Session 2: Session 2: Fundamentals and Fundamentals and

Classical methods of Classical methods of quantitative elemental analysisquantitative elemental analysis

http://bcs.whfreeman.com/qca7e http://www.good-weighing-practice.com/gwp/proper-weighing

Measurement of Mass and Measurement of Mass and Volume:Volume:

Recognising random and Recognising random and

systematic errorssystematic errors

Recap:Recap:Quantitative Analysis - Quantitative Analysis -

PrinciplesPrinciples1)1) Define sample amount (mass or volume)Define sample amount (mass or volume)

2)2) Measure quantity proportional to analyte Measure quantity proportional to analyte concentrationconcentration

Measured property must vary in a defined way: Measured property must vary in a defined way: calibrationcalibration with known standards necessary with known standards necessary

Analysis must be specific: Analysis must be specific: InterferencesInterferences must be must be known and if possible be eliminatedknown and if possible be eliminated

AccuracyAccuracy: : Proximity of measured value to Proximity of measured value to accepted (or "true") value: accepted (or "true") value: must be must be determineddetermined

PrecisionPrecision: : Closeness of measured values to one Closeness of measured values to one another:another: must be defined and reported must be defined and reported

Measuring MassMeasuring Mass

Classical two-pan balance

Modern electronic analytical balance

F = m x g

From: Harris, 6th edition

Electroma-gnetic force to counter

Random errors in weight Random errors in weight measurements: measurements:

tolerances of analytical balances tolerances of analytical balances and weights for calibrationand weights for calibration

Analytical balances need to be calibrated Analytical balances need to be calibrated regularlyregularly

Typically use stainless steel weights (d = 8.0 Typically use stainless steel weights (d = 8.0 g/ml)g/ml)

From: Harris, 6th edition

Specifications for balancesSpecifications for balances

CapacityCapacity ReadabilityReadability Repeatability (standard deviation); Repeatability (standard deviation);

larger than readabilitylarger than readability

Capacity x readability:

Analytical balances: tens to hundreds of g x 0.1 - 0.01 mg Ultra-micro balance: e.g. 6 g x 0.0001 mg

Avoiding systematic error in Avoiding systematic error in weight measurements:weight measurements:

buoyancybuoyancy Any object displaces a certain amount of airAny object displaces a certain amount of air This reduces the apparent mass that a balance measuresThis reduces the apparent mass that a balance measures If density of the object being weighed is significantly If density of the object being weighed is significantly

different from calibration weights, buoyancy correction is different from calibration weights, buoyancy correction is necessary:necessary:

d

d

d

dm

ma

w

a

1

1'

m = true mass; m’ = measured mass; da = density of air (0.0012 g/ml); dw = density of calibration weights (8.0 g/ml); d = density of weighed object

Correcting buoyancy errorsCorrecting buoyancy errors

Buoyancy correction in dependence on density of weighed object

Exercise:Exercise: A bottle weighed 7.6500 g empty and 9.9700 g after A bottle weighed 7.6500 g empty and 9.9700 g after

introduction of an organic liquid with a density of 0.92 g introduction of an organic liquid with a density of 0.92 g cmcm-3-3. The balance was equipped with stainless steel . The balance was equipped with stainless steel weights having a density of 8.0 g cmweights having a density of 8.0 g cm-3-3. Correct the . Correct the weight of the sample for the effects of buoyancy.weight of the sample for the effects of buoyancy.

Avoiding systematic errors in Avoiding systematic errors in weight measurementsweight measurements

Temperature effectsTemperature effects Convection air currents Convection air currents Warm air in balance weighs lessWarm air in balance weighs less Measured mass of object appears lowerMeasured mass of object appears lower Essential to weigh at room temperatureEssential to weigh at room temperature

Prevent object from picking up moisture:Prevent object from picking up moisture: Do not touch with bare fingersDo not touch with bare fingers Let cool in desiccatorLet cool in desiccator If weighing substances that are kept in If weighing substances that are kept in

fridge or freezer, let warm up before weighingfridge or freezer, let warm up before weighing Be aware of hygroscopic substancesBe aware of hygroscopic substances

Absolute error in weight as a function of time after object was removed from a 110°C oven (A: porcelain filtering crucible, B: weighing bottle containing 7.5 g of KCl.)

Measuring Volume

Typical pipettes: (a) volumetric (transfer pipette); (b) Mohr; (c) serological; (d) Eppendorf micropipette

Apparatus for volume measurementApparatus for volume measurement Pipettes, Burettes, Volumetric flasksPipettes, Burettes, Volumetric flasks

Calibrated either for containment (flasks) or delivery (pipettes, burettes) of specified volume

Characteristics of pipettesCharacteristics of pipettes

NameName FunctionFunction CapacitCapacity (cmy (cm33))

Type of Type of drainagedrainage

VolumetriVolumetricc

Delivery of fixed volumeDelivery of fixed volume 1-2001-200 FreeFree

MohrMohr Delivery of variable Delivery of variable volumevolume

1-251-25 To lower To lower calibration calibration lineline

SerologicSerologicalal

Delivery of variable Delivery of variable volumevolume

0.1-100.1-10 Blow out last Blow out last dropdrop

EppendorfEppendorf Delivery of fixed or Delivery of fixed or variable vol.variable vol.

0.001-10.001-1 Empty tip by Empty tip by air air displacementdisplacement

Random errors in volume Random errors in volume measurements:measurements:

Tolerances of Tolerances of Class AClass A Pipettes Pipettes

Capacity (cmCapacity (cm33)) Tolerances (cmTolerances (cm33))

0.50.5 0.0060.006

11 0.0060.006

22 0.0060.006

55 0.010.01

1010 0.020.02

2020 0.030.03

2525 0.030.03

5050 0.050.05

100100 0.080.08

Random errors in volume measurement:Random errors in volume measurement:Range and precision of typical Eppendorf Range and precision of typical Eppendorf

micropipettesmicropipettes

Volume range Volume range ((L)L)

Standard dev. Standard dev. ((L)L)

1-201-20 <0.04 @ 2 <0.04 @ 2 LL

<0.06 @ 20 <0.06 @ 20 LL

10-10010-100 <0.1 @ 15 <0.1 @ 15 LL

<0.15 @ 100 <0.15 @ 100 LL

20-20020-200 <0.15 @ 25 <0.15 @ 25 LL

<0.3 @ 200 <0.3 @ 200 LL

100-1000100-1000 <0.6 @ 250 <0.6 @ 250 LL

<1.3 @ 1000 <1.3 @ 1000 LL

500-5000500-5000 <3 @ 1000 <3 @ 1000 LL

<8 @ 5000 <8 @ 5000 LL

Burette Burette Vol. (mL)Vol. (mL)

Tolerance Tolerance (mL)(mL)

Volumetric Volumetric flask vol. flask vol. (mL)(mL)

Tolerance Tolerance (mL)(mL)

55 0.010.01 55 0.020.02

1010 0.020.02 1010 0.020.02

2525 0.030.03 2525 0.030.03

5050 0.050.05 5050 0.050.05

100100 0.200.20 100100 0.080.08

250250 0.120.12

500500 0.200.20

10001000 0.300.30

20002000 0.500.50

Tolerances of Class A burettes and Tolerances of Class A burettes and volumetric flasksvolumetric flasks

Avoiding systematic errors in Avoiding systematic errors in volume measurements:volume measurements:

Temperature effectsTemperature effects Volume occupied by a given mass of liquid, Volume occupied by a given mass of liquid,

as well as the device that holds the liquid, as well as the device that holds the liquid, varies with temperature varies with temperature

For dilute aqueous solution:For dilute aqueous solution: Coefficient of expansion = 0.025% / Coefficient of expansion = 0.025% / °C°C 1°C increase in temp. yields 0.025% increase in 1°C increase in temp. yields 0.025% increase in

volume.volume. Refer volumetric measurements to temperature at Refer volumetric measurements to temperature at

which they were made (standard temperature is 20 which they were made (standard temperature is 20 °C).°C).Exercise:

A 40.00 mL sample is taken from an aqueous solution at 5°C. What volume does it occupy at 20 °C?

Avoiding systematic errors in Avoiding systematic errors in volume measurement:volume measurement:

Calibration of Volumetric WareCalibration of Volumetric Ware

Measure mass of liquid of known density Measure mass of liquid of known density and temperature contained in or delivered and temperature contained in or delivered by a stated volumeby a stated volume

Buoyancy correction must be made (see Buoyancy correction must be made (see Table)Table)

Divide corrected mass by density of liquidDivide corrected mass by density of liquid Express results at standard temperature Express results at standard temperature

(20(20°C).°C).

Volume occupied by 1.0000g water weighed in Volume occupied by 1.0000g water weighed in air against stainless steel weightsair against stainless steel weights

Volume (mL/g) at TVolume (mL/g) at TTemperature Temperature T T ((°°C)C) Corrected for Corrected for

buoyancybuoyancyCorrected for buoyancy Corrected for buoyancy and change in container and change in container volume = volume @ volume = volume @ 2020°°CC

1010 1.00131.0013 1.00161.00161212 1.00151.0015 1.00171.00171414 1.00181.0018 1.00191.00191616 1.00211.0021 1.00221.00221818 1.00241.0024 1.00251.00252020 1.00281.0028 1.00281.00282222 1.00331.0033 1.00321.00322525 1.00401.0040 1.00361.00362626 1.00431.0043 1.00411.00412828 1.00481.0048 1.00461.00463030 1.00541.0054 1.00521.0052Exercise:

A 25 mL pipette has been measured to deliver 24.976 g of water weighed against stainless steel mass at 25°C. Use the data in the Table to determine the volume delivered by this pipette at 25°C and 20 °C.

Treatment of glasswareTreatment of glassware

Need to ensure that containers are Need to ensure that containers are clean and not contaminatedclean and not contaminated

Important that liquids interact in Important that liquids interact in defined way with glass surfaces: use defined way with glass surfaces: use detergents detergents

For trace analysis, it is common to For trace analysis, it is common to use an “acid wash”use an “acid wash”

If possible, use polypropylene or If possible, use polypropylene or teflon rather than glassteflon rather than glass

SummarySummary

All measurements carry All measurements carry errors/uncertaintyerrors/uncertainty Systematic errors can be correctedSystematic errors can be corrected

Accuracy of methods can be improvedAccuracy of methods can be improved Random errors cannot be correctedRandom errors cannot be corrected

Precision of method can be determined and must Precision of method can be determined and must be knownbe known

All quantitative data must be reported with All quantitative data must be reported with errorerror

Methods to solve a given analytical Methods to solve a given analytical question can be selected according to their question can be selected according to their performance characteristicsperformance characteristics

Analysts must be aware of the Analysts must be aware of the performance characteristics of their performance characteristics of their toolstools

Classical analytical Classical analytical methods:methods:

Gravimetric and volumetric Gravimetric and volumetric analysesanalyses

Gravimetric analysisGravimetric analysis Analyte is converted to a solid product of known Analyte is converted to a solid product of known

(pure) composition and weighed(pure) composition and weighed Conversion of the analyte can be accomplished Conversion of the analyte can be accomplished

in several ways:in several ways: Reduction of an ion to its elemental form (e.g. by Reduction of an ion to its elemental form (e.g. by

electrolysis)electrolysis) Roasting (hydrolysis/oxidation) of a compoundRoasting (hydrolysis/oxidation) of a compound Precipitation of an ion with a counterionPrecipitation of an ion with a counterion Precipitation of an organic moleculePrecipitation of an organic molecule

Methods exist for most inorganic anions and Methods exist for most inorganic anions and cations, Hcations, H22O, SOO, SO22, CO, CO22, and iodine, and iodine

Organic compounds can also be quantified Organic compounds can also be quantified

ExamplesExamples

Precipitate analyte using precipitating agentPrecipitate analyte using precipitating agent

Reducing agentReducing agent AnalyteAnalyte

SOSO22 Se, AuSe, Au

SOSO22 + H + H22NOHNOH TeTe

HH22NOHNOH SeSe

HH22CC22OO44 AuAu

HH22 Re, IrRe, Ir

HCOOHHCOOH PtPt

NaNONaNO22 AuAu

SnClSnCl22 HgHg

Precipitating agentPrecipitating agent Ion precipitated (Precipitate)Ion precipitated (Precipitate)

AgNOAgNO33 ClCl-- (AgCl), Br (AgCl), Br-- (AgBr), I (AgBr), I-- (AgI) (AgI)

HClHCl AgAg++ (AgCl), Hg (AgCl), Hg++ (Hg (Hg22ClCl22), ),

NaNa++ (NaCl) (NaCl)

HNOHNO33 SnSn4+4+ (SnO (SnO22))

HH22SOSO44 LiLi++, Mn, Mn2+2+, Sr, Sr2+2+, Cd, Cd2+2+, Pb, Pb2+2+, Ba, Ba2+2+ (all as sulfates) (all as sulfates)

HH22CC22OO4 4 (oxalate)(oxalate) CaCa2+2+, Sr, Sr2+2+, Th, Th4+4+ (as oxalates or oxides) (as oxalates or oxides)

Convert analyte (usually ions) to its elemental form using reducing Convert analyte (usually ions) to its elemental form using reducing agentsagents

Gravimetric analysis: Gravimetric analysis: precipitation of insoluble salts precipitation of insoluble salts

or complexesor complexes Involves precipitation, filtration, drying, Involves precipitation, filtration, drying,

weighingweighing e.g.: Sulfate with BaCle.g.: Sulfate with BaCl22 Ni(II) with dimethylglyoximeNi(II) with dimethylglyoxime 8-hydroxyquinoline (oxine): range of metal 8-hydroxyquinoline (oxine): range of metal

ions. Forms sparingly soluble complexesions. Forms sparingly soluble complexes

For accuracy, certain conditions must be For accuracy, certain conditions must be fulfilled:fulfilled:

The ion of interest must precipitate completely The ion of interest must precipitate completely (=quantitatively). The formed salt must have a very (=quantitatively). The formed salt must have a very low solubility productlow solubility product

Precipitate must be a pure compound (avoid co-Precipitate must be a pure compound (avoid co-precipitation) precipitation)

Precipitate must be easy to filterPrecipitate must be easy to filter

Accurate and precise (if done Accurate and precise (if done properly)properly)

Absolute method: No calibration Absolute method: No calibration requiredrequired

Apparatus required is relatively Apparatus required is relatively inexpensiveinexpensive

Why gravimetry is still in use, Why gravimetry is still in use, although time-consuming and although time-consuming and

challenging:challenging:

Exercise:Exercise:

Lead (as PbLead (as Pb2+2+) can be determined by precipitation ) can be determined by precipitation with sodium iodidewith sodium iodide

Write down the stoichiometric reaction formulaWrite down the stoichiometric reaction formula What mass of NaI is needed to convert 1.00 g of What mass of NaI is needed to convert 1.00 g of

Pb(NOPb(NO33))2 2 to PbIto PbI22?? What mass of What mass of PbIPbI2 2 will be formed?will be formed?

ExerciseExercise::A sample of metallic tin (2.00 g) was reacted with iodine A sample of metallic tin (2.00 g) was reacted with iodine (8.80 g) in a refluxing organic solvent, and an orange-(8.80 g) in a refluxing organic solvent, and an orange-yellow solid (yellow solid (AA) (8.62 g) was isolated. A qualitative ) (8.62 g) was isolated. A qualitative elemental analysis of elemental analysis of AA showed it contained only tin and showed it contained only tin and iodide. A sample of iodide. A sample of AA (2.0000 g ) was accurately weighed (2.0000 g ) was accurately weighed into a pre-weighed silica crucible and roasted in air (into a pre-weighed silica crucible and roasted in air (AA reacts with Hreacts with H22O) to produce SnOO) to produce SnO22 (0.4810 g). A second (0.4810 g). A second sample of sample of A A (2.0000 g) was dissolved in a small excess of (2.0000 g) was dissolved in a small excess of nitric acid, and excess silver nitrate added dropwise to nitric acid, and excess silver nitrate added dropwise to precipitate silver iodide, which was collected in a weighed precipitate silver iodide, which was collected in a weighed sinter crucible, dried in an oven at 110sinter crucible, dried in an oven at 110°C, and then cooled °C, and then cooled and weighed (mass of AgI obtained=2.9986 g). When a and weighed (mass of AgI obtained=2.9986 g). When a sample of sample of A A was exposed to air for several months, it was exposed to air for several months, it became hydrated as shown by a second analysis of the became hydrated as shown by a second analysis of the impure product,impure product, B B, which showed Sn=17.92%, I=76.64%, , which showed Sn=17.92%, I=76.64%, and H=0.61%.and H=0.61%.

(Sn=118.69, I=126.90, O=16.00, Ag=107.87, H=1.008)(Sn=118.69, I=126.90, O=16.00, Ag=107.87, H=1.008)

Exercise (continued):Exercise (continued):1.1. From the amount of SnOFrom the amount of SnO22 obtained in the original analysis, calculate the obtained in the original analysis, calculate the

percentage of tin in percentage of tin in AA

2.2. From the amount of AgI obtained, calculate the percentage of iodide in From the amount of AgI obtained, calculate the percentage of iodide in AA..

3.3. Assuming a molecular formula for Assuming a molecular formula for AA of Sn of SnaaIIbb, the molecular weight of , the molecular weight of AA is is therefore =118.69 x a + 126.90 x b.therefore =118.69 x a + 126.90 x b.

Percentage tin Percentage tin XX (1)(1)

Percentage iodidePercentage iodide YY (2)(2)

Use equations (1) and (2) and your calculated values for X and Y to Use equations (1) and (2) and your calculated values for X and Y to estimate the ratio b/a and determine the empirical formula of estimate the ratio b/a and determine the empirical formula of AA..

4.4. A mass spectrum of A mass spectrum of AA showed a cluster of peaks centred at a showed a cluster of peaks centred at a charge/mass ratio charge/mass ratio m/zm/z=626, and no other peaks at higher =626, and no other peaks at higher m/z.m/z. Assuming Assuming the observed the observed m/zm/z corresponds to the approximate molar mass, corresponds to the approximate molar mass, MM, what is , what is the molecular formula of the molecular formula of AA??

5.5. What is the percentage purity of the exposed sample, What is the percentage purity of the exposed sample, BB, compared with , compared with AA (regard (regard AA as pure), and how many water molecules are there in as pure), and how many water molecules are there in BB??

M

a 69.118100

M

b 90.126100

Volumetric analysisVolumetric analysis Amount of analyte determined by measurement of Amount of analyte determined by measurement of

volume of a reagent needed to react with analytevolume of a reagent needed to react with analyte TitrimetryTitrimetry: Determining the quantity of a reagent : Determining the quantity of a reagent

of known concentration that is required to react of known concentration that is required to react completely with the analytecompletely with the analyte Titration: Adding standard solution (titrant) to

solution of the analyte until reaction is complete. Solution dispensed from burette to determine volume of reagent required for reaction

Requires that Reaction has a large equilibrium constant Reaction proceeds rapidly

Titrimetric methodsTitrimetric methods Acid-base titrationsAcid-base titrations Precipitation titrationsPrecipitation titrations

Volhard (AgVolhard (Ag++ directly or Cl directly or Cl- - via back titration)via back titration) Complexometric titrationsComplexometric titrations

Cations with EDTACations with EDTA Redox titrationsRedox titrations

Manganometry, iodometryManganometry, iodometry Spectrophotometric titrations Spectrophotometric titrations

Measures changes in UV-Vis spectraMeasures changes in UV-Vis spectra Potentiometric titrations Potentiometric titrations

Measures changes in potential (e.g. with pH electrodes or Measures changes in potential (e.g. with pH electrodes or Ion-selective electrodes)Ion-selective electrodes)

General terminologyGeneral terminology

• Equivalence point: Point in a titration when quantity of added titrant is the exact amount necessary for stoichiometric reaction with analyte. This is the “ideal” point sought in a titration. In reality, we find the

• End point: Point reached when a (ideally sudden) physical change in the solution occurs, which indicates the absence of unreacted analyte. End points are often detected through an indicator

• Ideally, there is very little difference between the volumes for the equivalence and end points. This difference is the titration error

• Can be determined with a blank titration

• Back titration: Excess of a standard solution added to consume analyte is determined by addition of second standard. Required when direct reaction is slow or unstable

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 5 10 15 20 25 30 35 40 45 50

Typical titration curveTypical titration curvelo

g c

Volume/ amount of titrant added

Decrease in concentration of analyteNote: semi-log plot

Equivalence point

General terminology: General terminology: StandardisationStandardisation

Titrations require standard solutions: Reagent of known concentration used to carry out titration Primary standard: Solution of a highly purified

compound (>99.9%) that can be accurately weighed Serves as a reference material in a given volumetric titration

method. The accuracy of such methods is critically dependent on the properties of this compound

Must be stable (not decomposed during storage) Must be a compound that can be dried to remove residue

water Standard reference materials commercially available (SRMs)

Secondary standard: Solution of titrant that has been standardised by titrating a known amount of primary standard (also commercially available)

Precipitation TitrationsPrecipitation Titrations Based on reactions that give products of low Based on reactions that give products of low

solubilitysolubility One of the oldest analytical techniques (mid One of the oldest analytical techniques (mid

1800s)1800s) E.g. Volhard method for silver(I) titrationsE.g. Volhard method for silver(I) titrations

For direct analysis of silver ions or indirect detn. of For direct analysis of silver ions or indirect detn. of halideshalides

Titrant: NaSCNTitrant: NaSCN

Fe(III) acts as the indicatorFe(III) acts as the indicator

Red colour observed at [Fe(SCN)Red colour observed at [Fe(SCN)2+2+] = 6.4] = 6.4×10×10-6-6 M M

Reaction 1: Ag+ + SCN¯ ⇌ AgSCN(s)

Ksp=[Ag+][SCN¯]=1.1 ×10-12

Reaction 2: Fe3+ + SCN¯ ⇌ [Fe(SCN)]2+ (red)

Kf= [Fe(SCN)]2+ =1.4 ×102

[Fe3+][SCN¯]

Volhard method for AgVolhard method for Ag++

Exercise:Exercise: Titrate 50 mL of 0.05 M AgTitrate 50 mL of 0.05 M Ag++ with 0.1 M KSCN with 0.1 M KSCN What concentration of FeWhat concentration of Fe3+3+ should be used to reduce should be used to reduce

titration error to zero?titration error to zero? Note: For zero titration error, the Fe(SCN)Note: For zero titration error, the Fe(SCN)2+2+ colour should colour should

appear when appear when [Ag[Ag++] = [SCN] = [SCN--]]

Effect of solubility productEffect of solubility product

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 5 10 15 20 25 30 35 40 45 50

Ksp≈10-12

Ksp≈10-18

The higher the solubility, the more difficult becomes the end point recognition

Log

[Ag+

]

Compleximetric titrationCompleximetric titration Metal ion determinationMetal ion determination Metal ion reacts with ligand to form complexMetal ion reacts with ligand to form complex

Can form soluble complexes or precipitatesCan form soluble complexes or precipitates Equivalence point determined by indicatorEquivalence point determined by indicator

EDTA: Ethylenediamine tetraacetic acid; EDTA: Ethylenediamine tetraacetic acid; is a hexadentate ligandis a hexadentate ligand

(n-4)+

[M(H2O)6]n+ + [H2(EDTA)]2-

pK1 = 0.0pK2 = 1.5pK3 = 2.0pK4 = 2.66pK5 = 6.16pK6 = 10.24

+ 6 H2O + 2 H+

ISO 6059: Determination of Hardness in water

Stoichiometric formation Stoichiometric formation constants for EDTA complexesconstants for EDTA complexes

CationCation KKMYMY log Klog KMYMY CationCation KKMYMY log Klog KMYMY

AgAg++ 2.12.1×10×1077 7.327.32 CuCu2+2+ 6.36.3×10×101818 18.8018.80

MgMg2+2+ 4.94.9×10×1088 8.698.69 ZnZn2+2+ 3.23.2×10×101616 16.5016.50

CaCa2+2+ 5.05.0×10×101010 10.7010.70 CdCd2+2+ 2.92.9×10×101616 16.4616.46

SrSr2+2+ 4.34.3×10×1088 8.638.63 HgHg2+2+ 6.36.3×10×102121 21.8021.80

BaBa2+2+ 5.85.8×10×1077 7.767.76 PbPb2+2+ 1.11.1×10×101818 18.0418.04

MnMn2+2+ 6.26.2×10×101313 13.7913.79 AlAl3+3+ 1.31.3×10×101616 16.1316.13

FeFe2+2+ 2.12.1×10×101414 14.3314.33 FeFe3+3+ 1.31.3×10×102525 25.1025.10

CoCo2+2+ 2.02.0×10×101616 16.3116.31 VV3+3+ 7.97.9×10×102525 25.9025.90

NiNi2+2+ 4.24.2×10×101818 18.6218.62 ThTh4+4+ 1.61.6×10×102323 23.2023.20

Mn++Y4- ⇌ MY(n-4)+ [MY(n-4)+][Mn+][Y4-]

=KMY (Kf)

Titration curve shape depends on Titration curve shape depends on formation constantformation constant

CaCa2+2+ has smallest formation constant (weakest EDTA complex) has smallest formation constant (weakest EDTA complex) FeFe3+3+ has largest formation constant (strongest EDTA complex) has largest formation constant (strongest EDTA complex)

Mn++Y4- ⇌ MY(n-4)+ [MY(n-4)+][Mn+][Y4-]

=KMY (or Kf)

Titration curves for 50 mL of 0.01 mol/L cation solutions at pH 6.0.

Effect of pH Effect of pH

Depending on pH, only a certain portion of Depending on pH, only a certain portion of EDTA is present as YEDTA is present as Y4-4-: : [Y[Y4-4-] = ] = Y4-Y4- [EDTA] [EDTA]totaltotal

The value of The value of Y4-Y4- decreases with pH decreases with pHpHpH Y4-Y4- at 20 at 20°°CC

00 1.31.3×10×10-23-23

11 1.91.9×10×10-18-18

22 3.33.3×10×10-14-14

33 2.62.6×10×10-11-11

44 3.83.8×10×10-9-9

55 3.73.7×10×10-7-7

66 2.32.3×10×10-5-5

77 5.05.0×10×10-4-4

88 5.65.6×10×10-3-3

99 5.45.4×10×10-2-2

1010 0.360.36

1111 0.850.85

1212 0.980.98

1313 1.001.00

1414 1.001.00

Free ligandAHAH2AH3AH4

1,2-Diaminoethane-N,N,N',N'-tetraethanoic acid, Sequestric acid

pH12108642

% o

f lig

an

d

100

90

80

70

60

50

40

30

20

10

0

Y4-

HY3-

H2Y2-

H3Y-

Speciation curve

Effect of pHEffect of pH This leads to an apparent reduction in This leads to an apparent reduction in

stability:stability:

Significant for complexes with small Significant for complexes with small K K values:values:

][MY]][Y[M

4)-(n

4n

K][MY

[EDTA] ][M4)-(n

total-Y4n

K

Influence of pH on the titration of 0.01 Influence of pH on the titration of 0.01 mol/L Camol/L Ca2+2+ (50 mL) with 0.01 mol/L EDTA. (50 mL) with 0.01 mol/L EDTA.

Minimum pH needed for the satisfactory Minimum pH needed for the satisfactory titration of various cations with EDTAtitration of various cations with EDTA

Endpoint recognition in Endpoint recognition in Titrations with EDTATitrations with EDTA

Indicator for EDTA titrations: Eriochrome Black TIndicator for EDTA titrations: Eriochrome Black T Different forms of indicator ( -, 2-, 3- ) have different coloursDifferent forms of indicator ( -, 2-, 3- ) have different colours

H2O + H2In- ⇌ HIn2- + H3O+ Ka1 = 5×10-7; pKa = 6.3 (red) (blue)(blue)

H2O + HIn2- ⇌ In3- + H3O+ Ka2 = 2.8×10-12; pKa = 11.6 (orange)

M n-3

+ 2H++ Mn+

Kf for M(In) < Kf for M(EDTA): Solution stays red until no more M is left for complexation with ETpH must be > 6.3 to see colour change to blueblue

(red)

SummarySummary Both gravimetric and volumetric Both gravimetric and volumetric

methods require an understanding of the methods require an understanding of the underlying Chemistry underlying Chemistry

Gravimetry: absolute method, no Gravimetry: absolute method, no standardisation required (but accuracy standardisation required (but accuracy of a given method must be tested)of a given method must be tested)

Titrimetry: careful standardisation is Titrimetry: careful standardisation is required to achieve satisfactory accuracyrequired to achieve satisfactory accuracy

ExerciseExercise

Find and list gravimetric and/or Find and list gravimetric and/or volumetric methods that may be volumetric methods that may be commonly used in a commercial commonly used in a commercial Analytical LabAnalytical Lab