properties and the chemical concentrations are uniform in a compartment. In addition, the environmental properties are assumed to not change with time. Other assumptions made in the model include continuous emissions to the compartments, equilibrium between different phases within each compartment and first-order irreversible loss rate within each compartment [38]. 

The motions of the individual fluid parcels may be overlooked in favor of a more global, or Eulerian, description. In the case of single-phase systems, convective transport equations for scalar quantities are widely used for calculating the spatial distributions in species concentrations and/or temperature. Chemical reactions may be taken into account in these scalar transport equations by means of source or sink terms comprising chemical rate expressions. The pertinent transport equations run as... 

The example reactions considered in this section all have the property that the number of reactions is less than or equal to the number of chemical species. Thus, they are examples of so-called simple chemistry (Fox, 2003) for which it is always possible to rewrite the transport equations in terms of the mixture fraction and a set of reaction-progress variables where each reaction-progress variablereaction-progress variable —> depends on only one reaction. For chemical mechanisms where the number of reactions is larger than the number of species, it is still possible to decompose the concentration vector into three subspaces (i) conserved-constant scalars (whose values are null everywhere), (ii) a mixture-fraction vector, and (iii) a reaction-progress vector. Nevertheless, most commercial CFD codes do not use such decompositions and, instead, solve directly for the mass fractions of the chemical species. We will thus look next at methods for treating detailed chemistry expressed in terms of a set of elementary reaction steps, a thermodynamic database for the species, and chemical rate expressions for each reaction step (Fox, 2003). 

BAF defined as the ratio of the test chemical concentration in (a part of) an organism (e.g., bird, mammal or fish) to the concentration in its food (e.g., laboratory fodder, plants, invertebrates, birds, mammals) at steady state. BAFs are generally used for accumulation by birds, mammals and fish and are expressed on a weight basis. 

Upper Explosive Limit Also known as Upper Flammable Limit. Is the highest concentration (expressed in percent of vapor or gas in the air by volume) of a substance that will burn or explode when an ignition source is present. Theoretically above this limit the mixture is said to be too rich to support combustion. The difference between the LEL and the UEL constitutes the flammable range or explosive range of a substance. That is, if the LEL is one ppm and the UEL is five ppm, then the explosive range of the chemical is one ppm to Five ppm. (See also LEL)... 

It should be stressed that the pH value of an actual buffer solution prepared by mixing quantities of the weak acid or base and its conjugate base or acid based on the calculated ratio will likely be different from what was calculated. The reason for this is the use of approximations in the calculations. For example, the molar concentration expressions found in Equations (5.23) to (5.30), e.g., [H+], are approximations. To be thermodynamically correct, the activity of the chemical should be used rather than the concentration. Activity is directly proportional to concentration, the activity coefficient being the proportionality constant ... 

A sorption isotherm is completed for each solid particle type and SWMs/ COMs. A range of solid to solution concentrations (i.e., solid solution) was chosen for each solid phase and waste material leachate (e.g., 50-250 mg/l),with about five data points per range. All control and test samples were performed in duplicate. The solution used in the isotherms was prepared by a 24-h batch leaching experiment with the solid test material and distilled water. The material controls consisted of the test material leachate without the solid phase particles. Chemical analyses, expressed either as TOC or as individual organic compound (e.g., aliphatic and aromatic compounds) concentrations relative to the organic carbon content of the SWM/COM, revealed the actual concentrations of various organic constituents in the leachates. Solid phase controls were also prepared for each of the test soils/sediments in order to determine the concentrations of the constituents leached from the solid phase alone. 

Bioaccumulation is generally referred to as a process in which the chemical concentration in an organism achieves a level that exceeds that in the respiratory medium (e.g., water for a fish or air for a mammal), the diet, or both. The extent to which chemicals bioaccumulate is expressed by several quantities, including the bioconcentration factor (BCF), bioaccumulation factor (BAF), biomagnification factor (BMF), and trophic or food web magnification factor (TMF) [6]. The ecological, biological and chemical parameters involved in the transfer and accumulation of contaminants in food webs are complex. 

For local effects, in contrast, the determining factor for effects to occur at the site of first contact (mucous membrane of the respiratory tract, the eyes, or the skin) is generally the concentration of the chemical in the air rather than the total dose at the site of first contact. In such cases, a tolerable concentration (expressed as mg/m ) is estabhshed from the NOAEC, or LOAEC, derived in the inhalation smdy without an adjustment to a continuous exposure. 

In a seminal study in 1980, Horwitz and colleagues compiled interlaboratory reproducibility values for a large number of chemical analyses (around 7500, later expanded to more than 10,000) and observed that there was a trend to increased reproducibility (expressed as a relative standard deviation, RSD) with smaller concentration, expressed as a mass fraction (x) (i.e., 1 mg kg 1 = 1 ppm and x= 10 6). An empirical fit of the compiled data gave the Horwitz relationship... 

A mole (mol) is Avogadro s number of particles (atoms, molecules, ions, or anything else). Molarity (M) is the number of moles of a substance per liter of solution. A liter (L) is the volume of a cube that is 10 cm on each edge. Because 10 cm = 0.1 m, 1 L = (0.1 m)3 = 10-3 m3. Chemical concentrations, denoted with square brackets, are usually expressed in moles per liter (M). Thus means the concentration of H+ ... 

Special cases of chemical equilibrium in solution are considered in several later chapters, so here we deal only with gaseous reactions. When concentrations are expressed in moles/liter (as they usually are in solution) in Equation 16-1, the equilibrium constant is designated as Kc, whereas for concentrations expressed as partial pressures in atm (as they usually are for gases) the equilibrium constant is designated as Kt>. For gaseous equilibria, then, Equation 16-1 becomes... 

Evaluations of tile chemical properties of a soil could he perplexing if an attempt were made to include all components that affect plant growth. Methods ill use today rely on indicators for judging the interaction of many factors at one time. Soil reaction or hydrogen ion concentration (expressed as pH) is the most widely used indicator. Acidity alone may or... 

The extent of chemical bioaccumulation usually is expressed in the form of a bioaccumulation factor (BAF), which is the ratio of the chemical concentrations in the organism (CB) to those in water (Cw) ... 

Because chemical sorption to particulate and dissolved organic matter in the water column can reduce substantially the fraction of chemical in water that can be absorbed by aquatic organisms (see discussion on bioavailability), the BAF also can be expressed in terms of the freely dissolved chemical concentration (CWD) ... 

The chemical concentration in the organism usually is expressed in units of mass of chemical per kg of organism, whereas the concentration in water is expressed in mass per litre. The weight of the organism can be expressed on a wet weight (WW), dry weight (DW), or lipid weight (LW) basis. 

The BCF, like the BAF, also can be expressed in terms of the dissolved chemical concentration (CWD) ... 

The biota-sediment bioaccumulation factor describes bioaccumulation in sediment dwelling organisms and fish relative to chemical concentrations in sediment. It is the ratio of chemical concentration in an organism to that in the sediments in which the organism resides. This ratio is usually expressed in one of two ways ... 

Here C is the dissolved chemical concentration in water (g/L), CD is the chemical concentration in food (g/kg), CB is the chemical concentration in the organism (g/kg organism), t is time in days (Gobas et al., 1989), and dCB/ dt expresses the rate of net chemical accumulation the organism. The first order rate constants are klr for uptake from water via... 

The U.S. Environmental Protection Agency (Lee et al., 1993) designed Bedded Sediment Bioaccumulation tests to estimate the bioaccumulation of sediment-associated contaminants by benthic organisms. The bioaccumulation potential of a chemical in sediment usually is expressed as a Biota-Sediment Accumulation Factor (BSAF). The simplest test determines the BSAF as the ratio of the steady-state chemical concentration in the test organism and the chemical concentration in the sediment. 

The BCFs and BAFs are expressed in terms of the dissolved chemical concentration in water. The BCF represents a bioaccumulation estimate under laboratory conditions where organisms are exposed to waterbom chemical only. The BAF represents a field-based bioaccumulation estimate, where organisms are exposed to chemical through water and their diet. In this example, the benthic invertebrates are consuming algae, and the fish are consuming the benthic invertebrates. 

Composition of solutions Concentrations expressed in physical units Concentrations expressed in chemical units Comparison of the concentration scales Summary of concentration units Dilution problems... 

However, note that in Ney s approach, concentration expressed in parts per million (ppm) does not incorporate molecular weight. Therefore, it does not consider the identity or molecular character of the chemical. 

By using the fundamental. principles of conservation of mass and charge, and chemical equilibrium expressions, the concentration of individual species in solution (sometimes called the "species distribution") can be calculated. The available equilibrium models for lime or limestone based FGD are the Bechtel-modified Radian equilibrium program (BMREP) (4) and the species distribution model (SDM)... 

Given radiation chemical yields expressed as G values, it is possible to calculate the concentrations of oxidative and reductive species in pure water at a known absorbed dose. The SI unit of dose is the gray (Gy), which equals an energy deposition of 1 J kg-1. For example, the concentration of OH-produced in pure, neutral water by absorbing 1 kGy is ... 

The procedure to set up such an experimental design is as follows. Once the EC50s of the individual toxicants are established, the chemical concentrations can be expressed in terms of these EC50 as toxic units (c / EC50). Choose the toxic unit levels that need to be tested, for instance, 0, 0.25, 0.5, 1, 2, and 4 toxic units. Choose the ratios to be tested, for instance, 1 0, 213 1/3, l/2 l/2, 1/3 213, 0 1. Calculate the... 

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