1 titration curves k (a) + (r) (p) (a) + (r) (p) analyte titrant product titration curves: 1. strong...

1

TITRATION CURVESTITRATION CURVES K (A)(A) + + (R)(R) (P)(P)

analyte titrant product

Titration curves:Titration curves:1.1. Strong acidStrong acid with strong basestrong base,, Strong baseStrong base with strong acidstrong acid2.2. Weak acidWeak acid withstrong basestrong base,, Weak baseWeak base with strong acidstrong acid3.3. Polyprotic acidPolyprotic acid with strong basestrong base

Important points and Important points and regionsregions::2 points: before titrationbefore titration (at 0%) I.I. [[AA]]

at the end pointat the end point (at 100 %) III.III. [[AA]] = = [[RR]]2 regions: before the end pointbefore the end point (0.00..1 – 99.99…%) II.II. [[AA]] + + [[PP]]

after the end pointafter the end point (100.00..1 – ∞) IV.IV. [[PP]] + + [[RR]]

NEUTRALIZATIONNEUTRALIZATIONANALYSISANALYSIS

Introduction

Titrants

Titration curvesTitration curves

End point detection

Applications

Outline

NEUTRALIZATION ANALYSISNEUTRALIZATION ANALYSIS

2

TITRATION CURVESTITRATION CURVES

I.I. At the start:At the start:

HCl HCl + + NaOHNaOH Cl Cl–– ++ NaNa++(H(H22O) O) acidacid11 + + base base22 basebase11 ++ acid acid22

(very weak)(very weak)

e.g.

1.1. Strong acidStrong acid with strong basestrong base,, Strong baseStrong base with strong strong acidacid

II.II. Before the end point:Before the end point: [H+] = [H3O+]=[HCl]unreacted [OH–] = [NaOH]unreacted

pH = – lg [HCl]unreacted pOH = – lg [NaOH]unreacted

III.III. At the end point:At the end point: [H+] ≡ [OH–]

KW = 10–14 pH ≡ 7IV.IV. After the end point:After the end point:

[OH–] = [NaOH]excess [H+] = [H3O+]=[HCl]excess

pOH = – lg [NaOH]excess pH = – lg [HCl]excess

OutlineNEUTRALIZATIONNEUTRALIZATION

ANALYSISANALYSIS

Introduction

Titrants


End point detection

Applications

[H+] = [H3O+]=[HCl]0 [OH–] = [NaOH]0

pH = – lg [HCl]0 pOH = – lg [NaOH]0 pH = 14 – pOH

3


1. Effect of the temperature:1. Effect of the temperature:

100100°C°C [H[H++]·[OH]·[OH––] = K] = Kww = 10 = 10–1–122 NNeeutr. pointutr. point: pH = 6: pH = 6

25°C25°C [H[H++]·[OH]·[OH––] = K] = Kww = 10 = 10–14–14 NeutrNeutr. po. poiint: pH = nt: pH = 77Outline


Introduction

Titrants


End point detection

Applications

100 0

Titration curves:Titration curves:1.1. Strong acid Strong acid with strong basestrong base,, Strong baseStrong base with strong acidstrong acid2.2. Weak acidWeak acid withstrong basestrong base,, Weak baseWeak base with strong acidstrong acid3.3. Polyprotic acid Polyprotic acid with strong basestrong base

EFFECTS ON THE TITRATION CURVE: EFFECTS ON THE TITRATION CURVE:

4

EFFECTS ON THE TITRATION CURVEEFFECTS ON THE TITRATION CURVE

2. Dependence on the initial concentrations (e.g. 2. Dependence on the initial concentrations (e.g. [[HClHCl]]):):

[HCl]0

↓

0% 50% 90% 99% 99.9% 100% 100.1% 101% 110%

1 N 0 0,3 1 2 3 7 11 12 13

0,1 N 1 1,3 2 3 4 7 10 11 12

0,01 N 2 2,3 3 4 5 7 9 10 11

0,001 N 3 3,3 4 5 6 7 8 9 10

ΔpH33 – – 11114 4 – – 101055 – – 9966 – – 88

pH change around the end point

100 0


Introduction

Titrants


End point detection

Applications

Outline

5

3. Dependence on the acid strength (dissociation constants):3. Dependence on the acid strength (dissociation constants):

A. Weak acidA. Weak acid with strong basesstrong bases ,

EFFECTS ON THE TITRATION CURVEEFFECTS ON THE TITRATION CURVE

100 0


Introduction

Titrants


End point detection

Applications

Outline e.g. 10–1 N CHCH33COOHCOOH is titrated with NaOH NaOH (Ka = 2x10–5)

%% 0 50 90 99 99.9 100 100.1 101 110

pH 2.9 4.7 5.7 6.7 7.7 8.9 10 11 12

ΔpH pKInd ≈ 99 → PHENOLPHTALEINPHENOLPHTALEINB.B. Weak baseWeak base with strong acidstrong acid

%% 0 50 90 99 99.9 100 100.1 101 110

pH 11.1 9.3 8.3 7.3 6.3 5.1 4 3 2

ΔpH pKInd ≈ 5 5 → METHYL REDMETHYL RED

e.g. 10–1 N NHNH44OH OH is titrated with HCl HCl (Kb = 2x10–5)

6


I. At the start:I. At the start:

pH

II.II. Weak acid Weak acid with strong base strong base Weak baseWeak base with strong acidstrong acid

Weak acidWeak acid Weak baseWeak base

basebCKOH acida CKH COOHCHKH 3a OHNHKOH 4b

II. Before the end point:II. Before the end point:

pHBuffer (acid / salt)Buffer (acid / salt) Buffer (base / salt)Buffer (base / salt)

salt

acida C

CKH

salt

baseb C

CKOH

4

4b

NH

OHNHKOH

COOCH

COOHCHKH

3

3a

III. At the end point:III. At the end point:

pHHydrolysing salt (Brönsted base)Hydrolysing salt (Brönsted base) Hydrolysing salt (Brönsted acid)Hydrolysing salt (Brönsted acid)

salta

w CK

KOH

saltbCKOH COOCHKOH 3b

IV. After the end point:IV. After the end point:

pHExcess of strong baseExcess of strong base

[OH–] = Cexcess base [OH–] = [NaOH]excess[H+] = Cexcess acid [H+] = [HClexcess

Excess of strong acidExcess of strong acid

saltb

w CK

KH

salta CKH 4a NHKH

Introduction

Titrants


End point detection

Applications


ANALYSISANALYSIS

e.g. Titration of CHCH33COOHCOOH with NaOH NaOH , Titration ofTitration of NHNH44OH OH with HCl:HCl:

7

TITRATION CURVES TITRATION CURVES III.III. Polyprotic acid Polyprotic acid with strong basestrong base

1.1. HH33POPO44 + + OHOH–– HH22POPO44– – + H+ H22OO Ka1 = 7x10–3

2.2. HH22POPO44–– ++ OHOH–– HPOHPO44

2–2– ++ HH22OOKa2 = 6x10–8

3.3. HPOHPO442–2– + + OHOH–– POPO44

33–– + + HH22OOKa3 = 10–12

e.g. Titration of H H33POPO44 with NaOHNaOH

Introduction

Titrants


End point detection

Applications


ANALYSISANALYSIS

8

ACID / BASE INDICATORSACID / BASE INDICATORS

1.1. Azo-compoundsAzo-compounds

Genearal structure:Genearal structure:

Mechanism:Mechanism:

Yellow Yellow (basic)(aromatic)

YellowYellow(intermediate) (protonated)

RedRed (acidic)(quinoid)


Outline

Introduction

Titrants

Titration curves

End p. detectionEnd p. detection- chemical- chemical- instrumental

Applications

9

ACID / BASE INDICATORS ACID / BASE INDICATORS

2.2. PHTHALEIN-derivativesPHTHALEIN-derivativesGeneral structure:General structure:

Mechanism:Mechanism:

ColorlessColorless (acidic)

ColorlessColorless(intermediate)

PurplePurple (basic)

Thymol blueNEUTRALIZATIONNEUTRALIZATION

ANALYSISANALYSIS

Introduction

Titrants

Titration curves

End p. detectionEnd p. detection- chemical- chemical- instrumental

Applications

Outline

10

INSTRUMENTAL DETECTIONINSTRUMENTAL DETECTION

MethodMethod Sensing deviceSensing device Type of titrationType of titration

POTENTIOMETRYPOTENTIOMETRY

(Potential (Potential vs %)%)

Different types of

electrodeselectrodes

Neutralization titr.Neutralization titr.Complexometric titr.Complexometric titr.Precipitation titr.Precipitation titr.Redox titr.Redox titr.

The titration process is followed by electrochemical, photometric or other sensing devices.sensing devices.

(Summary)(Summary)

AMPEROMETRYAMPEROMETRY

(Current (Current vs %)%)

PtPt electrode

(dead stop…) Redox titr.Redox titr.

CONDUCTOMETRYCONDUCTOMETRY

(Conductivity (Conductivity vs %)%)Conductivity cellConductivity cell Neutralization titr.Neutralization titr.

Precipitation titr.Precipitation titr.

PHOTOMETRYPHOTOMETRY(A = (A = εε ·· c c ·· l l vs %)%)

SpectrophotometerSpectrophotometer Complexometric Complexometric titr.titr.

ENTALPHYMETRYENTALPHYMETRY(Q = f (c, (Q = f (c, ΔΔH) H) vs % %

ThermistorThermistor Neutralization titr.Neutralization titr.Complexometric titr.Complexometric titr.Precipitation titr.Precipitation titr.Redox titr.Redox titr.

Outline

INSTRUMENTALINSTRUMENTALDETECTIONDETECTION

Advantages

TypesTypes

Potentiometric end point detection

Conductometric end point detection

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POTENTIOMETRYPOTENTIOMETRY

Nernst equation:Nernst equation:

Electrode potentialElectrode potential developed developedbetween:

Neutralization titration:Neutralization titration:

Complexometric titration:Complexometric titration: E = EE = E00 + lg + lg [[MMn+n+]]

Indicator electrodeIndicator electrode Reference electrodeReference electrode

Glass electrodeGlass electrode

MetalMetalelectrodeelectrode

Precipitation titration:Precipitation titration:

Redox titration:Redox titration:

E = EE = E00 + 0.059 lg + 0.059 lg [[XX−−]]

E = EE = E00 + lg + lg [[oxox]][[redred]]

Ion-selectiveIon-selectiveelectrodeelectrode

Nobel metalNobel metalelectrodeelectrode

Common reference electrodes:Common reference electrodes:Solid metal / its „unsoluble” salt / saturated conc. of anion

e.g. Ag / AgCl / KCl Hg / Hg2Cl2 / KCl Hg / Hg2SO4 / K2SO4

E = EE = E00 + 0.059 lg + 0.059 lg [[HH++]]

0.0590.059 nn

0.0590.059 nn

Known, constant potentialconstant potential (Eref) IndependentIndependent of theof the analyte concentrationconcentration

PotentialPotential (Eind) varies Depends onDepends on the analyte concentrationconcentration

Outline


Advantages

Types

Potentiometric Potentiometric end point end point detectiondetection


clgn

059.0EE 0

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POTENTIOMETRYPOTENTIOMETRYNeutralization analysisNeutralization analysis

Indicator electrode:Indicator electrode:

GLASS ELECTRODEGLASS ELECTRODE

Electrochemical cell for measurement of pH:Electrochemical cell for measurement of pH:

Internal referenceInternal reference electrodeelectrode (Ag/AgCl/KCl)

InternalInternal ||

buffer sol.buffer sol. ||(KCl) (pH = 7) ||

pH-sensitivepH-sensitive || glass- glass- || membranemembrane ||

HH++ conc. conc. || to be to be ||determineddetermined ||

External referenceExternal reference |||| electrodeelectrode ||||(Hg/Hg2Cl2/KCl) ||||

███████████External Dry glass Internalhydrated hydratedgel layer gel layer

ExternalExternalreference electrodereference electrode Glass electrodeGlass electrode

H+ conc. to be determined

Outline


Advantages

Types



13

POTENTIOMETRYPOTENTIOMETRYGlass electrodeGlass electrode

Composition of glass:Composition of glass:E.g. 22 % Na2O, 6 % CaO, 72 % SiO2.

Ion-exchange reaction:Ion-exchange reaction:between

HH++ in the solutionin the solution and NaNa++ in the glass in the glass:

Combination glass electrode:Combination glass electrode:

Na+ mobile

KKHH++ + + NaNa++GlGl−− NaNa++ + + HH++GlGl–– K = LARGE!K = LARGE!

solution glass solution glass

Na+H+

solution membrane

Na+H+

H+

Na+Outline


Advantages

Types



14

POTENTIOMETRYPOTENTIOMETRYTitration curveTitration curve

Potentiometric titration curve:Potentiometric titration curve:

Measuring the potential potential of a suitable indicator electrode (pH)(pH) as a function ofof volume volume titrant.

Determination of the end point:Determination of the end point: from the derivativesderivatives

11stst derivative derivative

22ndnd derivative derivative

Titration curveTitration curve

Outline


Advantages

Types



15

CONDUCTOMETRIC CONDUCTOMETRIC TITRATION CURVESTITRATION CURVES

I.I. Titration of Titration of strong acidstrong acid (a) (a) with strong basestrong base e.g. e.g. HClHCl with NaOH NaOH (b)(b) with weak baseweak base e.g. e.g. HClHCl with NHNH44OHOH

II.II. Titration of Titration of weak acidweak acid (c) (c) with strong basestrong base e.g. e.g. CHCH33COOHCOOH with NaOH NaOH

(d)(d) with weak baseweak base e.g. e.g. CHCH33COOHCOOH with NH NH44OHOH

%%

%%

Outline


Advantages

Types

Potentiometric end point detection


16

APPLICATIONSAPPLICATIONS

I. I. Determination ofDetermination of strong strong acidsacids / / basesbases:: Equivalence point:Equivalence point: pH = 7pH = 7

TITRATIONSTITRATIONS

DirectDirect Back (indirect):Back (indirect):

AnalyteAnalyteTitrant in excessTitrant in excess

to measureto measureto calculateto calculate

Outline


Introduction

Titrants

Titration curves

End point detection

ApplicationsApplications

e.g. NaOHNaOH

VVphen.phen.

VVmeth.r.meth.r.

OHOH− − H H22OO

17


e.g. carboxylic acids of low carbon atoms carboxylic acids of low carbon atoms e.g. CH3COOH

II. (a)II. (a) Determination of Determination of weakweak acidsacids : :

e.g. CO CO22 (as carbonate or hydrogencarbonate)(as carbonate or hydrogencarbonate)

Application of COApplication of CO22 determination: determination:

DeterminationDetermination of of organic materialsorganic materials Determination ofDetermination of COCO22, HCO, HCO33

– – , CO, CO332–2–

content of natural waterscontent of natural waters

Distillation apparatusDistillation apparatus ((Maros- Schulek)Maros- Schulek)

Nonaqueous solvents:Nonaqueous solvents: KKa a << 10 10–7–7

>> 10 10–12–12

fatty acids fatty acids (e.g. fat, wax, oil)

Direct:Direct:

Back :Back : if the weak acid is if the weak acid is volatilevolatile

II.II. Determination ofDetermination of weakweak acidsacids : :

weak weak bases bases : : Equivalence point:Equivalence point: pH pH >> 7 7 (phenolphtalein indicator)(phenolphtalein indicator)

Outline


Introduction

Titrants

Titration curves

End point detection


Equivalence point:Equivalence point: pH pH << 7 7 (methyl red indicator)(methyl red indicator)

Ka ≥ 10–5. (10–7 - 10–4)

bubble-freebubble-free distillationdistillationCOCO22 known amount of Ba(OH)Ba(OH)22

back titrationback titration of excessexcess Ba(OH)Ba(OH)22

with with standard HClHCl

18


Direct:Direct: e.g. NHNH44OHOH

Back:Back: NHNH44++ -salt -salt NHNH33

Application of NHApplication of NH33 determination: determination:

N-containing organic compounds N-containing organic compounds (e.g. amino acids, proteins,…)(e.g. amino acids, proteins,…)

strong base (NaOH)

boiling

Kjeldahl methodKjeldahl method::

Decomposition Decomposition (mineralization)(mineralization) withwith cc. H cc. H22SOSO44, 300 , 300 °°CC

+ catalyst: + catalyst: Se, or CuSe, or Cu2+2+

Ox. number:Ox. number: – 3– 3 (NH(NH44))22SOSO44 (e.g.. – NH2, –N(CH3)2, =NH, –N<)

Ox. number:Ox. number: + 3, +1+ 3, +1 HNOHNO33 (+5)(+5) (e.g.,azo- (-N=N-), nitro-, nitrozo comp.) ReductionReduction

with Zn, Nawith Zn, Na22SS22OO44,..,..

Nonaqueous solvents:Nonaqueous solvents: KKb b << 10 10–7–7

>> 10 10–12–12

NHNH44++

distillation distillation into into known excessknown excess of acid of acid Outline


Introduction

Titrants

Titration curves

End point detection


II. (b)II. (b) Determination of Determination of weak weak basesbases : :Kb ≥ 10–5 (10–7 - 10–4)

NH3

known HCl

back titration of excess acid (HCl) with basic titrant (NaOH)

19


NOT MEASURABLE!NOT MEASURABLE!III.III. Determination of Determination of salts:salts:

E.g. Aniline Aniline ·· HCl HCl;; Benzidine Benzidine ··HH22SOSO44;; Papaverine Papaverine ·· HCl… HCl…

(a)(a) Neutral salts:Neutral salts:

(b)(b) Salt hydrolyzing to acid:Salt hydrolyzing to acid: Brönsted acid (strong acid + weak base)MA + H2O MOH + A– + HH++

if pK if pK >> 7 7! can be TITRATED! can be TITRATED with basewith base

(c)(c) Salt hydrolyzing to base:Salt hydrolyzing to base: Brönsted base Brönsted base (strong base + weak acid)MA + H2O HA + M+ + OHOH––

if pK if pK >> 7 7 can be TITRATED can be TITRATED with acid with acid

E.g.E.g. NaNa22BB44OO77 (B4O72– +7 H2O 4H3BO3 + 2OH–) methyl redmethyl red

Outline


Introduction

Titrants

Titration curves

End point detection


E.g. NaNa22COCO33 (CO32– + H2O HCO3

– + OH–) phenolphtphenolpht..(CO3

2– +2 H2O H2CO3 +2 OH–) methyl redmethyl red

NaHCONaHCO33 (HCO3– + H2O H2O + CO2 +OH–) methyl redmethyl red

NaHCONaHCO33

VVmeth.r.meth.r.

HCOHCO33− − H H22COCO33

VVphenphen = 0 = 0COCO3322− − HCOHCO33

−− H H22COCO33

NaNa22COCO33

VVphenphen VVmeth.r.meth.r.

20

(d)(d) Specific determinations:Specific determinations: NaOH NaOH – Na– Na22COCO33

in the presence of each otherin the presence of each other

Outline


Introduction

Titrants

Titration curves

End point detection



OHOH−−, , COCO3322− − HCOHCO33

−− H H22COCO33

A. OH– + H+ H2O

Warder’s method :Warder’s method :

CO32– + H+ HCO3

–..B. HCO3

– + H+ H2CO3 methyl redmethyl red

one sample :one sample :

two samples :two samples :B. OH− + H+ H2O

CO32– +2H+ H2CO3

methyl redmethyl red

phenolpht.phenolpht.

Winkler’s method :Winkler’s method :A. + BaCl2

CO32– +Ba2+ BaCO3

OH– + H+ H2O phenolpht.


NaHCONaHCO33 – Na– Na22COCO33 in the presence of each otherin the presence of each other

Warder’s method :Warder’s method :two samples :two samples :

A. CO32– + H+ HCO3

–

phenolpht...B. HCO3

– + H+ H2CO3

CO32− +2H+ H2CO3

methyl redmethyl red

COCO3322−− HCOHCO33

− − HCOHCO33−− H H22COCO33


1 titration curves k (a) + (r) (p) (a) + (r) (p) analyte titrant product titration curves: 1. strong...

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