ENVIRONMENTAL BIOLOGY (TKA3104) LECTURE NOTES -1Unit of Concentration

8/9/2019 ENVIRONMENTAL BIOLOGY (TKA3104) LECTURE NOTES -1Unit of Concentration

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CHAPTER 1

Unit of Concentration


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Concentration

In chemistry, concentration is the measure ofhow much of a given substance there is mixedwith another substance. This can apply to anysort of chemical mixture, but most frequently the

concept is limited to homogeneoussolutions,where it refers to the amount of solutein asubstance.

To concentrate a solution, one must add moresolute, or reduce the amount of solvent (forinstance, by selective evaporation). By contrast,to dilute a solution, one must add more solvent,or reduce the amount of solute.
http://en.wikipedia.org/wiki/Chemistryhttp://en.wikipedia.org/wiki/Chemical_substancehttp://en.wikipedia.org/wiki/Solutionhttp://en.wikipedia.org/wiki/Solutehttp://en.wikipedia.org/wiki/Solventhttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Evaporationhttp://en.wikipedia.org/wiki/Solventhttp://en.wikipedia.org/wiki/Solutehttp://en.wikipedia.org/wiki/Solutionhttp://en.wikipedia.org/wiki/Chemical_substancehttp://en.wikipedia.org/wiki/Chemistry


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Unless two substances are fully misciblethere exists aconcentration at which no further solute will dissolve in asolution. At this point, the solution is said to be saturated.If additional solute is added to a saturated solution, it will

not dissolve (except in certain circumstances, whensupersaturation may occur). Instead, phase separationwill occur, leading to either coexisting phases or asuspension. The point of saturation depends on manyvariables 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.
http://en.wikipedia.org/wiki/Saturation_(chemistry)http://en.wikipedia.org/wiki/Supersaturationhttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Suspension_(chemistry)http://en.wikipedia.org/wiki/Suspension_(chemistry)http://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Supersaturationhttp://en.wikipedia.org/wiki/Saturation_(chemistry)


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Qualitative description

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 highconcentration. Those terms relate the amount ofa substance in a mixture to the observableintensity of effects or properties caused by that

substance. For example, a practical rule is thatthe more concentrated a chromatic solution is,the more intensely colored it is (usually).
http://en.wikipedia.org/wiki/Qualitativehttp://en.wikipedia.org/wiki/Colorhttp://en.wikipedia.org/wiki/Colorhttp://en.wikipedia.org/wiki/Qualitative


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These glasses containing red dye demonstrate qualitative changes inconcentration. The solutions on the left are more dilute, compared tothe more concentrated solutions on the right.
http://en.wikipedia.org/wiki/File:Dilution-concentration_simple_example.jpg


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Quantitative notation

For scientific or technical applications, aqualitative account of concentration is almostnever sufficient; therefore quantitative measuresare needed to describe concentration. There are

a number of different ways to quantitativelyexpress concentration; the most common arelisted below. They are based on mass, volume,or both. Depending on what they are based on itis not always trivial to convert one measure to

the other, because knowledge of the densitymight be needed to do so. At times thisinformation may not be available, particularly ifthe temperature varies.
http://en.wikipedia.org/wiki/Quantitativehttp://en.wikipedia.org/wiki/Quantitative


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Mass versus volume

Some units of concentration particularly the mostpopular one, molarity require knowledge of asubstance's volume, which unlike mass is variabledepending on ambient temperature and pressure. In fact(partial) molar volume can even be a function ofconcentration itself. This is why volumes are notnecessarily completely additive when two liquids are addedand mixed. Volume-based measures for concentration aretherefore not to be recommended for non-dilute solutionsor problems where relatively large differences in

temperature are encountered (e.g. for phase diagrams). Unless otherwise stated, all the following measurements of

volume are assumed to be at astandard statetemperatureand pressure (for example 25 degreesCelsiusat 1atmosphereor 101.325kPa). The measurement of mass

does not require such restrictions.
http://en.wikipedia.org/wiki/Partial_molar_volumehttp://en.wikipedia.org/wiki/Phase_diagramhttp://en.wikipedia.org/wiki/Standard_statehttp://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Atmosphere_(unit)http://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Standard_statehttp://en.wikipedia.org/wiki/Phase_diagramhttp://en.wikipedia.org/wiki/Partial_molar_volume


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Mass can be determined at a precision of < 0.2 mg on aroutine basis with an analytical balance and more preciseinstruments exist. Both solids and liquids are easilyquantified by weighing.

The volume of a liquid is usually determined by calibratedglassware such as burettes and volumetric flasks. For verysmall volumes precision syringes are available. The use ofgraduated beakers and cylinders is not recommended astheir indication of volume is mostly for decorative ratherthan quantitative purposes. The volume of solids,

particularly of powders, is often difficult to measure, whichis why mass is the more usual measure. For gases theopposite is true: the volume of a gas can be measured in agas burette, if care is taken to control the pressure, but themass is not easy to measure due to buoyancy effects.
http://en.wikipedia.org/wiki/Weighing_scalehttp://en.wikipedia.org/wiki/Weighing_scale


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Molarity

Molarity (in units of mol/L, molar, or M) or molarconcentration denotes the number of moles of agiven substance per liter of solution. A capital

letter M is used to abbreviate the units of mol/L.For instance:

The actual formula for molarity is:
http://en.wikipedia.org/wiki/Molarityhttp://en.wikipedia.org/wiki/Molar_concentrationhttp://en.wikipedia.org/wiki/Molar_concentrationhttp://en.wikipedia.org/wiki/Mole_(unit)http://en.wikipedia.org/wiki/Literhttp://en.wikipedia.org/wiki/Literhttp://en.wikipedia.org/wiki/Mole_(unit)http://en.wikipedia.org/wiki/Molar_concentrationhttp://en.wikipedia.org/wiki/Molar_concentrationhttp://en.wikipedia.org/wiki/Molarity


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Such a solution may be described as "0.50 molar." It mustbe emphasized that a 0.5 molar solution contains 0.5moles of solute in 1.0 liter of solution. This is notequivalentto 1.0 liter of solvent. A 0.5 mol/L solution will containeither slightly more or slightly less than 1 liter of solventbecause the process of dissolution causes the volume ofthe liquid to increase or decrease.

Following the SI system of units, the National Institute ofStandards and Technology, the United States authority onmeasurement, considers the term molarity and the unit

symbol M to be obsolete, and suggests instead theamount-of-substance concentration (c) with unitsmol/m3 or other units used alongside the SI such as mol/L.This recommendation has not been universallyimplemented in academia or chemistry research yet.
http://en.wikipedia.org/wiki/SIhttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/Measurementhttp://en.wikipedia.org/wiki/Measurementhttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/SI


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Preparation of a solution of known molarity involves adding anaccurately weighed amount of solute to a volumetric flask, addingsome solvent to dissolve it, then adding more solvent to fill to thevolume mark.

When discussing the molarity of minute concentrations, such as in

pharmacological research, molarity is expressed in units of millimolar(mmol/L, mM, 1 thousandth of a molar), micromolar (mol/L, M, 1millionth of a molar) or nanomolar (nmol/L, nM, 1 billionth of a molar).

Although molarity is by far the most commonly used measure ofconcentration, particularly for dilute aqueous solutions, it does sufferfrom a number of disadvantages. Masses can be determined with greatprecision as balances are often very precise. Determining volume is

often not as precise. In addition, due to a thermal expansion, themolarity of a solution changes with temperature without adding orremoving any mass. For non-dilute solutions another problem is thatthe molar volume of a substance is itself a function of concentration sothat volume is not strictly additive.
http://en.wikipedia.org/wiki/Volumetric_flaskhttp://en.wikipedia.org/wiki/Pharmacologyhttp://en.wikipedia.org/wiki/Weighing_scalehttp://en.wikipedia.org/wiki/Thermal_expansionhttp://en.wikipedia.org/wiki/Thermal_expansionhttp://en.wikipedia.org/wiki/Weighing_scalehttp://en.wikipedia.org/wiki/Pharmacologyhttp://en.wikipedia.org/wiki/Volumetric_flask


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Molality

Molality (mol/kg, molal, or m) denotes the number of molesof soluteper kilogram of solvent(not solution). Forinstance: adding 1.0 mole of solute to 2.0 kilograms ofsolvent constitutes a solution with a molality of 0.50 mol/kg.Such a solution may be described as "0.50 molal". Theterm molal solutionis used as a shorthand for a "one molalsolution", i.e. a solution which contains one mole of thesolute per 1000 grams of the solvent.

Following the SI system of units, the National Institute ofStandards and Technology, the United States authority on

measurement, considers the unit symbol mto be obsolete,and suggests instead the term 'molality of substance B'(mB) with units mol/kg or a related unit of the SI. Thisrecommendation has not been universally implemented inacademia yet.
http://en.wikipedia.org/wiki/Molalityhttp://en.wikipedia.org/wiki/Mole_(unit)http://en.wikipedia.org/wiki/Kilogramhttp://en.wikipedia.org/wiki/Mole_(unit)http://en.wikipedia.org/wiki/SIhttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/Measurementhttp://en.wikipedia.org/wiki/Measurementhttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttp://en.wikipedia.org/wiki/SIhttp://en.wikipedia.org/wiki/Mole_(unit)http://en.wikipedia.org/wiki/Kilogramhttp://en.wikipedia.org/wiki/Mole_(unit)http://en.wikipedia.org/wiki/Molality


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Note that molality is sometimes represented by thesymbol (m), while molarity by the symbol (M). Thetwo symbols are not meant to be confused, andshould not be used as symbols for units. The SI

unit for molality is mol/kg. (The unit mmeansmeter.)

Like other mass-based measures, thedetermination of molality only requires a good

scale, because the masses of both solvent andsolute can be obtained by weighing, and molalityis independent of the physical conditions liketemperature and pressure, providing advantagesover molarity.
http://en.wikipedia.org/wiki/SIhttp://en.wikipedia.org/wiki/SI


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In a dilute aqueous solution near roomtemperature and standard atmosphericpressure, the molarity and molality will be very

similar in value. This is because 1 kg of waterroughly corresponds to a volume of 1 L at theseconditions, 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.
http://en.wikipedia.org/wiki/Aqueous_solutionhttp://en.wikipedia.org/wiki/Aqueous_solution


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Mole fraction

The mole fraction, (also called molar fraction)denotes the number of moles of solute as aproportion of the total number of moles in a

solution. For instance: 1 mole of solute dissolvedin 9 moles of solvent has a mole fraction of 1/10or 0.1. Mole fractions are dimensionlessquantities. (The mole percentageor molar

percentage, denoted "mol %" and equal to 100%times the mole fraction, is sometimes quotedinstead of the mole fraction.)
http://en.wikipedia.org/wiki/Mole_fractionhttp://en.wikipedia.org/wiki/Mole_fraction


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This measure is used very frequently in the construction ofphase diagrams. It has a number of advantages:

the measure is not temperature dependent (such asmolarity) 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 =0.1and =0.9, the roles of 'solvent' and 'solute' arereversed.
http://en.wikipedia.org/wiki/Phase_diagramhttp://en.wikipedia.org/wiki/Phase_diagram


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As both mole fractions and molality are onlybased on the masses of the components it iseasy to convert between these measures. This

is not true for molarity, which requiresknowledge of the density.


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Mass percentage (fraction)

Mass percentagedenotes the mass of a substance in amixture as a percentage of the mass of the entiremixture. (Mass fraction xm can be used instead of masspercentage by dividing mass percentage to 100.) Forinstance: if a bottle contains 40 grams of ethanol and 60

grams of water, then it contains 40% ethanol by mass or0.4 mass fraction ethanol. Commercial concentratedaqueous reagents such as acids and bases are oftenlabeled in concentrations of weight percentagewith thespecific gravity also listed. In older texts and references

this is sometimes referred to as weight-weightpercentage(abbreviated as w/wor wt%). In waterpollution chemistry, a common term of measuring totalmass percentage of dissolved solids in an aqueousmedium is total dissolved solids.
http://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Mass_fraction_(chemistry)http://en.wikipedia.org/wiki/Gramhttp://en.wikipedia.org/wiki/Ethanolhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Specific_gravityhttp://en.wikipedia.org/wiki/Water_pollutionhttp://en.wikipedia.org/wiki/Water_pollutionhttp://en.wikipedia.org/wiki/Total_dissolved_solidshttp://en.wikipedia.org/wiki/Total_dissolved_solidshttp://en.wikipedia.org/wiki/Water_pollutionhttp://en.wikipedia.org/wiki/Water_pollutionhttp://en.wikipedia.org/wiki/Specific_gravityhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Ethanolhttp://en.wikipedia.org/wiki/Gramhttp://en.wikipedia.org/wiki/Mass_fraction_(chemistry)http://en.wikipedia.org/wiki/Mass


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Mass-volume percentage

Mass-volume percentage, (sometimes referredto as weight-volume percentage or percentweight per volume and often abbreviated as %

m/v or % w/v) describes the mass of the solutein g per 100 mL of the resulting solution. Mass-volume percentage is often used for solutionsmade from a solid solute dissolved in a liquid.

For example, a 40% w/v sugar solution contains40 g of sugar per 100 mL of resulting solution.
http://en.wikipedia.org/wiki/Percentagehttp://en.wikipedia.org/wiki/Percentage


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Volume-volume percentage

Volume-volume percentage(sometimes referredto as percent volume per volume andabbreviated as % v/v) describes the volume of

the solute in mL per 100 mL of the resultingsolution. This is most useful when a liquid - liquidsolution is being prepared, although it is used formixtures of gases as well. For example, a 40%

v/v ethanol solution contains 40 mL ethanol per100 mL total volume. The percentages are onlyadditive in the case of mixtures of ideal gases.
http://en.wikipedia.org/wiki/Ideal_gashttp://en.wikipedia.org/wiki/Ideal_gas


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Normality

Normality highlights the chemical nature ofsalts: in solution, salts dissociate intodistinct reactive species (ions such as H+,

Fe3+

, or Cl-

). Normality accounts for anydiscrepancy between the concentrations ofthe various ionic species in a solution. Forexample, in a salt such as MgCl2, there

are two moles of Cl- for every mole ofMg2+, so the concentration of Cl- is said tobe 2 N (read: "two normal").


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Definition

A normal is one gram equivalent of asolute per liter of solution. The definition ofa gram equivalent varies depending on the

type of chemical reaction that is discussed- it can refer to acids, bases, redoxspecies, and ions that will precipitate.
http://en.wikipedia.org/wiki/Gram_equivalenthttp://en.wikipedia.org/wiki/Gram_equivalent


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Usage

It is critical to note that normality measures a single ionwhich takes part in an overall solute. For example, onecould determine the normality of hydroxide or sodium inan aqueous solution of sodium hydroxide, but the

normality of sodium hydroxide itself has no meaning.Nevertheless it is often used to describe solutions ofacids or bases, in those cases it is implied that thenormality refers to the H+ or OH ion. For example, 2Normal sulfuric acid (H2SO4), means that the normality of

H+

ions is 2, or that the molarity of the sulfuric acid is 1.Similarly for 1 Molar H3PO4 the normality is 3 as itcontains three H+ ions.
http://en.wikipedia.org/wiki/Sulfuric_acidhttp://en.wikipedia.org/wiki/Sulfuric_acid


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Specific cases

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

In acid-base chemistry, normality is used to express the concentrationof protons or hydroxide ions in the solution. Here, the normality differsfrom the molarity by an integer value - each solute can produce nequivalents of reactive species when dissolved. For example: 1 Maqueous Ca(OH)2 is 2 N (normal) in hydroxide.

In redox reactions, normality measures the quantity of oxidizing orreducing agent that can accept or furnish one mole of electrons. Here,the normality scales from the molarity, most commonly, by a fractionalvalue. 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 scalesfrom the molarity again by an integer value.
http://en.wikipedia.org/wiki/Redoxhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Precipitation_(chemistry)http://en.wikipedia.org/wiki/Precipitation_(chemistry)http://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Redox


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Practical uses

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

following equation:NaVa=NbVb

However, normality cannot reliably represent anunambiguous measure of the concentration of a solution.Since the measure of normality depends on the reactionthat the solute participates in, the same concentration ofsolute can possess two differentnormalities for twodifferent reactions. For example, Mg2+ is 2 N with respectto a Cl- ion, but it is only 1 N with respect to an O2- ion.


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Accordingly, normality is no longer used torepresent the concentration of a solution assuch. Instead, a solution should be labeled

according to its molarity, and it is then possibleto calculate the normality for a particular titration

using the equation above.


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Equivalents

Expression of concentration in equivalents perliter (or more commonly, milliequivalents perliter) is based on the same principle asnormality. A normal solution is one equivalentper liter of solution (Eq/L). The use ofequivalents and milliequivalents as a means ofexpressing concentration is losing favor, butmedical reporting of serum concentrations in

mEq/L still occurs.
http://en.wikipedia.org/wiki/Equivalent_(chemistry)http://en.wikipedia.org/wiki/Equivalent_(chemistry)


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Formal

The formal(F) is yet another measure ofconcentration similar to molarity. Formalconcentrations are sometimes used whensolving chemical equilibrium problems. It is

calculated based on the formula weights ofchemicals per liter of solution. The differencebetween formal and molar concentrations is thatthe formal concentration indicates moles of theoriginal chemical formula in solution, without

regard for the species that actually exist insolution. Molar concentration, on the other hand,is the concentration of species in solution.


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For example: if one dissolves sodium carbonate(Na2CO3) in a litre of water, the compounddissociates into the Na+ and CO3

2- ions. Some ofthe CO3

2- reacts with the water to form HCO3-

and H2CO3. If the pH of the solution is low, thereis practically no Na2CO3 left in the solution. So,although we have added 1 mol of Na2CO3 to thesolution, it does not contain 1 M of that

substance. (Rather, it contains a molarity basedon the other constituents of the solution.)However, it was once said that such solutionscontain 1 F of Na2CO3.


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"Parts-per" notation

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

Parts perhundred(denoted by '%' [the per cent symbol], and veryrarely 'pph') - denotes the amount of a given substance in a totalamount of 100 regardless of the units of measure as long as they arethe same. e.g. 1 gram per 100 gram. 1 part in 102.

Parts perthousand(denoted by '' [the per mille symbol], andoccasionally 'ppt', though this should be avoided) denotes the amount

of a given substance in a total amount of 1000 regardless of the unitsof measure as long as they are the same. e.g. 1 milligram per gram, or1 gram per kilogram. 1 part in 103.
http://en.wikipedia.org/wiki/Parts-per_notationhttp://en.wikipedia.org/wiki/100_(number)http://en.wikipedia.org/wiki/1000_(number)http://en.wikipedia.org/wiki/1000_(number)http://en.wikipedia.org/wiki/100_(number)http://en.wikipedia.org/wiki/Parts-per_notation


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Parts permillion('ppm') denotes the amount of a given substance ina total amount of 1,000,000 regardless of the units of measure usedas long as they are the same. e.g. 1 milligram per kilogram. 1 part in106.

Parts perbillion('ppb') denotes the amount of a given substance in atotal 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 pertrillion('ppt') denotes the amount of a given substance in a

total amount of 1,000,000,000,000 regardless of the units ofmeasure as long as they are the same. e.g. 1 milligram perkilotonne. 1 part in 1012.

Parts perquadrillion('ppq') denotes the amount of a given

substance in a total amount of 1,000,000,000,000,000 regardless ofthe units of measure as long as they are the same. e.g. 1 milligramper megatonne. 1 part in 1015.
http://en.wikipedia.org/wiki/1000000_(number)http://en.wikipedia.org/wiki/1000000000_(number)http://en.wikipedia.org/wiki/1000000000000_(number)http://en.wikipedia.org/wiki/1000000000000000_(number)http://en.wikipedia.org/wiki/1000000000000000_(number)http://en.wikipedia.org/wiki/1000000000000_(number)http://en.wikipedia.org/wiki/1000000000_(number)http://en.wikipedia.org/wiki/1000000_(number)


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Frequently used standards of concentration

Measurement Notation Generic formula Typical units

atomic percentage (A) at.% %

atomic percentage (B) at.% %

Mass percentage wt% %

Mass-volume percentage -% though

strictly%g/mL

Volume-volumepercentage

- %

Molarity Mmol/L (or M or

mol/dm3)

Molinity - mol/kg

Molality m mol/kg (or m**)
http://en.wikipedia.org/wiki/Atomic_percenthttp://en.wikipedia.org/wiki/Atomic_percenthttp://en.wikipedia.org/wiki/Atomic_percenthttp://en.wikipedia.org/wiki/Atomic_percent


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Molar fraction (chi) (decimal)

Formal F mol/L (or F)

Normality N N

Parts per hundred % (or pph) dg/kg

Parts per thousand (or ppt*) g/kg

Parts per million ppm mg/kg

Parts per billion ppb g/kg

Parts per trillion ppt* ng/kg

Parts per quadrillion ppq pg/kg

ENVIRONMENTAL BIOLOGY (TKA3104) LECTURE NOTES -1Unit of Concentration

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