# concentration

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## concentration

concentration, in chemistry, measure of the relative proportions of two or more quantities in a mixture. The concentration of a solute is very important in studying chemical reactions because it determines how often molecules collide in solution and thus indirectly determines the rates of reactions and the conditions at equilibrium (see chemical equilibrium).

Concentration may be expressed in a number of ways. The simplest statement of the concentrations of the components of a mixture is in terms of their percentages by weight or volume. Mixtures of solids or liquids are frequently specified by weight percentage concentrations, such as alloys of metals or mixtures used in cooking, whereas mixtures of gases are usually specified by volume percentages. Very low concentrations may be expressed in parts per million (ppm), as in specifying the relative presence of various substances in the atmosphere.

In addition to these means of expressing concentration, several others are defined especially for describing solutions: molarity, molality, mole fraction, formality, and normality. Some of these define the concentration of the solute in reference to the amount of solvent, others in reference to the total amount of solution. The molarity of a solution is the number of moles of solute per liter of solution; e.g., a solution of glucose in water containing 180.16 grams (1 gram-molecular weight, or mole) of glucose per liter of solution is referred to as one molar (1 M). The molality of a solution is the number of moles of solute per 1,000 grams of solvent; a solution prepared by dissolving 180.16 grams of glucose in 1,000 grams of water is one molal (1 m). The mole fraction of a solution is the ratio of moles of solute to the total number of moles in the solution. Since ionic compounds, such as sodium chloride, NaCl, do not occur as molecules, their concentrations cannot be expressed in terms of molarity, molality, or mole fraction. Instead, the concentration of an ionic compound in solution may be given by its formality, the number of gram-formula weights of the compound per liter of solution; e.g., a solution containing 58.44 grams (one gram-formula weight) of NaCl per liter of solution is one formal (1 F). In considering the reactions of certain solutions in combination, for example the neutralization of acids and bases, a useful expression of the concentration is the normality of each solution, the number of gram-equivalent weights of solute per liter of solution (see equivalent weight); e.g., a solution containing 49.04 grams (one gram-equivalent weight) of sulfuric acid, H2SO4, per liter of solution is one normal (1 N). Concentrations of solutions may also frequently be given in terms of the weight of solute in a given volume of solvent or solution.

## concentration

The diameter of the telescopic image of a point-source of light, such as a star. The image is increased from point size by unavoidable optical effects in the telescope, specifically diffraction (see Airy disk), and for ground-based telescopes it is further increased by distortion due to atmospheric conditions at the time of observation, i.e. by the seeing.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Concentration

in chemistry, the value representing the relative quantity of a given component (independent constituent) in a physicochemical system (mixture, solution, melt). The most commonly used methods of expressing concentration are (1) mass fraction—the ratio of the mass of the given component to the mass of the entire system; this ratio multiplied by 100 yields the concentration in weight percent; (2) atomic, or mole, fraction—the ratio of the number of gram atoms (moles) of a given component to the total number of gram atoms (moles) of the system; this ratio multiplied by 100 yields the concentration in atom (mole) percent; and (3) volume fraction—the ratio of the volume of the given component to the total volume of the system; this ratio multiplied by 100 yields the concentration in volume percent.

The concentration of liquid systems is often expressed by the weight of the substance dissolved in 100 g (sometimes in 1 l) of solvent or by the number of moles of substance per 1,000 moles of solvent. In the study of solutions, the concepts of molarity (the number of moles of solute per 1 l of solvent) and molality (the number of moles of solute per 1,000 grams of solvent) are often used. In volumetric analysis, the concentration is expressed by normality (the number of gram equivalents of the active constituent per 1 l of solution) and by titer (the number of grams of active substance or the substance being determined per 1 ml of solution).

In practice, concentration is determined using both the standard methods of quantitative analysis and certain instrumental methods, which make it possible to perform rapid and sufficiently accurate calculations of the content of the main component (for example, determination of the concentration of aqueous solutions of acids, alkalis, salts, and ethyl alcohol by measuring density with the aid of a hydrometer).

### REFERENCES

Anosov, V. Ia, and S. A. Pogodin. Osnovnye nachalafizikokhimicheskogo analiza. Moscow-Leningrad, 1947. Pages 81–83.
Terminologiia termodinamiki; sborniki rekomenduemykh terminov, fasc. 7. Edited by A. M. Terpigorev. Moscow, 1952.
Kireev, V. A. Kurs fizicheskoi khimii. Moscow, 1955. Pages 340–44.

S. A. POGODIN

## concentration

[‚kän·sən′trā·shən]
(chemistry)
In solutions, the mass, volume, or number of moles of solute present in proportion to the amount of solvent or total solution.
(hydrology)
The ratio of the area of the sea covered by ice to the total area of sea surface.
(mathematics)
An operation that provides a relatively sharp boundary to a fuzzy set; for a fuzzy set A with membership function mA, a concentration of A is a fuzzy set whose membership function has the value [mA (x)]αfor every element x, where α is a fixed number that is greater than 1.
(mining engineering)
Separation and accumulation of economic minerals from gangue.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## concentration

1. Chemistry the strength of a solution, esp the amount of dissolved substance in a given volume of solvent, usually expressed in moles per cubic metre or cubic decimetre (litre).
2. Economics the degree to which the output or employment in an industry is accounted for by only a few firms
3. another name (esp US) for Pelmanism
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

## concentration

In communications, the combining of multiple channels into one.
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References in periodicals archive ?
If the containment spray system has been operated along with the initiation of leakage of fission products, the airborne concentration of fission products will be minimum (Figures 9, 10, and 11).
After taking measures to reduce the spread of contamination, IDPH uses monitoring instruments to assess airborne concentrations and the extent of contamination.
Airborne concentrations of MWCNT-OH and CB were very low during weighing and transferring, with the highest particle number concentrations detected in the 500-nm range (3,065 particles/L for MWCNT-OH and 1,428 particles/L for CB).
Federal rules prohibit airborne concentrations of the smallest, most readily inhaled dust particles--those 10 micrometers in diameter or smaller--to exceed an average of 15 [micro]g/[m.sup.3] over any 24-hour period or 150 [micro]g/[m.sup.3] over any single hour.
Streifel (1990), "Elevated Airborne Concentrations of Fungi in Residential and Office Environments," American Industrial Hygiene Association Journal, 51:601-604.
Although there are significant difficulties in measuring airborne concentrations of most bioaerosols, the health effects associated with exposure to aerosolized bacteria have received less research attention than those associated with exposure to other organic dusts, such as molds and animal allergens.
A new study reveals that as airborne concentrations of fine dust particles climb, so do three blood factors that increase an individual's heart attack risk.
Generally, passive dosimeters are accurate within [+ or -] 25 percent of airborne concentrations, depending on temperature and humidity.
For reservoirs in some of the watersheds studied, however, PAHs never peaked and continue to climb--even though airborne concentrations in these regions probably fell during the recent past.
Although significant, none of these drops were nearly as dramatic as the drop in airborne concentrations. The team speculates that poor nutrition--as indicated in other Mexico City studies showing low intake of key nutrients such as iron--may have contributed to higher blood lead in the children.
Without ventilation, the 24-hour average airborne concentrations of these VOCs often exceeded the 25 milligrams per cubic meter of air that can cause irritation, headaches, and other symptoms in healthy adults.

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