Cosimo A. De Caro, A. Aichert, C. Gordon

Mettler-Toledo GmbH, AnalyticalCH-8603 Schwerzenbach, Switzerland

Karl Fischer Titration Of Food : Towards Complete Automation

• Introduction• Why water determination in food ?• Sample preparation • The analytical process• Karl Fischer titration

Manual vs. automated KF titrationAutomated volumetric KF titration Automated coulometric KF titration

• Conclusions

Program

Why water determination in food?

■ Lifetime (storage)

■ Physical properties (e.g. viscosity)Sugar: too much moisture --> it will not flow

■ Microbiological stability

■ Content of dried substance, Concentration or purity

■ Quality standards

■ Nutrition value

■ LawButter: max 16.5% water content

Karl Fischer Titration

Reaction:

SO2 + 3 RN + ROH + H2O + I2 = (RNH)SO4R + 2 (RNH)IMethylsulfate

Intermediate: (RNH)SO3R Methylsulfite

• The solvent e.g. methanol is involved in the reaction

• A suitable base keeps the pH 5 - 7

Recent work on KF reaction:A. Cedergren, H. Isengard, H. Hoffmann, A. Margolis, S. Grünke, E. Scholz, S. Schöffski, G. Wünsch.

Volumetric / Coulometric Titration

Volumetric Karl Fischer Analysis:

● Iodine is added by burette during titration.● Water as a major component:

100 ppm - 100 %

Coulometric Karl Fischer Analysis:

● Iodine is generated electrochemicallyduring titration.

● Water in trace amounts:5-10 ppm - 5 %

+-

Karl Fischer Titration

• System tightness Check carefully

• Ambient moisture Drift determination

• Stability of titrant Standardization

• Side reactions Check literature

• Sample handling Accuracy, precision

• Free water only Sample Preparation

Important points:

Which water is determined?

Surface moisture

Particle

Moisture/water

Example: Sugar

Entrapped Water

Water

Particle

Example: Food

Crystal water

Water in the crystal lattice

Example: Salt

Capillary-bound water

Example: Vegetables

Water

Karl Fischer Titration: Sample Preparation

Only free water:

■ Dissolve

■ Crush

■ Extraction

■ Evaporation

Analytical procedure

Sampling

Sample preparation

MeasurementAnalytical principle

Evaluation

Analy

tical

metho

d

Analytical procedure

Sampling

Sample preparation

Measurement(Titration method)

Evaluation

Analy

tical

metho

d

Manual KF titration

From: G. Wieland, 1985

Manual versus automatic Titration

Some Considerations

■ Colour Indication■ End vs. Equivalence Point Titration■ Glassware (burettes)■ Weighing■ Uncertainty Assessment■ Calculations■ Reporting■ Handling of Chemicals

Manual versus automatic Titration

■ Colour perception:

■ Glassware accuracy:10.00 ± 0.02 mL (Class A) at 25°CDifferent classes of burettesCalibrated at different temperatures

■ Volume reading error: 1-3 drops ~ 30-90 µL

Overtitrated?Titrated?Titrated?

Analytical procedure

Sampling

Sample preparation

Measurement(Titration method)

Analy

tical

metho

d

Evaluation

Motorized piston burette

DV/DK Titration System, 1972

Indication: Double pin Pt polarized electrode

Excess of iodine after the endpoint: Ion conductivity

e – e –

+ –

I2 + 2e- = 2 I-2 I- = I2 + 2e-

KF dedicated titrators

DL18 KF Titrator, 1984

DL38 KF titrator, 1998

Advantages

Pretitration, drift determination and compensation, titrant standardization, sample analysis and stand-by ➨ increase of the reproducibility.

Each sample is titrated in exactly the same way.➨ enhances the repeatability.

Minimum increment is much smaller than in manual titration.➨ higher resolution of the titration curve

increases the accuracy of the results.

Errors in calculations and documentation are minimized➨ Fully compliant with QM

Analytical procedure: sample series

Sampling

Sample preparation

Measurement(Titration method)

Analy

tical

metho

d

Evaluation

Automation of volumetric KF titrations

• Beakers with foil and rubber seal

• Foil is pierced by electrode.

• Glass micro titration beaker10-15 mL solvent

• Optimum tightness:rubber sealrubber lips on top of sealmagnetic stirrer(drift < 15-20 µg / min)

Automation of volumetric KF titrationsSample n Sample size

(g)Water content

(%)srel(%)

Titrationtime (s)

Mixtime (s)

Titer determination:Sodium tartrate

3 0.05 Titer:1.00103

0.29 210 60

Test:Glycerine

5 0.2-0.4 5.52 0.50 500 60

Honey (Guatemala) 5 0.07 19.11 0.56 240 60Honey (Switzerland) 6 0.15 18.16 0.63 220 60

Honey (France) 5 0.09 17.52 0.94 240 60

Gin 2 0.04 70.9 -- 420 60

Schnapps (Williams) 6 0.04 67.81 0.81 330 5

srel : relative standard deviation

Total analysis time = Titration time + Mix time

Analytical procedure

Sampling

Sample preparation

Measurement(Titration method)

Evaluation

Analy

tical

metho

d

Stromboli is a volcano in Italy...

for automated Karl Fischer Titration

DL39 and STROMBOLI

...but also an oven sample changer

■ Large glass vials of nominal volume of 25 mL■ Large volume provided for samples with low

moisture content or low density.

Easy sample handling

■ Sealing of vials:an aluminium foil and a rubber seal

■ Simple sealing of vials■ Vials and rubber seals

can be cleanedand reused

■ The vial is introduced into the oven by a lift .

■ The glass capillary pierces the aluminium foil and penetrates the vial.

■ Moisture is transferred with dry carrier gas via the outlet into the titration cell.

■ After completion of titration, the vial is removed from the oven.

Sample processing

■ Drift determination:Compensate moistureintroduced by the carrier gas.

■ Waiting time for a stable drift: 10 - 15 minutes

■ Expected drift value:4 - 10 µg/min

■ Blank determination:Compensate the moisturefrom the vial.

■ Expected blank value:100 - 300 µg

Drift and Blank Value Determination

Automated KF coulometric titration

Stromboli KF Oven Sample Changer

result srel recovery temp time% % % °C min

Water Standard 5.55% 5,52 1,6 99.8 255 18KF-OvenSet value: 5.53%

Water Standard 1% 0,99 0,8 99 180 15KF-OvenSet value: 1.00±02%

Automated KF coulometric titrationSample n Sample

size(g)

Watercontent

(%)

srel(%)

Set T(°C)

Titrationtime(min)

Mixtime(min)

Garlicpowder

3 0.02-0.05 10.14 2.17 180 9-12 5

Solublecoffee

4 0.04-0.06 4.84 1.64 180 11-13 5

Cinnamon(powder)

7 0.04-0.06 9.71 0.41 180 8-9 5

Solubletee (powder)

6 0.04-0.05 10.88 0.89 180 7-10 5

Cocoa(powder)

6 0.03-0.08 2.13 4.58 180 6-8 5

srel : relative standard deviation

Side reactionTest samplemanual KF oven

Comparison: External extraktion in methanol � 610 ppm

0 5 10 15 20 min

µg Water250 °C

160 °C

120 °C

100

200

300

115 mg660 ppm

Automated KF coulometric titration

KF determination in soluble tea, sample no. 5

What are the benefits in automated KF water analysis?

Summary

■ Increased sample throughput and efficiencySaves time and money

■ Identical and reproducible analysis conditions for all samples.

Enhances precision and reliability of results

■ Reduced reagent ans solvent consumptionEnvironmental friendly, less expenses

Questions ?

Thank you for your attention !