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In [[science]] and [[engineering]], '''concentration''' is the measure of how much of a given substance there is mixed with another substance.   
In [[science]], [[engineering]] and often in common usage, '''concentration''' is the measure of how much of a given substance there is mixed with another substance.   


==Notation==
==Notation==
   
   
There are a number of different ways to quantitatively express concentration; the most common are listed below.
There are a number of different ways to quantitatively express concentration, including those listed below:


===Molarity===  
===Molarity, molality and normality of liquid solutions===  


[[Molarity]] (in units of mol/L, molar, or M) or [[molar concentration]] denotes the number of [[mole (unit)|mole]]s of a given substance per [[liter]] of solution. A capital letter M is used to abbreviate the units of mol/L.  
*[[Molarity]] or [[molar concentration]] (in units of mol/L) denotes the number of [[mole (unit)|mole]]s 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]] 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>{{cite web|url=http://physics.nist.gov/Pubs/SP811/sec11.html|title=NIST Guide to SI Units accessdate =February 1, 2009}} (Scroll down to item 18)</ref>. This recommendation has not been universally implemented in academia or chemistry research yet.


Following the [[SI]] system of units, the [[National Institute of Standards and Technology]], the [[United States]] authority on [[measurement]], considers the term molarity and the unit symbol M to be obsolete, and suggests instead the '''amount-of-substance concentration''' (''c'') with units mol/m<sup>3</sup> or other units used alongside the SI such as mol/L<ref> {{cite web
*[[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 for [[metre]]).
|url=http://physics.nist.gov/Pubs/SP811/sec11.html
|title=NIST Guide to SI Units
|accessdate=2007-09-03 }}</ref>. This recommendation has not been universally implemented in academia or chemistry research yet.


When discussing the molarity of minute concentrations, such as in [[pharmacology|pharmacological]] research, molarity is expressed in units of millimolar (mmol/L, mM, 1 thousandth of a molar), micromolar (μmol/L, μM, 1 millionth of a molar) or nanomolar (nmol/L, nM, 1 billionth of a molar).
===Molality===
[[Molality]] (mol/kg, molal, or ''m'') denotes the number of [[mole (unit)|mole]]s of ''solute'' per [[kilogram]] of ''solvent'' (not ''solution'').  For instance: adding 1.0 [[mole (unit)|mole]] of solute to 2.0 kilograms of solvent constitutes a solution with a molality of 0.50 mol/kg. Such a solution may be described as "0.50 molal". The term ''molal solution'' is used as a shorthand for a "one molal solution", i.e. a solution which contains one mole of the solute per 1000 grams of the solvent.
Following the [[SI]] system of units, the [[National Institute of Standards and Technology]], the [[United States]] authority on [[measurement]], considers the unit symbol ''m'' to be obsolete, and suggests instead the term 'molality of substance B' (''m<sub>B</sub>'') with units mol/kg or a related unit of the SI<ref> {{cite web|url=http://physics.nist.gov/Pubs/SP811/sec11.html |title=NIST Guide to SI Units |accessdate=2007-12-17 }}</ref>. This recommendation has not been universally implemented in academia yet.
Note that molality is sometimes represented by the symbol (''m''), while molarity by the symbol (M). The two symbols are not meant to be confused, and should not be used as symbols for units. The [[SI]] unit for molality is mol/kg. (The unit ''m'' means meter.)


===Mole fraction===  
===Mole fraction===  

Revision as of 13:54, 2 February 2009

(This needs a lot of work)


In science, engineering and often in common usage, concentration is the measure of how much of a given substance there is mixed with another substance.

Notation

There are a number of different ways to quantitatively express concentration, including those listed below:

Molarity, molality and normality of liquid solutions

  • 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 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 cB  with SI units of mol/m3 or other SI acceptable units.[1]. This recommendation has not been universally implemented in academia or chemistry research yet.


Mole fraction

The mole fraction Χ, (also called molar fraction) denotes the number of moles of a component as a proportion of the total number of moles . The mole percentage or molar percentage, denoted "mol %" and equal to 100% times the mole fraction, is sometimes quoted instead of the mole fraction.)

Mass percentage and mass fraction)

Mass percentage denotes the mass of a substance in a mixture as a percentage of the mass of the entire mixture.

Volume percentage and volume fraction

"Parts-per" notation

For more information, see: Parts-per notation.

The parts-per notation is used in some areas of science and engineering because it does not require conversion from weights or volumes to more chemically relevant units such as normality or molarity. It describes the amount of one substance in another. It is the ratio of the amount of the substance of interest to the amount of that substance plus the amount of the substance it is in.

  • Parts per hundred (denoted by '%' [the per cent symbol], and very rarely 'pph') - denotes the amount of a given substance in a total amount of 100 regardless of the units of measure as long as they are the same. e.g. 1 gram per 100 gram. 1 part in 102.
  • Parts per thousand (denoted by '‰' [the per mille symbol], and occasionally 'ppt', though this should be avoided) denotes the amount of a given substance in a total amount of 1000 regardless of the units of measure as long as they are the same. e.g. 1 milligram per gram, or 1 gram per kilogram. 1 part in 103.
  • Parts per million ('ppm') denotes the amount of a given substance in a total amount of 1,000,000 regardless of the units of measure used as long as they are the same. e.g. 1 milligram per kilogram. 1 part in 106.
  • Parts per billion ('ppb') denotes the amount of a given substance in a total amount of 1,000,000,000 regardless of the units of measure as long as they are the same. e.g. 1 milligram per tonne. 1 part in 109.
  • Parts per trillion ('ppt') denotes the amount of a given substance in a total amount of 1,000,000,000,000 regardless of the units of measure as long as they are the same. e.g. 1 milligram per kilotonne. 1 part in 1012.
  • Parts per quadrillion ('ppq') denotes the amount of a given substance in a total amount of 1,000,000,000,000,000 regardless of the units of measure as long as they are the same. e.g. 1 milligram per megatonne. 1 part in 1015.

Clarity of notation

The notation is used for convenience and the units of measure must be denoted 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, the concentrations are commonly expressed as μg/m³ or mg/m³ (e.g., μg or mg of particulates per cubic metre of ambient air). This expression eliminates the need to take into account the impact of temperature and pressure on the density and hence weight of the gas.

The usage is generally quite fixed inside most specific branches of science, leading some researchers to believe that their own usage (mass/mass, volume/volume or others) is the only correct one. This, in turn, leads them not to specify their usage in their research, and others may therefore misinterpret their results. For example, electrochemists often use volume/volume, while chemical engineers may use mass/mass as well as volume/volume. Many academic papers of otherwise excellent level fail to specify their usage of the part-per notation. The difference between expressing concentrations as mass/mass or volume/volume is quite significant when dealing with gases and it is very important to specify 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

  1. ↑ NIST Guide to SI Units accessdate =February 1, 2009. (Scroll down to item 18)
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