determination of water by the karl fischer titration: theory

1Taiwan, September 2002

Determination of Water

by the

Karl Fischer Titration:

Theory


Motivation Volumetric KF titration

one an two-component reagentsresolution and detection limits

Coulometric KF titrationcell with or without diaphragmresolution and detection limits

Indication, control algorithm, termination parameters

KF titration: important points

Support

Program


Why measure water or moisture?

Butter: max 16.5% water content by law

Sugar: too much moisture will not flow

Drugs: too much moisture decomposition

Compact Disc: too much moisture bad music quality

Brake Fluid: too much water brake do not work

Kerosene: too much water blocked tubing

Flour: too little moisture dust explosion


Methods for the Determination of Water

Chromatography

Spectroscopy (IR, MS)

Thermogravimetry / DSC

Balance with IR /Halogen / Microwave heater

Drying oven

Karl Fischer Titration


Coulometric KF Volumetric KF

Karl Fischer Titration: Why?

Fast (e.g. 1...2 minutes) Selective for water Accurate and precise (0.3% srel) Wide measuring range : ppm to %


Karl Fischer

Bunsen reaction: 2 H2O + SO2 + I2 = H2SO4 + 2 HI

Pyridine happened to be around in the Lab

German petrochemist,1901 – 1958

Publication:1935


KF Titration

KF Reaction

SO2 + RN + ROH ------> (RNH)SO3R

a sulfite compound

(RNH)SO3R + H2O + I2 + 2RN ------> (RNH)SO4R + 2(RNH)I a sulfate compound

Summary

H2O + I2 + SO2 + 3RN + ROH ----->(RNH)SO4R + 2(RNH)I

The solvent (generally methanol) is involved in the reaction A suitable base keeps the pH 5 - 7


Solvent

pH range

optimal

slow

side reactions

pH

log K

2 4 6 8 10

2

4

0

buffer neededoptimal pH 5 - 7


Volumetric / Coulometric Titration

Volumetric Karl Fischer TitrationIodine is added by burette during titration.Water as a major component:100 ppm - 100 %

Coulometric Karl Fischer TitrationIodine is generated electrochemically during titration.Water in trace amounts:1 ppm - 5 %

+-


Volumetric KF Titration

Iodine is added by burette during titration. Water as a major component: 100 ppm - 100 %


Volumetric KF Titration

One - component reagent Titrant:

I2 , SO2, imidazole, methanol and diethylene glycol monoethyleter

Solvent:Methanol

Two - component reagent Titrant:

I2 and Methanol Solvent:

SO2, Imidazole, Methanol

-> fast reaction, chemically stable, higher cost


Volumetric KF Reagents

Titrant Concentration

1-2-5 mg H2O/mL

Titer stability

-----> Check by Standardization

Standardization materials

Water 100%Sodium tartrate 15.66%

Standard solution 5 mg/mLWater Standard 1% (10 mg/g)


Air humidity:

0.5 - 3 mg water / 10 mL air

Air Humidity

Conditioning of the titration stand

Well sealed titration cell

Tropical countries: Air conditioning

Protect titration stand, titrant and solvent from ingress of water.


Drift determination

Automatic drift compensation in the result calculation.

The titration stand is not 100 % tight against air humidity.

Drift determination

The drift is the amount of water entering into the titration stand per minute.

1 - 20 µg H20 / minute


Volumetric Karl Fischer Titration

Titrant: 5 mg H20/mLResolution: 2.5 µg H20/stepDetection limit: 125 µg H20For 5 g sample: 25 ppm

Resolution of burette: 10,000 stepsDetection limit : 50 x ResolutionBurette size: 5 mL

Titrant: 2 mg H20/mLResolution: 1 µg H20/stepDetection limit: 50 µg H20For 5 g sample: 10 ppm

Resolution and Detection Limit


Coulometric KF Titration

Iodine is generated electrochemically during titration Water in trace amounts: 1 ppm - 5 %

- +


Anolyte(sulfur dioxide, imidazole, iodide, different solvent for different application - methanol, ethanol with chloroform, octanol, ethyleneglycol )

Double platinum pin electrode

Catholyte(similar or

modified solution)

Generator electrode

Diaphragm

AnodeCathode+–


Titration cell and reagents



Same reaction as volumetric KF Titration but Iodine is produced just in time from iodide

+–

H+

-H

I--

I 2 I- I2 + 2 e-

AnodeIodine production by oxidation

Side reaction:Reduction of sulfur components.After 1 - 2 weeks, smells like mercaptansChange catholyte every week!

H2 2 H+ + 2 e-

Cathode


Coulometry Theory

One Coulomb C is the quantity of charge transported by an electric current of one Ampere (A) during one second (s).

1 C = 1 A • 1 s

Absolute method, no standardization!

Charles Augustin de Coulomb14.6.1736 - 23.8.1806

To produce one mol of a chemicalcompound, using one electron,96484 C are required.

2 I- ions react to form I2 which in turnreacts with water

1 mol of water (18g) is equivalent to 2 x 96484 C or 10.72 C/mg water.


Filling the Titration Cell

Anolyte:Fill in ~ 100 mL anolyte

Catholyte:Fill in 5 mL catholyte.

Anode+–

Cathode

Catholyte

Anolyte

The level of the anolyte should be 3 - 5 mm higher than the level of catholyte sothat the flow is from the anolyte compartment to catholyte compartment.

Low drift value

With stirring the level difference of anolyte and catholyte will be stable.


Filling the Titration Cell

Anode+–

Cathode

Catholyte

Anolyte

Catholyte always contains water!

If the catholyte level ishigher or at the same levelas the anolyte,there is a flow of moisture into the anolyte compartment.

High drift value


With or Without Diaphragm

What are the differences?


With Diaphragm

+–

Without Diaphragm

I- -I

Iodine is only in the anodecompartment and reacts with water.

+–

I- -I

-I I-

It is possible that iodine can go to the cathodeand convert to iodide.

With or Without Diaphragm


Without Diaphragm

Prevention:

– bigger sample error has no effect

– high stirrer speed iodine reacts faster with water

Iodine I2 can go to the cathode and convert to iodide.

– Small cathode surface less chance to contact iodine

Only a little less accuratefor samples

with very low water content.

+–

I- -I

-

H+ H

– high iodine production speed hydrogen protects cathode


Without Diaphragm

The hydrogen produced at the cathode is a very good reducing agent.

Easily reducible samples (nitro compounds) get reduced, which produces water.

too high result

Not recommended for easily reducible samples: e.g. nitrobenzene, unsaturated fatty acids, etc.

–

I- -I

-H+ H

R-NO2 R-NH2 + H2O+


Without Diaphragm

– A little bit less accuracy for very small water content (< 50 µg/sample).

– Not recommended for easily reducible samples: nitro compounds, unsaturated fatty acids, etc.

+ Titration cell easier to clean.+ Long-term drift value more stable.+ Only one reagent.

Titration cell without diaphragm is the standard set-up for: Hydrocarbons, halogenated hydrocarbons, alcohols, esters, ethers, acetamides, mineral oils, edible oils, ethereal oils


Resolution: 0.1 µg water

Detection limit: 5 µg waterfor 5 g sample 1 ppm

Measuring range:10 µg - 100 mg water/sample1 ppm - 5 % water

+-

Coulometric Karl Fischer Titration

Resolution and Detection Limit


srel > 5 %

srel 5 - 0.5 %

srel < 0.5 %

Not suitable for coulometry

coulometry

Not suitable for volumetry

srel > 5 %

srel 5 - 0.5 %

srel < 0.5 %

volumetry

1 ppm

10 ppm

100 ppm

1000 ppm

1 %

10 %

100 %

Coulometry versus Volumetry

Repeatability


Ipol = 20µAU = 650mV

2

KF Indication Principle (1/2)

Bivoltametric indicationconstant current at the double platinum pin electrode ==> polarization current (Ipol)

During titration:

I2 reacts with water

no free I2 in the solution

high potential


Ipol = 20µAU = 84mV

2

+ -

ee

I2

I2 + 2e- -> 2 I-2 I- -> I2 + 2e-

2I- I2

KF Indication Principle (2/2)

At endpoint all water has reacted with I2

After the endpoint

free I2 in the solution

I2 is reduced to I- at the cathode ionic conductivity occurs and the

measured potential drops potential change = endpoint


V/mL

E/mV

EP

KF Fuzzy logic

V/mL

E/mV

Control range EP

KF Classical

KF Control: Titrator Algorithm

Karl Fischer Fuzzy Logic Control DL31/38 No control band required

(typical 300 mV)

The titrant addition rate depends on: the distance to the endpoint EP the potential change/increment

Advantages: Simpler control: Only two control parameters

Vmin , Vmax (smallest/largest increment) Faster, more accurate, and better precision

even at low water content(toluene: n = 5, 115 ppm, srel 0.17% )


E (mV)

t(s)

EP

15 s

KF Control: Termination Parameters (1/3)

Delay time

the actual potential is lower than the EP for a defined time after the last titrant increment

typical delay : 15 - 20 sec

Note:Adapt the smallest increment to the drift and to the concentration of the titrant



Absolute drift stop

the actual drift is less then the predefined value

typical value : 30 g/min

Note:Adapt the value to the initial drift

t(s)

Drift (µg/min)

abs. drift stop= 30 µg/min

EP


Drift (µg/min)

t(s)

Initial drift

Rel. Drift stop= 20 µg/min


Relative drift stop

the sum of the initial and the relative drift has been reached

typical value : 15 g/min

independent from the initial drift and of titrant concentration

ideal with side reactions that cannot be suppressed otherwise


Karl Fischer Titration : Checks

Relevant points to be checked System tightness : Check carefully Ambient moisture : Drift determination Stability of titrant : Standardisation Side reactions : Check literature Sample handling : Accuracy, precision Free water only : Sample preparation


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determination of water by the karl fischer titration: theory

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