Associate profAssociate prof . L.V. Vronska. L.V. VronskaAssociate profAssociate prof . M.M. Mykhalkiv

LL ee cc tt uu rr ee 6 6

Titrimetric analysis. Titrimetric analysis. Acid–Base titration.Acid–Base titration.

OutlineOutline1.1. Titrimetric method of the analysis: the basic concepts and Titrimetric method of the analysis: the basic concepts and

classification.classification.

2.2. Technics of titrimetric analysis.Technics of titrimetric analysis.

3.3. Types of titrimetric determinations.Types of titrimetric determinations.

4.4. Calculations in titrimetric analysis.Calculations in titrimetric analysis.

5.5. Protolytometry or acid–base titration (a neutralization Protolytometry or acid–base titration (a neutralization method): the basic concepts, titrants, defined substances.method): the basic concepts, titrants, defined substances.

6.6. Indicator choice, calculation of errors of titration in a Indicator choice, calculation of errors of titration in a method protolytometry.method protolytometry.

7.7. Nonaqueous acid–base titration.Nonaqueous acid–base titration.

1. Titrimetric method of the 1. Titrimetric method of the analysis: the basic concepts analysis: the basic concepts

and classification.and classification.

TitrimetryTitrimetry - any method in which volume is the signal.

TitrimetryTitrimetry, in which we measure the volume of a reagent reacting stoichiometrically with the analyte.

Titration setup: the titrant drops from the burette into the analyte solution in the flask. An indicator present then changes color permanently at the endpoint.

Titration of an acid solution of unknown concentration with a base solution of known concentration

Titration is a procedure for determining the concentration of a

solution by allowing a carefully measured volume to react with a

standard solution of another substance, whose concentration is

known.

StandardizationStandardization is the process of establishing a technical standard is the process of establishing a technical standard

In chemistry, an In chemistry, an aliquotaliquot is usually a portion of a total amount of a is usually a portion of a total amount of a

solution.solution.

Titrant - the reagent added to a solution containing

the analyte and whose volume is the signal.

A reagent, called the titrant, of known

concentration (a standard solution) and volume is

used to react with a solution of the analyte, whose

concentration is not known.

equivalence point - the point in a titration where

stoichiometrically equivalent amounts of analyte

and titrant react.

End pointEnd point - the point in a titration where we stop adding titrant.

IndicatorIndicator - a colored compound whose change in color signals the end point of a titration.

Titration errorTitration error - the determinate error in a titration due to the difference between the end point and the equivalence point.

A primary standardA primary standard is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. A primary standard in chemistry is a reliable, readily quantified substance.

Secondary reagentSecondary reagent - a reagent whose purity must be established relative to a primary reagent

A burette (also buret) is a vertical cylindrical piece of laboratory glassware with a volumetric graduation on its full length and a precision tap, or stopcock, on the bottom (or calibrated glass tube).

Even the thickness of the lines printed on the burette matters; the bottom of the meniscus of the liquid should be touching the top of the line you wish to measure from.

stopcock buret

Buret with bottle of standard solution

Gay-Lussac burette

Buret with rubber shutter

Mohr burette

Microburet:а) Shilov air-powered Buret; б) stopcock buret

A pipetteA pipette (also called a (also called a pipet, pipettorpipet, pipettor or or chemical dropperchemical dropper) is a laboratory ) is a laboratory instrument used to transport a measured instrument used to transport a measured volume of liquidvolume of liquid

Erlenmeyer flask or conical flask.

There are different ways of There are different ways of preparation of standard solutions: preparation of standard solutions:

on accurately weighed sample (on accurately weighed sample (primary primary standardstandard))

by means of standard substance or a by means of standard substance or a standard solutionstandard solution ((secondary standardsecondary standard))

“ “standard titrimetric substance” standard titrimetric substance” ((primary standardprimary standard))

Accurately weighed sample (primary Accurately weighed sample (primary standard)standard)

Because a volumetric flask contains a solution, it is useful Because a volumetric flask contains a solution, it is useful in preparing solutions with exact concentrations. The in preparing solutions with exact concentrations. The reagent (the accurately weighed sample) is transferred to reagent (the accurately weighed sample) is transferred to the volumetric flask, and enough solvent is added to the volumetric flask, and enough solvent is added to dissolve the reagent. After the reagent is dissolved, dissolve the reagent. After the reagent is dissolved, additional solvent is added in several portions, mixing additional solvent is added in several portions, mixing the solution after each addition. The final adjustment of the solution after each addition. The final adjustment of volume to the flask’s calibration mark is made using a volume to the flask’s calibration mark is made using a dropping pipet. To complete the mixing process, the dropping pipet. To complete the mixing process, the volumetric flask should be inverted at least ten times.volumetric flask should be inverted at least ten times.

Volumetric flask

Volumetric flask — for preparing liquids with volumes of high precision. It is a flask with an approximately pear-shaped body and a long neck with a circumferential fill line.

Proper means of reading the meniscus on a volumetric flask or pipet.

When filling a pipet or volumetric flask, set the liquid’s level exactly at the calibration mark. The liquid’s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware’s calibration mark. The meniscus should be adjusted with the calibration mark at eye level to avoid parallax errors.

Calculate concentration of primary Calculate concentration of primary standardstandard

VM

mC

M

VmEm

NC

V/mT

secondary reagentsecondary reagent - a reagent whose purity must be established relative to a primary reagent

To prepare the solution we place calculated amount of substance, weighed to the nearest tenth of a gram, in a bottle or beaker and add approximately volume of water

by means of standard substance or a by means of standard substance or a standard solutionstandard solution (secondary standard)(secondary standard)

2

112 V

VCC N

N

А) a measured volume of another primary standard solution

Establishment of secondary standard Establishment of secondary standard concentrationconcentration

where:CCN2N2 and VV22 are concentration and volume

of secondary standard solutionCCN1N1 and VV11 are concentration and volume

of primary standard solution

BB) ) a weighed quantity of a primary standarda weighed quantity of a primary standard

VE

mC

mN

1000

where: CN and V are concentration

and volume of secondary standard solution

m and Em are mass and equivalent weight of primary standard

Typical instrumentation for performing an automatic titration. Courtesy of Fisher Scientific.

Features of a primary standard include:Features of a primary standard include:

1. It should have 1. It should have crystal structurecrystal structure and correspond the and correspond the chemical formula chemical formula

2. 2. High purityHigh purity (it is the absence of impurity in a substance) (it is the absence of impurity in a substance)

3. 3. StabilityStability (low reactivity) (low reactivity)

4. 4. Low hygroscopicityLow hygroscopicity (it is the ability of a substance to (it is the ability of a substance to attract water molecules from the surrounding environment attract water molecules from the surrounding environment through either absorption or adsorption) and through either absorption or adsorption) and efflorescenceefflorescence (in chemistry, is the loss of water (or a solvent) of (in chemistry, is the loss of water (or a solvent) of crystallization from a hydrated or solvated salt to the crystallization from a hydrated or solvated salt to the atmosphere on exposure to air).atmosphere on exposure to air).

5. 5. High solubilityHigh solubility (if used in titration) (if used in titration)

6. 6. High equivalent weight.High equivalent weight.

7.7. Not to contain extraneous Not to contain extraneous impurityimpurity more than more than admissible borders for substances of mark “chemically admissible borders for substances of mark “chemically pure”.pure”.

8. Methods of 8. Methods of purificationpurification of standard substance of standard substance from from impurityimpurity (crystallisation, extraction, sublimation etc.) (crystallisation, extraction, sublimation etc.) should be availableshould be available in analytical laboratory. in analytical laboratory.

Some examples of primary standards according to the Some examples of primary standards according to the

European Pharmacopoeia 5, ch. 4.2:European Pharmacopoeia 5, ch. 4.2:

Arsenic trioxide for making sodium arsenite solution for Arsenic trioxide for making sodium arsenite solution for

standardisation of sodium periodate solution (also for standardisation of sodium periodate solution (also for

iodine and cerium (IV) sulfate solutions, since 2002 iodine and cerium (IV) sulfate solutions, since 2002

standardised by sodium thiosulfate)standardised by sodium thiosulfate)

Benzoic acid for standardisation of waterless basic Benzoic acid for standardisation of waterless basic

solutions: ethanolic sodium and potassium hydroxide, solutions: ethanolic sodium and potassium hydroxide,

TBAH, and alkali methanolates in methanol, isopropanol, TBAH, and alkali methanolates in methanol, isopropanol,

or DMFor DMF

Potassium bromate (KBrOPotassium bromate (KBrO33) for standardisation of sodium ) for standardisation of sodium

thiosulfate solutionsthiosulfate solutions

Potassium hydrogen phthalate (usually called KHP) for Potassium hydrogen phthalate (usually called KHP) for standardisation of aqueous base and perchloric acid in standardisation of aqueous base and perchloric acid in acetic acid solutionsacetic acid solutions

Sodium carbonate for standardisation of aqueous acids: Sodium carbonate for standardisation of aqueous acids: hydrochloric, sulfuric acid and nitric acid solutions (but hydrochloric, sulfuric acid and nitric acid solutions (but not acetic acid)not acetic acid)

Sodium chloride for standardisation of silver nitrate Sodium chloride for standardisation of silver nitrate solutionssolutions

Sulfanilic acid for standardisation of sodium nitrite Sulfanilic acid for standardisation of sodium nitrite solutionssolutions

Zinc powder, after being dissolved in sulfuric or Zinc powder, after being dissolved in sulfuric or hydrochloric acid, for standardisation of EDTA solutionshydrochloric acid, for standardisation of EDTA solutions

“ “standard titrimetric standard titrimetric substance” (primary standard)substance” (primary standard)

More often in an More often in an ampoule contains 0,1 ampoule contains 0,1 mol (0,1 equivalents) mol (0,1 equivalents) of substances, it is of substances, it is necessary for necessary for preparation of 0,1 preparation of 0,1 mol/L solution.mol/L solution.

Preparation rules primary solutions Preparation rules primary solutions and definition of their titre.and definition of their titre.

1. The initial substance which is used for preparation of a standard solution, should be chemically pure.

2. The initial substance should easily and quickly react with standartized solution.

3. The solution of initial substance don’t change itself concentration long time.

4. It is necessary to use reactions between initial and defined substance, which are possible in direct titration.

5. Titration process should end quickly and accurately. The end point of titration should will be defined easily and precisely.

6. To establish of titre it is desirable either a method of accurately weighed sample or dissolution of precisely weighed initial substance in certain volume.

7. For the prevention of errors by titration it is necessary to choose volume of the primary standard aliquot or weighed of standard substance, that the volume of the secondary standard which will react in titration was not less than 20 mL (buret on 25 mL) or 40 mL (buret on 50 mL).

8. Titration should be carry out until then it will not be received yet three reproduced results.

9. Prepared of standartized solution should be stored in conditions which exclude absorption of air moisture by them, and also evaporation. A titre should not change at standing in time.

10.Wares and measuring devices which are used in titrimetry, should be washed up, calibrated, prepared for titration and should be stored in a pure place.

11.Accuracy measurement of volumes and the calculations, should correspond to accuracy of weighing.

Titrimetric methods are classified into four groups based on the type of

reaction involved. acid–base titrations, in which an acidic or basic , in which an acidic or basic

titrant titrant reacts with an analyte that is a base or an reacts with an analyte that is a base or an acid; acid;

complexometric titrations involving a metal– involving a metal–ligand complexation reaction; ligand complexation reaction;

redox titrations, where the titrant is an oxidizing or , where the titrant is an oxidizing or reducing agent; reducing agent;

precipitation titrations, in which the analyte and , in which the analyte and titrant react to form a precipitate.titrant react to form a precipitate.

Requirements to reactions in titrimetric analysisRequirements to reactions in titrimetric analysis all reactions involving the titrant and analyte all reactions involving the titrant and analyte

must be of known must be of known stoichiometrystoichiometry, , quantitativelyquantitatively the titration reaction must occur the titration reaction must occur rapidlyrapidly a suitable method must be available for a suitable method must be available for

determining the end point with an acceptable determining the end point with an acceptable level of level of accuracyaccuracy

Reactions should proceed Reactions should proceed by by room temperatureroom temperature Titration should not Titration should not be be accompanied by accompanied by

collateral reactionscollateral reactions which deform the results of which deform the results of the analysisthe analysis

Reactions should be Reactions should be specificspecific

2. Techniques of titrimetric 2. Techniques of titrimetric analysis.analysis.

Washing up and drying wareWashing up and drying ware Preparation of standard solutionsPreparation of standard solutions Sample preparationSample preparation Titration: Titration:

- Measurement of volumes- Measurement of volumes

- An indicator choice- An indicator choice CalculationsCalculations

Common examples of glassware used to measure volume::

The Buret (or burette)

Graduated cylinders

beaker

volumetric flask

measuring pipet

transfer pipet;

CalibrationCalibration:: volumetric flask - an injection method - an injection method

pipettes, pipettes, burettes - a pouring out method - a pouring out method

Methods to determine the end Methods to determine the end pointpoint

visual indicatorsvisual indicators:: Colour changeColour change: In some reactions, the solution changes colour without : In some reactions, the solution changes colour without

any added indicator. This is often seen in redox titrations, for instance, any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce when the different oxidation states of the product and reactant produce different colours.different colours.

PrecipitationPrecipitation: If the reaction forms a solid, then a precipitate will form : If the reaction forms a solid, then a precipitate will form during the titration. A classic example is the reaction between Agduring the titration. A classic example is the reaction between Ag++ and and ClCl-- to form the very insoluble salt AgCl. This usually makes it difficult to form the very insoluble salt AgCl. This usually makes it difficult to determine the endpoint precisely. As a result, precipitation titrations to determine the endpoint precisely. As a result, precipitation titrations often have to be done as "back" titrations (see below).often have to be done as "back" titrations (see below).

Physical and chemicalPhysical and chemical methods with the subsequent methods with the subsequent analysis of analysis of curves of the titrationcurves of the titration showing changes which occur in the course of showing changes which occur in the course of titration (change of physical and chemical parametres standard titration (change of physical and chemical parametres standard solutions)solutions)

3. Types of titrimetric 3. Types of titrimetric determinations.determinations.

Titration can be:Titration can be: direct titrationdirect titration back-titration (back-titration (on residueon residue)) substitute-titration (substitute-titration (displacement titrationdisplacement titration)) revertive titrationrevertive titration

direct titrationdirect titration – titrant add to an analyte solution and – titrant add to an analyte solution and react with determined substrancereact with determined substrance

Requirements to reactions in direct titrationRequirements to reactions in direct titration reaction involving the titrant and analyte must be of reaction involving the titrant and analyte must be of

known known stoichiometrystoichiometry, , quantitativelyquantitatively the titration reaction must occur the titration reaction must occur rapidlyrapidly a suitable method must be available for determining the a suitable method must be available for determining the

end point with an acceptable level of end point with an acceptable level of accuracyaccuracy Reactions should proceed Reactions should proceed by by room temperatureroom temperature Titration should not Titration should not be be accompanied by collateral accompanied by collateral

reactionsreactions which deform the results of the analysis which deform the results of the analysis Reactions should be Reactions should be specificspecific a suitable a suitable indicator is availableindicator is available

А + Т = А + Т = productproduct

Back titration.Back titration. A titration in which a reagent is added to a solution containing the analyte, and the excess reagent remaining after its reaction with the analyte is determined by a titration.

This titration is used, when:This titration is used, when: the titration reaction is the titration reaction is too slowtoo slow,, a suitable a suitable indicator is not availableindicator is not available,, there is there is no useful directno useful direct titration reaction titration reaction the standard solution the standard solution lacks of stabilitylacks of stability (fugitive) (fugitive)

АА + + ТТexcessexcess = product = product11 + + ТТresidueresidue

ТТresidueresidue + + ТТpaddingpadding = product = product22

displacement titration. displacement titration. A titration in which the analyte A titration in which the analyte displaces a species, usually from a complex, and the amount displaces a species, usually from a complex, and the amount of the displaced species is determined by a titration.of the displaced species is determined by a titration.

This titration is used, when:This titration is used, when: the analytes are unstable substance the analytes are unstable substance It is impossible to indicate the equivalent (end) point in direct reaction It is impossible to indicate the equivalent (end) point in direct reaction Analyte doesn’t react with titrantAnalyte doesn’t react with titrant reaction involving the titrant and analyte mustn’t be of known reaction involving the titrant and analyte mustn’t be of known

stoichiometrystoichiometry, , quantitativelyquantitatively

АА + Т + Т1(1(paddingpadding compoundscompounds)) = = АА1(1(substituentsubstituent))

АА1(1(substituentsubstituent)) + + ТТ = = productproduct

CrCl2 + FeCl3 = CrCl3 + FeCl2

analyte substitute5FeCl2 + KMnO4 + HCl = 5FeCl3 + KCl + MnCl2 + 4H2O

Revertive titrationRevertive titration..

A standard solution is titrated by solution A standard solution is titrated by solution of investigated substance in of investigated substance in reversive reversive titrationtitration

АА(in burette)(in burette) + + ТТ(in flask)(in flask) = product = product

4. Calculations in titrimetric 4. Calculations in titrimetric analysis.analysis.

Weight of investigated Weight of investigated substance by results of substance by results of direct, displacement or direct, displacement or reversive titration:reversive titration:

a

kmTПH

V1000VEVKC

m

a

kTПAT

V

VVKTm

Weight of investigated substance by results Weight of investigated substance by results of back titration:of back titration:

a

kmTПHTПH

V1000

VE)VKCVKC(m 2211

Titre of titrant by investigated substance:Titre of titrant by investigated substance:

1000)A(E)T(С

Т mН

АТ

Acid–base titration - titration in which the reaction between the analyte and titrant is an

acid–base reaction.ProtolytometryProtolytometry is is titrimetric method oftitrimetric method of analysis which uses analysis which uses

solutions ofsolutions of acids or bases acids or bases as titrantsas titrants. In this method of the . In this method of the analysis defined substances are the substances, capable to analysis defined substances are the substances, capable to react with acids and the bases.react with acids and the bases.

The basic reaction of a method:The basic reaction of a method:

НН++ + ОН + ОН-- = Н = Н22ОО or or

HAHA + + BB = = BHBH++ + + AA--

5. PROTOLYTOMETRY OR ACID–BASE 5. PROTOLYTOMETRY OR ACID–BASE TITRATION (A NEUTRALIZATION METHOD): TITRATION (A NEUTRALIZATION METHOD): THE BASIC CONCEPTS, TITRANTS, DEFINED THE BASIC CONCEPTS, TITRANTS, DEFINED

SUBSTANCES.SUBSTANCES.

MethodsMethods of acid-base titration or of acid-base titration or acid–base titrimetry::

acidimetryacidimetry ( (titrantstitrants - - HCl, HHCl, H22SOSO44))

alkalimetryalkalimetry ( (titrantstitrants - - NaOH, KOHNaOH, KOH))

All All titrants titrants are secondary standard are secondary standard solutions, therefore demand of solutions, therefore demand of standardization (definition of precise standardization (definition of precise concentration).concentration).

Standardization of acidic titrants - solutions Standardization of acidic titrants - solutions of of acidacid HClHCl, , HH22SOSO44

StandardStandard (reference) substances (reference) substances – – sodiumsodium tetraboratictetraboratic NaNa22BB44OO7755HH22O or O or

NaNa22BB44OO7710H10H22OO, , sodium carbonatesodium carbonate NaNa22COCO33::

NaNa22BB44OO77 + 2HCl + 5H + 2HCl + 5H22O = 2NaCl + 4HO = 2NaCl + 4H33BOBO33

NaNa22COCO33 + 2HCl = 2NaCl + CO + 2HCl = 2NaCl + CO22 + H + H22OO

Standardization of Standardization of НСНСl solution l solution on sodium on sodium tetraboratetetraborate. .

Weigh exact Weigh exact shotshot of of NaNa22BB44OO7755HH22O O or or NaNa22BB44OO771010HH22OO and place and place itit in a measured in a measured flask, dissolve in hot water, after a solution is flask, dissolve in hot water, after a solution is cooled and diluted of solution by water to cooled and diluted of solution by water to necessarynecessary volume and it is mixed. volume and it is mixed.

In a flask for titration place an In a flask for titration place an aliquotaliquot of of prepared primary standard solution prepared primary standard solution NaNa22BB44OO7755HH22O O or or NaNa22BB44OO771010HH22OO, add some , add some drops of the methyl orange. The received drops of the methyl orange. The received solution is solution is titratetitrated by solution of d by solution of НСНСl l to change to change of colour with yellow to orange with a rose of colour with yellow to orange with a rose shade.shade.

Standardization of Standardization of НСНСl solution l solution on sodium on sodium tetraboratetetraborate. .

By 3-4 results of titration calculate average volume of used By 3-4 results of titration calculate average volume of used titranttitrant and calculate concentration of hydrochloric acid. and calculate concentration of hydrochloric acid.

HCl

OBNaN

HClN V

VCC OBNa 742742

Standardization of Standardization of HCl HCl solution on sodium solution on sodium carbonatecarbonate

In a flask for titration place exact In a flask for titration place exact shotshot of of sodium sodium carbonate, dissolve in necessary volume of water, add carbonate, dissolve in necessary volume of water, add some drops methyl orange and some drops methyl orange and titrate titrate this solution by this solution by chloric acid. chloric acid.

Such titration repeat for 3-4 times. Each time calculate Such titration repeat for 3-4 times. Each time calculate concentration of HCl:concentration of HCl:

By 3-4 results of titration calculate average By 3-4 results of titration calculate average concenration of chloric acid.concenration of chloric acid.

HClCOmNa

CONa

HClN VE

mC

32

321000

_

Standardization of basic titrants - solutions Standardization of basic titrants - solutions of of bases ofbases of NaOH, KOHNaOH, KOH

StandardStandard (reference) substances (reference) substances – – oxalate oxalate acidacid HH22CC22OO442H2H22OO, , succinatesuccinate acidacid

HH22CC44HH44OO44::

HH22CC22OO44 + 2NaOH = Na + 2NaOH = Na22CC22OO44 + 2H + 2H22OO

HH22CC44HH44OO44 + 2NaOH = Na + 2NaOH = Na22CC44HH44OO44 + 2H + 2H22OO

Standardization of Standardization of NaOH solution NaOH solution on oxalic on oxalic acid. acid.

Weigh exact Weigh exact shotshot of of HH22CC22OO442H2H22O O and place and place itit in a measured flask, dissolve in hot water, after in a measured flask, dissolve in hot water, after a solution is cooled and diluted of solution by a solution is cooled and diluted of solution by water to water to necessarynecessary volume and it is mixed. volume and it is mixed.

In a flask for titration place an In a flask for titration place an aliquotaliquot of of prepared primary standard solution prepared primary standard solution HH22CC22OO442H2H22O O , add some drops of the , add some drops of the phenolphthalein. The received solution is phenolphthalein. The received solution is titratetitrated by solution of NaOH d by solution of NaOH to change of to change of colourless to rose (or red).colourless to rose (or red).

Standardization of Standardization of NaOH solution NaOH solution on oxalic on oxalic acid. acid.

By 3-4 results of titration calculate average volume of By 3-4 results of titration calculate average volume of used used titranttitrant and calculate concentration of NaOH. and calculate concentration of NaOH.

NaOH

OCHN

NaOHN V

VCC OCH 422422

_

According to force of acid and the base such According to force of acid and the base such types of the acid-base interaction are possible:types of the acid-base interaction are possible:

Between strong acid and the strong basisBetween strong acid and the strong basis

NaOHNaOH + + HClHCl = = HH22OO + + NaClNaCl Between weak acid and the strong basisBetween weak acid and the strong basis NaOHNaOH + + CHCH33COOHCOOH ↔ ↔ CHCH33COONaCOONa + + HH22OO Between strong acid and the weak basisBetween strong acid and the weak basis

NHNH44OHOH + + HClHCl = = NHNH44ClCl + + HH22OO Between weak acid and the weak basisBetween weak acid and the weak basis

CHCH33COOH + NHCOOH + NH44OH = CHOH = CH33COONHCOONH44 + H + H22OO

Indicators of acid-base titrationIndicators of acid-base titration

The substances which colouring changes depending on size The substances which colouring changes depending on size change change рН рН of solution.of solution.

Requirements to indicators:Requirements to indicators: Indicator colouring at near values Indicator colouring at near values рНрН should differ well should differ well Change of colouring of the indicator should occur sharply in a Change of colouring of the indicator should occur sharply in a

small interval of small interval of рНрН Indicator colouring should be as it is possible more intensivelyIndicator colouring should be as it is possible more intensively The quantity of base or acid, necessary for change of The quantity of base or acid, necessary for change of

colouring of the indicator, should be very smallcolouring of the indicator, should be very small Change of colouring of the indicator Change of colouring of the indicator mustmust to be reversible to be reversible

6. INDICATOR CHOICE, CALCULATION 6. INDICATOR CHOICE, CALCULATION OF ERRORS OF TITRATION IN A OF ERRORS OF TITRATION IN A METHOD PROTOLYTOMETRY.METHOD PROTOLYTOMETRY.

1894 – the ionic theory of indicators1894 – the ionic theory of indicators

Indicators of an acid-base titration method Indicators of an acid-base titration method are weak acids or the bases at which not are weak acids or the bases at which not ionised molecules and ions have different ionised molecules and ions have different colouringcolouring

HIndHInd ↔ ↔ HH++ + + IndInd--

LLacmusacmus redred bluebluePhenol-Phenol-phthaleinphthalein colourlesscolourless rose rose

Ind: -Ind: - one-colourone-colour ((phenolphthaleinphenolphthalein )) - - two-colourtwo-colour ((methyl orangemethyl orange , , lacmuslacmus))

LimitationLimitation of of ionic theory of indicatorsionic theory of indicators :: Ascertaining of different colouring of acidic Ascertaining of different colouring of acidic

and basic forms, but is not present an and basic forms, but is not present an explanation of presence and colouring change.explanation of presence and colouring change.

The structure and colouring are not connected.The structure and colouring are not connected. Colouring change is ionic process but why it Colouring change is ionic process but why it

often is long in time?often is long in time?

Advantages of the ionic theory:Advantages of the ionic theory: possibility of possibility of quantitative interpretation of results of change quantitative interpretation of results of change of colouring.of colouring.

The theory of The theory of chromophorechromophore – – colouring of colouring of organic compounds is connected with presence organic compounds is connected with presence of of a a chromophorechromophore groups groups at molecules of indicators : at molecules of indicators :

AAuxochrome groupuxochrome groupss haven’t colouring, but haven’t colouring, but with with a a chromophorechromophore groups strengthen action of groups strengthen action of the last, causing deeper intensity of colouring.the last, causing deeper intensity of colouring.

-N=N-

O

- N=N-; -N=O; =C=S;

NCH3

CH3

NC2H5

C2H5

-OH; -NH2; -OCH3; ;

Colouring changeColouring change is a consequence of isomeric is a consequence of isomeric transformation which changes an indicator transformation which changes an indicator structurestructure

The The colourlesscolourless form The form The yellowyellow form form

NO

O

OH

O

O

N-O-H

LimitationLimitation of of chromophorechromophore theorytheory of of indicatorsindicators

Does not give an explanation why Does not give an explanation why tautomerictautomeric transformations and change of colouring of a transformations and change of colouring of a solution of indicators occurs at change solution of indicators occurs at change рНрН a a solution.solution.

Colouring changes instant, where as intra-Colouring changes instant, where as intra-molecular transformations generally long molecular transformations generally long processes is frequent.processes is frequent.

Does not give a quantitative estimation of Does not give a quantitative estimation of connection of colouring change with change connection of colouring change with change рНрН..

The The ionic- ionic- chromophorechromophore theory theory The acid-base indicators are weak acids and the The acid-base indicators are weak acids and the

bases, and the neutral molecule of the indicator bases, and the neutral molecule of the indicator and its ionised form contain different and its ionised form contain different chromophorechromophore groups groups

colourless colourless yellow yellowyellow yellow

NO

O

OH

O

O

N-O-H O

O

N-O-

+ H+

The The ionic- ionic- chromophorechromophore theory theory

1 1 22

ННIndInd00 ↔ ↔ HIndHInd ↔ ↔ HH++ + + IndInd--

the the acidacid the the basebase

formform formform

formbase

formacid

С

СpKpH

.

.lg

1pKpH

рТрТ of most often used indicators in the acid-base of most often used indicators in the acid-base titration:titration:

Methyl orangeMethyl orange 4,04,0 Methyl redMethyl red 5,5 5,5 LLacmusacmus 7,07,0 PhenolphthaleinPhenolphthalein 9,0 9,0

pTpT of of the indicator is value of the indicator is value of рНрН at which colour at which colour of the indicator sharply changes and stop to add of the indicator sharply changes and stop to add titranttitrant (there is end point of titration) (there is end point of titration)

Factors which influence the indicator Factors which influence the indicator indication.indication.

At increase tAt increase tоо the temperature indicator becomes the temperature indicator becomes less sensitive to less sensitive to НН++ -ions for indicators-bases -ions for indicators-bases

Presence of organic solvent (alcohol, acetone), Presence of organic solvent (alcohol, acetone), albuminous molecules, salts changes albuminous molecules, salts changes рКрК of of the the indicatorindicator

It is necessary to define It is necessary to define titretitre a working solution a working solution in the same conditions at which the test analysis in the same conditions at which the test analysis is conductedis conducted

it isn’t recommended to take a lot quantity of it isn’t recommended to take a lot quantity of indicatorindicator

2. 2. Indicator choice, calculation of errors Indicator choice, calculation of errors of titration in a method protolytometry.of titration in a method protolytometry.

Indicator choice spend two methods:Indicator choice spend two methods:

On reaction productsOn reaction products

On titration curvesOn titration curves

Titration curve for 0,1 mol/L for 0,1 mol/L hydrochlorichydrochloric acid by acid by 0,1 mol/L sodium 0,1 mol/L sodium hydroxidehydroxide

Dependence of inflection points on Dependence of inflection points on concentration of defined substanceconcentration of defined substance

(0,1 mol/L and 0,01 mol/L)(0,1 mol/L and 0,01 mol/L)

Dependence of inflection pointsDependence of inflection points

on force of acidon force of acid

Dependence of inflection pointsDependence of inflection pointson force of acidon force of acid

Titration curve for weak acid (CHfor weak acid (CH33COOH) by COOH) by

weak base (NHweak base (NH44OH)OH)

Titration curve for for HH33POPO44 by by NaOHNaOH

Factors which influence on Factors which influence on inflection pointsinflection points

constants of acid or baseconstants of acid or base ttemperaturemperatura ofa of ssolutionolutionss concentration of defined substancesconcentration of defined substances concentration of used titrantsconcentration of used titrants

Choice of the indicator:Choice of the indicator:The The pT pT of indicator (interval of transition of of indicator (interval of transition of

colouring - colouring - pH range) should be ) should be in limit in limit of of inflection pointsinflection points on a on a titration curve

Choice of the indicator:Choice of the indicator:The The pT pT of indicator (interval of transition of of indicator (interval of transition of

colouring - colouring - pH range) should be ) should be in limit in limit of of inflection pointsinflection points on a on a titration curve

Titration curve of 0.100 M HClwith 0.200 M NaOH

Titration curve of 0,1 М 0,1 М CHCH33COOH COOH

by 0,2 M NaOHby 0,2 M NaOH

Titration curve of 0,1 М 0,1 М NHNH33

by 0,1 M HClby 0,1 M HCl

Titration curve of weak acid by weak base

Titration curve for 50.00 mL of 0.100 M CH3COOH with 0.100 M NaOH showing the range of pHs (or pT) and volumes of titrant over which the indicators bromothymol blue and phenolphthalein are expected to change color.

Titration curve of mix0,1 M HCl + 0,1 M CH0,1 M HCl + 0,1 M CH33COOH COOH

by by 0,1 М 0,1 М NaOHNaOH

Acetic acid Ка=1,74·10-5

Titration curve of mixacetic and malatic acidsacetic and malatic acids

Malatic acid Ка=1,50·10-4

Acetic acid Ка=1,74·10-5

К1:К2<104

Titration curve of maleinic acidmaleinic acid

Titration curve 0,1 М 0,1 М oxalic acid oxalic acid byby 0,1 М 0,1 М NaOH NaOH

Determinate the end-point by potentiometric wayDeterminate the end-point by potentiometric way

Titration errorTitration error - the determinate error in a titration due to the difference between the end

point and the equivalence point.

Indicator’s error ““++” – ” – if have excess of base when define if have excess of base when define

acidacid ““--” – ” – if have rest of acid when define acidif have rest of acid when define acid

HHydroxoniumydroxonium errorerror

%100VC

V10x

kk

2pT

Indicator’s error ::

HHydroxyl ydroxyl errorerror

Acidic error

Bases error

%10010x рТрКHA

a

%100VC

V10x

kk

214pT

%10010x 14рТрКMeOH

b

7. NONAQUEOUS ACID–BASE TITRATION.7. NONAQUEOUS ACID–BASE TITRATION.

Titration in water solutions is limited by factors:Titration in water solutions is limited by factors: It is impossibleIt is impossible to titrate for a mix of acids or the to titrate for a mix of acids or the

bases if constants of dissociation differ less, than bases if constants of dissociation differ less, than on four orderon four order

It is impossibleIt is impossible to to titratetitrate for a mix of strong and for a mix of strong and weak acids (bases)weak acids (bases)

It is impossibleIt is impossible to titrate very weak acids (bases)to titrate very weak acids (bases) It is impossibleIt is impossible to to titratetitrate separately for a mix of separately for a mix of

acids (bases) with near constants of dissociationacids (bases) with near constants of dissociation It is impossibleIt is impossible to define substances which are to define substances which are

insoluble in water. insoluble in water.

Choice of solventsChoice of solvents::

The constant of aThe constant of autoprotolysisutoprotolysis solvent should be solvent should be as small as possibleas small as possible

For titration of the For titration of the weakweak basesbases should be to take should be to take a solvent with the expressed a solvent with the expressed progenicprogenic properties properties (the acid nature of solvent)(the acid nature of solvent)

For titration of For titration of weak acidsweak acids should be to take a should be to take a solvent with expressed solvent with expressed protophilicprotophilic properties, properties, (the basis nature of solvent)(the basis nature of solvent)

Dielectric Dielectric inductivityinductivity of solvent should be as it of solvent should be as it is possible aboveis possible above

The weak bases often are titrated in the acetic The weak bases often are titrated in the acetic acid medium acid medium

(strengthening of force of the bases)(strengthening of force of the bases)

TitrantTitrant: : perchlorate acidperchlorate acid HClO HClO44

StandardizationStandardization:: on potassium hydrogenphthalate, on potassium hydrogenphthalate, or on sodium salicylate if have solution of or on sodium salicylate if have solution of HClOHClO44 in in

CHCH33OHOH

Nonaqueous acid–base titration of of weak basesweak bases by perchlorate acidby perchlorate acid

Indicators:Indicators: crystal violetcrystal violet ( (violet violet – – blue orblue or

greengreen), ),

thymolthymol dark blue dark blue ( (yellowyellow – – roserose).).

The weak acids often are titrated in the The weak acids often are titrated in the medium medium dimethyl formamide, ethylene diamine, dimethyl formamide, ethylene diamine,

butylamine, pyridine butylamine, pyridine (strengthening of force of the acids)(strengthening of force of the acids)

Titrant:Titrant: sodium hydroxidesodium hydroxide NaOHNaOH in the in the solution of solution of benzenebenzene with with methanolmethanol

sodium sodium methylate methylate CHCH33ONaONa inin

methanolmethanol oror in the solution of in the solution of benzenebenzene with with methanolmethanol..

StandardizationStandardization of NaOH and CHof NaOH and CH33ONaONa onon

benzoic acidbenzoic acid

Nonaqueous acid–base titration of of weak weak acidsacids by by NaOH or CHNaOH or CH33ONaONa

Indicators:Indicators: thymol bluethymol blue ( (red-yellowred-yellow andand yellow-blueyellow-blue) ) oror physico-chemical physico-chemical methods methods ((potentiometrypotentiometry).).

In nonaqueous acid–base titration determinate determinate

the end-point by potenthiometric way the end-point by potenthiometric way

In nonaqueous acid–base titration determinate determinate

the end-point by potenthiometric way the end-point by potenthiometric way

Thanks for your attention!