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In [[science]], [[engineering]] and in general common usage, '''concentration''' is the measure of how much of a given substance there is in a given mixture of substances. 


==Notation==
There are many different notations that may be used to quantitatively express concentration. The most commonly used notations are discussed below:
===Mole fraction and mole percent===
{{main|Mole fraction}}
The ''mole fraction'' is a measure of the concentration of a component substance in a mixture of substances. It is defined as the number of [[Mole (unit)|moles]] of a component substance in a mixture divided by the total number of moles of the mixture.<ref>{{cite book|author=N.A. Gokcen and R.G. Reddy|title=Thermodynamics|edition=2nd Edition|publisher=Plenum Press|year=1996|id=ISBN 0-306-45380-0}}</ref>
The ''mole percent'' or ''molar percent'' is usually denoted by mole % and is equal to 100 times the mole fraction.
===Mass fraction and mass percent===
The ''[[mass]] fraction'' is also a measure of the concentration of a component substance in a mixture of substances. It is defined as the mass of a component substance in a mixture divided by the total mass of the mixture. It is most commonly referred to as the [[weight]] fraction.
The ''mass percent'' is equal to 100 time the mass fraction. It is most commonly referred to as the ''weight percent'' and is usually denoted as wt %, weight % or percent by weight.
===Volume fraction and volume percent===
The ''[[volume]] fraction'' is another measure of the concentration of a component substance in a mixture of substances. It is defined as the volume of a component substance in a mixture divided by the total volume of the mixture.
The ''volume percent'' is equal to 100 time the volume fraction and is usually denoted as vol %,  volume % or percent by volume.
===Mass per volume===
There are a number of concentration expressions that involve an amount of mass contained in an amount of volume. For example:
*Certain [[air pollutant concentrations]] are expressed as mg/m<sup>3</sup> of ambient air at a stated [[temperature]] and [[pressure]]
*[[Liquified petroleum gas]] (LPG) contains [[mercaptan]] [[odorant]]s at a concentration expressed as 1.5 [[U.S. customary units|pounds]] per 10<sup>4</sup> [[U.S. customary units|gallons]] (1.5 [[U.S. customary units|lb]]/10<sup>4</sup> [[U.S. customary units|gal]]) or 0.18 [[kilogram]]s per 10<sup>4</sup> [[litre]]s (0.18 [[kilogram|kg]]/10<sup>4</sup> [[Litre|L]]).
===Parts-per notation===
{{main|Parts-per notation}}
The parts-per notation is used in science and engineering as a measure of the concentration of a component substance in a mixture of substances and usually when the concentration is very small. The most commonly used parts-per notations are:
*''Parts per million'' (ppm): In most countries, 1 million is 1×10<sup>6</sup> and thus 1 part per million  parts (1 ppm) has a numerical value of 1×10<sup>-6</sup>.
*''Parts per billion'' (ppb): In the [[United States]], 1 billion is 1×10<sup>9</sup> and thus one part per billion parts (1 ppb) has a numerical value of 1×10<sup>-9</sup>. This terminology should be used with great caution because
**In [[France]] and frequently in continental [[Europe]], 1×10<sup>9</sup> is 1 ''milliard''.
**In the [[United Kingdom]] and in other nations using [[British English]], 1×10<sup>9</sup> is 1000 million and 1 billion is 1×10<sup>12</sup>.
*''Parts per trillion'' (ppt): In the United States, 1 trillion is 1×10<sup>12</sup> and thus one part per trillion parts (1 ppt) has a numerical value of 1×10<sup>-12</sup>. This terminology should also be used with great caution because:
**In the United Kingdom and other nations using British English, France and continental Europe, 1×10<sup>12</sup> is 1 billion and 1 trillion is 1×10<sup>18</sup>
**Concentrations are sometimes expressed as ppt meaning parts per thousand which conflicts with ppt meaning parts per trillion.
There are other parts-per notations that are not commonly used (see [[Parts-per notation]]).
===Molarity, molality and normality ===
{{main|Concentration (chemistry)}}
''[[Molarity]]'', ''[[normality]]'' and ''[[molality]]'' are terms used in chemistry to denote the concentration of [[solute]]s in [[solution]]s or [[solvent]]s.
====Molarity====
''Molarity'' or ''molar concentration'' (in units of mol/L) denotes the number of moles of a given [[solute]] per litre of [[solution]]. The units of mol/L are commonly replaced by the symbol M.
The [[National Institute of Standards and Technology]] (NIST) of the United States considers the term molarity and the symbol M to be obsolete and recommends using the term ''amount-of-substance concentration of B'' (or ''concentration of B'') and the symbol ''c<sub>B&nbsp;</sub>'' with SI units of mol/m<sup>3</sup> or other SI acceptable units.<ref name=NIST>[http://physics.nist.gov/Pubs/SP811/sec11.html NIST Guide to SI Units]  NIST website, accessed February 1, 2009. (Scroll down to item 18)</ref>. This recommendation has not been universally implemented in academia or chemistry research yet.
====Normality====
''Normality'', as a concentration term with symbol N, has been used for decades in [[chemistry]]. In solution, [[Salt (chemistry)|salts]] are [[Dissociation|dissociated]] into reactive solute species ([[Ion|ions]] such as [[Hydrogen|H]]<sup>+</sup>, [[Iron|Fe]]<sup>3+</sup>, or [[Chlorine|Cl]]<sup>− </sup>). A normal solution has one [[gram equivalent]] of a solute ion per liter of solution. The definition of a gram equivalent depends on the type of solute: [[acid]], [[base]], [[redox]] species, or ions that will precipitate. Note that normality measures a single ion which is part of an overall solute. For example, one could determine the normality of the [[hydroxide]] ion or [[sodium]] ion in an [[aqueous]] solution of the overall solute [[sodium hydroxide]] (NaOH), but the normality of sodium hydroxide itself has no meaning.
However, both NIST<ref name=NIST/><ref>[http://physics.nist.gov/Pubs/SP811/sec08.html#8.6.5 Concentration of B; amount-of-substance concentration of B] NIST website, accessed February 1, 2009</ref> and the International Union of Applied Chemistry (IUPAC)<ref>[http://old.iupac.org/publications/analytical_compendium/ Chapter 6, Section 6.3, Use of equivalence concept] IUPAC website, accessed February 1, 2009 (Scroll down to ''Normal solution'')</ref> now consider that the term normality is obsolete.
NIST recommends using the term ''amount-of-substance concentration of B'' (or ''concentration of B'') and the symbol ''c<sub>B&nbsp;</sub>'' with SI units of mol/m<sup>3</sup> or other SI acceptable units just as it does for the term molarity.<ref name=NIST/>
====Molality====
''Molality'' or ''molal concentration'' (in units of mol/kg) denotes the number of moles of solute per kilogram of [[solvent]] (not solution). The units of mol/kg are commonly replaced by the symbol m (not to be confused with symbol m for [[metre]]). NIST also considers the term molality and the symbol m to be obsolete and recommends using the term ''molality of solute B'' and the symbol ''b<sub>B</sub>'' or ''m<sub>B</sub>'' with SI units of mol/kg or other SI acceptable units.<ref name=NIST/>
==Clarity of notation==
Some of the concentration notations must be carefully defined or referenced for clarity though this is frequently not the case even in technical publications. 
In [[atmospheric chemistry]] and in [[air pollution]] regulations, the parts per notation is commonly expressed with a '''v''' following, such as '''ppmv''', to indicate parts per million by volume. This works fine for gas concentrations (e.g., ppmv of carbon dioxide in the ambient air) but, for concentrations of non-gaseous substances such as aerosols, cloud droplets, and particulate matter in the ambient air at stated ambient conditions of temperature and pressure, the concentrations are commonly expressed as μg/m<sup>3</sup> or mg/m<sup>3</sup> (i.e., μg or mg of particulates per cubic metre of ambient air). Since the concentration of gaseous substances in the air may also be expressed as μg/m<sup>3</sup> or mg/m<sup>3</sup>, it is quite important to state those ambient conditions so that such gaseous concentrations can be converted to ppmv as is sometimes needed.   
The usage of terminology is generally fixed inside most specific branches of science, leading some workers in those specific sciences to believe that their own terminology is the only correct one. This, in turn, leads them not to clearly define or reference their terminology in their publications and others may therefore misinterpret their results. Many scientific, engineering and other technical publications, that are otherwise excellent, fail to define their usage of the parts-per notation. The difference between expressing concentrations as ppm by volume or ppm by mass or weight is very significant when dealing with gases and it is most important to define which is being used. It is quite simple, for example, to distinguish ppm by volume from ppm by mass or weight by using ''ppmv'' or ''ppmw''.
==References==
{{reflist}}

Revision as of 22:31, 4 February 2009

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