In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently is used in relation to solutions, where it refers to the amount of solute dissolved in a solvent.

To concentrate a solution, one must add more solute, or reduce the amount of solvent (for instance, by selective evaporation). By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute.

There exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. If additional solute is added to a saturated solution, it will not dissolve. Instead, phase separation will occur, leading to either coexisting phases or a suspension. The point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute.

Qualitative description

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Often in informal, non-technical language, concentration is descibed in a qualitative way, through the use of adjectives such as "dilute" or "weak" for solutions of relatively low concentration and of others like "concentrated" or "strong" for solutions of relatively high concentration. Those terms relate the amount of a substance in a mixture to the observable intensity of effects or properties caused by that substance. For example, a practical rule is that that the more concentrated a chromatic solution is, the more intensely coloured it is.

Quantitative notation

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For scientifical or technical applications, a qualitative account of concentration is almost never sufficient, therefore Quantitative notations are needed to describe concentration. There are a number of different ways to quantitatively express concentration; the most common are listed below.

Note: Many units of concentration require measurement of a substance's volume, which is variable depending on ambient temperature and pressure. Unless otherwise stated, all the following measurements are assumed to be at standard state temperature and pressure (that is, 25 degrees Celsius at 1 atmosphere or 101.325 kPa).

Mass percentage

Mass percentage denotes the mass of a substance in a mixture as a percentage of the mass of the entire mixture. For instance: if a bottle contains 40 grams of ethanol and 60 grams of water, then it contains 40% ethanol by mass. Commercial concentrated aqueous reagents such as acids and bases are often labeled in concentrations of weight percentage with the specific gravity also listed. In older texts and references this is sometimes referred to as weight-weight percentage (abbreviated as w/w). In water pollution chemistry, a common term of measuring total mass percentage of dissolved solids in an aqueous medium is total dissolved solids.

Mass-volume percentage

Mass-volume percentage, (sometimes referred to as weight-volume percentage and often abbreviated as % m/v or % w/v) denotes the mass of the substance in a mixture as a percentage of the volume of the entire mixture. Mass-volume percentage is often used for solutions made from solid reagents. It is the mass of the solute in grams divided by the volume of solution in millilitres and multiplied by one hundred.

Volume-volume percentage

Volume-volume percentage or % (v/v) describes the volume of the solute in mL per 100 mL of the resulting solution. This is most useful when a liquid - liquid solution is being prepared. For example, beer is about 5% ethanol by volume. This means every 100 mL beer contains 5 mL ethanol (ethyl alcohol).

Molarity

Molarity (M) denotes the number of moles of a given substance per litre of solution. For instance: 4.0 litres of liquid, containing 2.0 moles of dissolved particles, constitutes a solution of 0.5 M. Such a solution may be described as "0.5 molar." (Working with moles can be highly advantageous, as they enable measurement of the absolute number of particles in a solution, irrespective of their weight and volume. This is often more useful when performing stoichiometric calculations.). See molar solution for further information.

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/m³ or other approved SI units such as mol/L ^*.

Molality

Molality (m) denotes the number of moles of a given substance per kilogram of solvent. For instance: 2.0 kilograms of solvent, containing 1.0 moles of dissolved particles, constitutes a molality of 0.5 mol/kg. Such a solution may be described as "0.5 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.

NIST considers the symbol m to be obsolete, and instead suggests using SI units such as mol/kg.

The advantage of molality is that it does not change with the temperature as it deals with the mass of solvent, rather than the volume of solution. Volume typically increases with increase in temperature resulting in decrease in molarity. Molality of a solution is always constant irrespective of the physical conditions like temperature and pressure.

Molinity

Molinity is a rarely-used term that denotes the number of moles of a given substance per kilogram of solution. For instance: imagine 2.0 kg of solvent, plus 1.0 mol of dissolved particles, weighs a total of 2.5 kg. The molinity of the solution is therefore 1 mol / 2.5 kg = 0.4 mol/kg.

Note: molarity and molinity are calculated using the volume of the entire solution, but molality is calculated using the mass of solvent only.

Normality

This type of concentration highlights the chemical nature of salts: in solution, salts break apart into distinct reactive species (ions such as Na⁺, Fe³⁺, or Ag²⁺, but those distinct species can never be isolated and measured independently - they only come as part of a charge-balanced salt. Thus, the need for normality, which is a measure of reactive species in a solution.

Definition:

A normal is one gram equivalent of a solute per liter of solution. The definition of a gram equivalent varies depending on the type of chemical reaction that is discussed - it can refer to acids, bases, redox species, and ions that will precipitate.

Usage:

It is critical to note that normality measures a single ion which takes part in an overall solute. For example, one could determine the normality of hydroxide or sodium in an aqueous solution of sodium hydroxide, but the normality of sodium hydroxide itself has no meaning.

Specific Cases:

As ions in solution can react through different pathways, there are three common definitions for normality as a measure of reactive species in solution:

• In acid-base chemistry, Normality is used to express the concentration protons or hydroxide ions in the solution. Here, the normality differs from the molarity by an integer value - each solute can produce n equivalents of reactive species when dissolved. For example: 1 M aqueous Ca(OH)₂ is 2 N (normal) in hydroxide.

• In redox reactions, normality measures the quantity of oxidizing or reducing agent that can accept or furnish one mole of electrons. Here, the normality scales from the molarity, most commonly, by a fractional value. Calculating the normality of redox species in solution can be challenging.

• In precipitation reactions, normality measures the concentration of ions which will precipitate in a given reaction. Here, the normality scales from the molarity again by an integer value.

Practical Uses:

The measure of normality is extremely useful for titrations - given two species that are known to react with a known ratio, one simply needs to scale the volumes of solutions with known normalities to get a complete reaction with the following equation:

N_aV_a=N_bV_b

Mole fraction

The mole fraction χ, chi (also called molar fraction) denotes the number of moles of solute as a proportion of the total number of moles in a solution. For instance: 1 mole of solute dissolved in 9 moles of solvent would have a mole fraction of 1/10 or 0.1.

Formal

The formal (F) is yet another measure of concentration similar to molarity. It is rarely used. It is calculated based on the formula weights of chemicals per litre of solution. The difference between formal and molar concentrations is that the formal concentration indicates moles of the original chemical formula in solution, without regard for the species that actually exist in solution. Molar concentration, on the other hand, is the concentration of species in solution.

For example: if one dissolves sodium carbonate (Na₂CO₃) in a litre of water, the compound dissociates into the Na⁺ and CO₃^2- ions. Some of the CO₃^2- reacts with the water to form HCO₃^- and H₂CO₃. If the pH of the solution is low, there is practically no Na₂CO₃ left in the solution. So, although we have added 1 mol of Na₂CO₃ to the solution, it does not contain 1 M of that substance. (Rather, it contains a molarity based on the other constituents of the solution.) However, one can still say that the solution contains 1 F of Na₂CO₃.

"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 chemicaly 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. e.g. 10 parts per million (ppm) sugar in water means that there are 10 mg of sugar in 999,990 mg of water.

• Parts per hundred (denoted by '%' 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 gm in a total weight of 100 gm. This is the common percent. 1 part in 10².

• Parts per thousand (denoted by '‰' per mil symbol, and occasionally 'ppt') denotes 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 liter in a total volume of 1000 liters. 1 part in 10³.

• Parts per million ('ppm') denotes 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 mg in a total weight of 1,000,000 mg. 1 part in 10⁶.

• Parts per billion ('ppb') denotes 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 gallon in a total volume of 1 billion gallons. 1 part in 10⁹.

• Parts per trillion ('ppt') denotes 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 pound in a total weight of 1 trillion pounds. 1 part in 10¹².

• Parts per quadrillion ('ppq') denotes 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. There are currently no analytical techniques that can measure ppq concentrations. 1 part in 10¹⁵.

Warning: although 'ppt' is usually used to denote 'parts per trillion', it is also on occasion used to denote 'parts per thousand'. If there is any chance of ambiguity, one should describe the abbreviation in full.

According to the U.S. National Institute of Standards and Technology (NIST) Guide for the Use of the International System of Units (SI), "the language-dependent terms part per million, part per billion, and part per trillion ... are not acceptable for use with the SI to express the values of quantities." ^* which lists examples of alternative expressions.

Notes for clarity: 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.

Techniques used to determine concentration

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• Spectrophotometry
• Chromatography
• Various titration methods

Table of concentration measures

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Frequently used standards of concentration

Measurement	Notation	Generic formula	Typical units
Mass percentage	-	$\backslash left\; (\; \backslash frac\{\backslash mathrm\{grams\}\backslash \; \backslash mathrm\{solute\}\; \backslash times\; 100\}\{\backslash mathrm\{grams\}\backslash \; \backslash mathrm\{solution\}\}\; \backslash right\; )$	%
Mass-volume percentage	-	$\backslash left\; (\; \backslash frac\{\backslash mathrm\{grams\}\backslash \; \backslash mathrm\{solute\}\; \backslash times\; 100\}\{\backslash mathrm\{millilitres\}\backslash \; \backslash mathrm\{solution\}\}\; \backslash right\; )$	% though strictly %kg/L
Volume-volume percentage	-	$\backslash left\; (\; \backslash frac\{\backslash mathrm\{millilitres\}\backslash \; \backslash mathrm\{solute\}\; \backslash times\; 100\}\{\backslash mathrm\{millilitres\}\backslash \; \backslash mathrm\{solution\}\}\; \backslash right\; )$	%
Molarity	M	$\backslash left\; (\; \backslash frac\{\backslash mathrm\{moles\}\backslash \; \backslash mathrm\{solute\}\}\{\backslash mathrm\{litres\}\backslash \; \backslash mathrm\{solution\}\}\; \backslash right\; )$	mol/L (or M)
Molinity	-	$\backslash left\; (\; \backslash frac\{\backslash mathrm\{moles\}\backslash \; \backslash mathrm\{solute\}\}\{\backslash mathrm\{kilograms\}\backslash \; \backslash mathrm\{solution\}\}\; \backslash right\; )$	mol/kg
Molality	m	$\backslash left\; (\; \backslash frac\{moles\backslash \; solute\}\{kilograms\backslash \; solvent\}\; \backslash right\; )$	mol/kg (or m)
Molar fraction	χ (chi)	$\backslash left\; (\; \backslash frac\{moles\backslash \; solute\}\{moles\backslash \; solution\}\; \backslash right\; )$	(fraction)
Formal	F	$\backslash left\; (\; \backslash frac\{moles\backslash \; undissolved\backslash \; solute\}\{litres\backslash \; solution\}\; \backslash right\; )$	mol/L (or F)
Normality	N	$\backslash left\; (\; \backslash frac\{gram\backslash \; equivalents\}\{litres\backslash \; solution\}\; \backslash right\; )$	N
Parts per hundred	% (or pph)	$\backslash left\; (\; \backslash frac\{dekagrams\backslash \; solute\}\{kilograms\backslash \; solution\}\; \backslash right\; )$	da.g/kg
Parts per thousand	‰ (or ppt*)	$\backslash left\; (\; \backslash frac\{grams\backslash \; solute\}\{kilograms\backslash \; solution\}\; \backslash right\; )$	g/kg
Parts per million	ppm	$\backslash left\; (\; \backslash frac\{milligrams\backslash \; solute\}\{kilograms\backslash \; solution\}\; \backslash right\; )$	mg/kg
Parts per billion	ppb	$\backslash left\; (\; \backslash frac\{micrograms\backslash \; solute\}\{kilograms\backslash \; solution\}\; \backslash right\; )$	μg/kg
Parts per trillion	ppt*	$\backslash left\; (\; \backslash frac\{nanograms\backslash \; solute\}\{kilograms\backslash \; solution\}\; \backslash right\; )$	ng/kg
Parts per quadrillion	ppq	$\backslash left\; (\; \backslash frac\{picograms\backslash \; solute\}\{kilograms\backslash \; solution\}\; \backslash right\; )$	pg/kg

* Although 'ppt' is usually used to denote 'parts per trillion', it is on occasion used for 'parts per thousand'. Sometimes 'ppt' is also used as an abbreviation for precipitate.

Note (1) : The table above is described in terms of solvents and solutes; however the units given often also apply to other types of mixture.

Note (2) : The use of billion, trillion, quadrillion above follows the short scale usage of these words.

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