In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties A chemical substance is a Material with a definite chemical composition. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in a substance. In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances

To concentrate a solution, one must add more solute, or reduce the amount of solvent (for instance, by selective evaporation). Evaporation is the process by which Molecules in a Liquid state (e By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute.

Unless two substances are fully miscible there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. In Chemistry, saturation has five different meanings In Physical chemistry, saturation is the point at which a Solution of a substance If additional solute is added to a saturated solution, it will not dissolve (except in certain circumstances, when supersaturation may occur). The term supersaturation refers to a Solution that contains more of the dissolved material than could be dissolved by the Solvent under normal circumstances Instead, phase separation will occur, leading to either coexisting phases or a suspension. In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties In Chemistry, A suspension is a Heterogenous fluid containing Solid particles that are sufficiently large for Sedimentation. The point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute.

Analytical concentration includes all the forms of that substance in the solution.

Qualitative description

These glasses containing red dye demonstrate qualitative changes in concentration. The solutions on the left are more dilute, compared to the more concentrated solutions on the right.

Often in informal, non-technical language, concentration is described 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 the more concentrated a chromatic solution is, the more intensely colored it is.

Quantitative notation

For scientific or technical applications, a qualitative account of concentration is almost never sufficient; therefore quantitative measures are needed to describe concentration. A quantitative attribute is one that exists in a range of magnitudes and can therefore be measured. There are a number of different ways to quantitatively express concentration; the most common are listed below. They are based on mass, volume, or both. Depending on what they are based on it is not always trivial to convert one measure to the other, because knowledge of the density might be needed to do so. At times this information may not be available, particularly if the temperature varies.

Mass versus volume

Some units of concentration — particularly the most popular one, molarity — require knowledge of a substance's volume, which unlike mass is variable depending on ambient temperature and pressure. In fact (partial) molar volume can even be a function of concentration itself. This is why volumes are not necessarily completely additive when two liquids are added and mixed. Volume-based measures for concentration are therefore not to be recommended for non-dilute solutions or problems where relatively large differences in temperature are encountered (e. g. for phase diagrams). In Physical chemistry, Mineralogy, and Materials science, a phase diagram is a type of graph used to show the equilibrium conditions

Unless otherwise stated, all the following measurements of volume are assumed to be at a standard state temperature and pressure (for example 25 degrees Celsius at 1 atmosphere or 101. In Chemistry, the standard state of a material is its state at 1 bar (100 Kilopascals exactly The Celsius Temperature scale was previously known as the centigrade scale. The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm 325 kPa). The measurement of mass does not require such restrictions.

Mass can be determined at a precision of < 0. 2 mg on a routine basis with an analytical balance and more precise instruments exist. A weighing scale (usually just "scale" in common usage except in Australian English where "scales" is more common is a Measuring instrument for Both solids and liquids are easily quantified by weighing.

The volume of a liquid is usually determined by calibrated glassware such as burettes and volumetric flasks. For very small volumes precision syringes are available. The use of graduated beakers and cylinders is not recommended as their indication of volume is mostly for decorative rather than quantitative purposes. The volume of solids, particularly of powders, is often difficult to measure, which is why mass is the more usual measure. For gases the opposite is true: the volume of a gas can be measured in a gas burette, if care is taken to control the pressure, but the mass is not easy to measure due to buoyancy effects.

Molarity

See also: Molar solution Molar concentration

Molarity (mol/L, molar, or M) or molar concentration denotes the number of moles of a given substance per liter of solution. A molar Solution is one that contains one mole of solute per Litre of solution In Chemistry, molar concentration, also called molarity, is a measure of the Concentration of a Solute in a Solution, or of any The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this The litre or liter (see spelling differences) is a unit of Volume. M = mol/L. For instance:

The actual formula for Molarity is:

Such a solution may be described as "0. 50 molar. " It must be emphasized that a 0. 5 molar solution contains 0. 5 moles of solute in 1. 0 liter of solution. This is not equivalent to 1. 0 liter of solvent. A 0. 5 mol/L solution will contain either slightly more or slightly less than 1 liter of solvent because the process of dissolution causes the volume of the liquid to increase or decrease.

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/m³ or other units used alongside the SI such as mol/L^[1]. The United States of America —commonly referred to as the Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as This recommendation has not been universally implemented in academia yet.

Preparation of a solution of known molarity involves adding an accurately weighed amount of solute to a volumetric flask, adding some solvent to dissolve it, then adding more solvent to fill to the volume mark. A volumetric flask is a piece of laboratory Glassware used in Analytical chemistry for the preparation of Solutions.

When discussing the molarity of minute concentrations, such as in a lot of pharmacological research, molarity is expressed in millimolars (mmol/L, mM, 1 thousandth of a molar), micromolars (μmol/L, μM, 1 millionth of a molar) or nanomolars (nmol/L, nM, 1 billionth of a molar).

Although molarity is by far the most commonly used measure for concentration, particularly for dilute aqueous solutions, it does suffer from a number of disadvantages. Masses can be determined with great precision as balances are often very precise. A weighing scale (usually just "scale" in common usage except in Australian English where "scales" is more common is a Measuring instrument for Determining volume is often not as precise. In addition, a volume of a liquid changes with temperature so that the molarity also changes without adding or removing any mass. For non-dilute solutions another problem is that the molar volume of a substance is itself a function of concentration so that volume is not strictly additive.

Molality

Molality (mol/kg, molal, or m) denotes the number of moles of solute per kilogram of solvent (not solution). The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this For instance: adding 1. 0 mole of solute to 2. The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this 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_B) with units mol/kg or a related unit of the SI^[2]. The United States of America —commonly referred to as the Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as 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. )

Like other mass-based measures, the determination of molality only requires a good scale, because the masses of both solvent and solute can be obtained by weighing, and molality is independent of the physical conditions like temperature and pressure, providing advantages over molarity.

In a dilute aqueous solution near room temperature and standard atmospheric pressure, the molarity and molality will be very similar in value. This is because 1 kg of water roughly corresponds to a volume of 1 L at these conditions, and because the solution is dilute, the addition of the solute makes a negligible impact on the volume of the solution.

However, in all other conditions, this is usually not the case.

Mole fraction

The mole fraction Χ, (also called molar fraction) denotes the number of moles of solute as a proportion of the total number of moles in a solution. In Chemistry, the mole fraction of a component in a Mixture is the relative proportion of molecules belonging to the component to those in the mixture For instance: 1 mole of solute dissolved in 9 moles of solvent has a mole fraction of 1/10 or 0. 1. Mole fractions are dimensionless quantities. (The mole percentage or molar percentage, denoted "mol %" and equal to 100% times the mole fraction, is sometimes quoted instead of the mole fraction. )

This measure is used very frequently in the construction of phase diagrams. In Physical chemistry, Mineralogy, and Materials science, a phase diagram is a type of graph used to show the equilibrium conditions It has a number of advantages:

• the measure is not temperature dependent (such as molarity) and does not require knowledge of the densities of the phase(s) involved
• a mixture of known mole fraction can be prepared by weighing off the appropriate masses of the constituents
• the measure is symmetrical: in the mole fractions Χ=0. 1 and Χ=0. 9, the roles of 'solvent' and 'solute' are reversed.

As both mole fractions and molality are only based on the masses of the components it is easy to convert between these measures. This is not true for molarity, which requires knowledge of the density.

Mass percentage (fraction)

Mass percentage denotes the mass of a substance in a mixture as a percentage of the mass of the entire mixture. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object (Mass fraction x_m can be used instead of mass percentage by dividing mass percentage to 100. In Chemistry the mass fraction is the fraction of one substance (x_A with mass m_A to the total mixture mass m_{tot} would be defined ) For instance: if a bottle contains 40 grams of ethanol and 60 grams of water, then it contains 40% ethanol by mass or 0. For other uses of the words gram or gramme see Gram (disambiguation. Water is a common Chemical substance that is essential for the survival of all known forms of Life. 4 mass fraction ethanol. Commercial concentrated aqueous reagents such as acids and bases are often labeled in concentrations of weight percentage with the specific gravity also listed. Specific gravity is defined as the ratio of the Density of a given solid or liquid substance to the density of water at a specific temperature and pressure typically In older texts and references this is sometimes referred to as weight-weight percentage (abbreviated as w/w or wt%). In water pollution chemistry, a common term of measuring total mass percentage of dissolved solids in an aqueous medium is total dissolved solids. Water pollution is the contamination of Water bodies such as Lakes Rivers Oceans and Groundwater caused by human activities Total Dissolved Solids (often abbreviated TDS) is an expression for the combined content of all Inorganic and organic substances contained in a liquid

Mass-volume percentage

Mass-volume percentage, (sometimes referred to as weight-volume percentage or percent weight per volume and often abbreviated as % m/v or % w/v) describes the mass of the solute in g per 100 mL of the resulting solution. In Mathematics, a percentage is a way of expressing a number as a Fraction of 100 ( per cent meaning "per hundred" Mass-volume percentage is often used for solutions made from a solid solute dissolved in a liquid. For example, a 40% w/v sugar solution contains 40 g of sugar per 100 mL of resulting solution.

Mass-volume ratio

Often used in medicine and pharmacology, a ratio of the weight of a drug dissolved in a volume of water, is presented as, grams of solute : mL of water. Practitioners use the term "dilution" when referring to this arcane unit. The most ubiquitous example is epinephrine solutions where a 1:100,000 solution has 1 g epinephrine in 100,000 mL water. This is equivalent to 0. 01 g/L epinephrine solution.

When the volume considered is a gas, a specific approach is needed: the gas' pressure and temperature conditions must be considered. A typical use is in air pollution emission quantification. It is very common to find values such as 50 g/Nm³ or 50 g/m³N. The "N" before or after the cubic meter indicates that the gas is under the Standard conditions for temperature and pressure. In Physical sciences standard conditions for temperature and pressure are Standard sets of conditions for experimental measurements to allow comparisons to be made

Volume-volume percentage

Main article: Volume percent

Volume-volume percentage (sometimes referred to as percent volume per volume and abbreviated as % v/v) describes the volume of the solute in mL per 100 mL of the resulting solution. Volume percent is a common expression of a Solution 's Concentration. This is most useful when a liquid - liquid solution is being prepared, although it is used for mixtures of gases as well. For example, a 40% v/v ethanol solution contains 40 mL ethanol per 100 mL total volume. The percentages are only additive in the case of mixtures of ideal gases. These four properties that constitute an ideal gas can be easily remembered by the acronym RIPE which stands for - R andom Motion (molecules are in constant random motion

Normality

Normality highlights the chemical nature of salts: in solution, salts dissociate into distinct reactive species (ions such as H⁺, Fe³⁺, or Cl^-). Normality accounts for any discrepancy between the concentrations of the various ionic species in a solution. For example, in a salt such as MgCl₂, there are two moles of Cl^- for every mole of Mg²⁺, so the concentration of Cl^- as well as of Mg²⁺ is said to be 2 N (read: "two normal"). Further examples are given below.

Definition

A normal is one gram equivalent of a solute per liter of solution. In chemistry a gram equivalent is the mass in grams of a compound's Equivalent weight. 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. Nevertheless it is often used to describe solutions of acids or bases, in those cases it is implied that the normality refers to the H⁺ or OH⁻ ion. For example, 2 Normal sulfuric acid (H₂SO₄), means that the normality of H⁺ ions is 2, or that the molarity of the sulfuric acid is 1. Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. Similarly for 1 Molar H₃PO₄ the normality is 3 as it contains three H⁺ ions.

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 of 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. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J 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. Precipitation is the formation of a Solid in a Solution during a Chemical 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

However, normality cannot reliably represent an unambiguous measure of the concentration of a solution. Since the measure of normality depends on the reaction that the solute participates in, the same concentration of solute can possess two different normalities for two different reactions. For example, Mg²⁺ is 2 N with respect to a Cl^- ion, but it is only 1 N with respect to an O^2- ion.

Accordingly, normality is no longer used to represent the concentration of a solution as such. Instead, a solution should be labeled according to its molarity, and it is then possible to calculate the normality for a particular titration using the equation above. NIST has also stipulated that this unit is obsolete and recommends discontinuing its use.

Formal

The formal (F) is yet another measure of concentration similar to molarity. Formal concentrations are sometimes used when solving chemical equilibrium problems. It is calculated based on the formula weights of chemicals per liter 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, it was once said that such solutions contain 1 F of Na₂CO₃.

"Parts-per" notation

Main article: 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. "Parts-per" notation is used especially in Science and Engineering, to denote Ratios (relative proportions in measured quantities particularly "Parts-per" notation is used especially in Science and Engineering, to denote Ratios (relative proportions in measured quantities particularly 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.

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. Atmospheric chemistry is a branch of Atmospheric science in which the Chemistry of the Earth's atmosphere and that of other planets is studied Air pollution is the human introduction into the atmosphere of Chemicals Particulate matter, or Biological materials that cause harm or discomfort 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. Electrochemistry is a branch of Chemistry that studies Chemical reactions which take place in a Solution at the interface of an electron conductor Chemical engineering is the branch of Engineering that deals with the application of Physical science (e 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.

Table of concentration measures

Frequently used standards of concentration

\left ( \frac{\rm number~of~atoms~of~dopant \times 100}{\rm number~of~atoms~of~solution} \right )</math>	%
atomic percentage (B)	at. Atomic percent or at% is a measure of Concentration of Dopant, used in Chemistry, physics of Solid state lasers and Spectroscopy %		%
Mass percentage	wt%		%
Mass-volume percentage	-		% though strictly %g/mL
Volume-volume percentage	-		%
Molarity	M		mol/L (or M or mol/dm3)
Molinity	-		mol/kg
Molality	m		mol/kg (or m**)
Molar fraction	Χ (chi)		(decimal)
Formal	F		mol/L (or F)
Normality	N		N
Parts per hundred	% (or pph)		da. g/kg
Parts per thousand	‰ (or ppt*)		g/kg
Parts per million	ppm		mg/kg
Parts per billion	ppb		µg/kg
Parts per trillion	ppt*		ng/kg
Parts per quadrillion	ppq		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. Precipitation is the formation of a Solid in a Solution during a Chemical reaction.

** Obsolete unit symbols.

See also

• Chemical equilibrium
• Serial dilution

References