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 !