The influence of conductivity on Karl Fischer titration

Dr. Silke Grünke, Merck KGaA, Analytics & Reagents,Laboratory manager special reagents

26-27 March 2002, Reims

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Introduction

- The mechanism of the Karl Fischer reaction

KF reaction: Oxidation of alkyl sulphite

1) ROH + SO2 + RN [RNH]SO3R 2) [RNH]SO3R + I2,I3- + H2O + 2 RN [RNH]SO4R + 2 [RNH]I

Competing equilibria in KF solutions:

SO2

ROSO2- HSO3

-

+ ROH+ H2O

HSO3- + I2,I3- SO3 + HI+ ROH + H2O + base

ROSO2O- SO42- B.SO3

Hydrogen sulphite can be oxidized in addition to alkyl sulphite:

27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 3

for buffering

pH 5 – 8

e.g. pyridine, imidazole

Introduction

- The composition of Karl Fischer solutions

main components

I2 SO2 base solvent

determines

-stoichiometry,- reaction rate

- indication

- dissolving properties

for stabile one component reagents:

Less reactive alcohol

e.g. DEGEE

For optimum KF titration:

reactive and polare alcohol

methanol most suitable

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Task- Alternative solvents for methanol

Reagents for special applications

• for aldehydes & ketones (methanol free)

• dissolving properties adapted on samplese.g. addition of toluene, formamide, less polar alcohols

Reduction of toxic properties

• 2-methoxyethanol replaced by DEGEE

• substitution of methanol by ethanol or 1-propanol

without methanol:

observation overtitration, poor reproducibility

reasons ? lower reaction rate

indication problems !

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The indication of the Karl Fischer titration- principle

voltametrically or amperometrically

by polarizing a double platinum electrode

U[mV]

U(stop)

t[s]

titration curve: voltage versus titration time

excess of I2/I3- leads to strong decrease of the resistance (voltage) at the endpoint

anode : 2 I- I2 + 2 e-

cathode : I2 + 2 e- 2 I-

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The indication of the Karl Fischer titration- influence of the solvent

electrochemical behaviour of KF solutions

standard potential E(I2/I-) conductivity

0

200

400

600

800

1000

0 25 50 75 100 125I(pol) [uA]

U [m

v]

1-propanol, c(I2) = 6*10^-4 mol/L

methanol, c(I2) = 6*10^-4 mol/LI-U range

parameter I(pol) – E(stop)

adjusted on the solvent

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one component titration- comparison of the course of conductivity

start: low titration rate

no reactive components with fresh solvent

low reaction rate low conductivity

titration: titration rate increases

addition of reagent

reaction rate increases conductivity increases

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one component titration- comparison of the course of conductivity

0

2000

4000

6000

8000

10000

12000

14000

16000

0 1 2 3 4 5 6 7 8 9 10

titrant [mL]

cond

uctiv

ity [µ

S/cm

]

KF reagent 109248KF reagent 109234CombiTitrant 188005

Titration with different one component reagents and methanol

type of solvent and baseinfluence conducivity

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one component titration- comparison of the course of conductivity

0

2000

4000

6000

8000

10000

12000

14000

0 1 2 3 4 5 6 7 8 9 10

titrant 188005 [mL]

cond

uctiv

ity [µ

S/cm

]

methanol

methanol/chloroform

methanol/toluene

ethanol

DEGEE

CombiSolvent

Titration with CombiTitrant 5 and several solvents

if indication problems:

adjust instrument parameters or composition of reagents

titration not possible

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one component titration- adjustment of composition of solvents

to increase conductivity of solvents

addition of salts e.g. R4N+Br-

ethanolic solvent for titrations with CombiTitrant(contains SO2 and base)

0,000

0,500

1,000

1,500

2,000

2,500

3,000

3,500

0 50 100 150 200 250 300 350 400 450 500

t [sec]

V [m

l]

methanol

CombiSolvent

use of diluted KF solvents

e.g. CombiSolvents, low concentrations of SO2 and base

advantages:

acceleration of reaction rate

increased initial conductivity

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one component titration- adjustment of composition of solvents

CombiSolvent oils for mineral oils

contains toluene

CombiSolvent fats for food industry

contains butyl acetate and decanol

water determination in oils & fatsaddition of toluene, chloroform, long chained alcohols

solvent mixtures of less polarity

consider influence on indication

diluted KF solvents for one component titration of oils & fats

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two component titration- comparison of the course of conductivity

0

5000

10000

15000

20000

25000

30000

0 2 4 6 8 10 12titrant [mL]

cond

uctiv

ity [µ

S/c

m] methanol

ethanol

Two component titration based on methanol and ethanol

start: reactive components in KF solvent ensure optimum conditions

titration:almost constant conductivity

ethanol: conductivity decreases (factor 3 to methanol)

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Coulometry- influence of conductivity

generation of iodine ! indication of the endpoint

conductivity more important than for volumetry

conductivity of catholyte (cell with frit) or working medium (fritless cell) decreases during determination

long chained alcohols, xylene or chloroform can be added

with diaphragmaddition of max. 40 % / volume to

CombiCoulomat frithigher robustness

without diaphragmaddition of max. 20 % / volume to

CombiCoulomat fritlesshigh current at generator electrode

limit 5-6 mS/cm before current breaks down

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Coulometry- development of new reagents

goal increased conductivityespecially for reagents for fritless cell

reached by addition of salts

problems solubility

side reactions, stability

formation of agglomerates

development activities in progress

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Conclusion

A minimum conductivity is necessary

for indicating the endpoint of KF titration.

replacement or reduction of methanol:

- consider influence on indication

- solvent determines E(I2/I3-) and conductivity

adjustment of instrument parameters and/or compositions of reagents are necessary e.g. advantage of diluted KF solvents for one component titration (apura CombiSolvents)

development of coulometric reagents with increased conductivity