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The influence of conductivity on Karl Fischer titration
Dr. Silke Grünke, Merck KGaA, Analytics & Reagents,Laboratory manager special reagents
26-27 March 2002, Reims
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 2
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 4
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 !
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 5
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-
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 6
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 7
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 8
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 9
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 10
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 11
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 12
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)
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 13
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 14
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
27/03/2002 © Merck Editor: Influence of conductivity on KF titration Page 15
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
Recommended