Karl Fischer Titration Of Food : Towards Complete Automation

Transcription

1 Karl Fischer Titration Of Food : Towards Complete Automation Cosimo A. De Caro, A. Aichert, C. Gordon Mettler-Toledo GmbH, Analytical CH-8603 Schwerzenbach, Switzerland

2 Program Introduction Why water determination in food? Sample preparation The analytical process Karl Fischer titration Manual vs. automated KF titration Automated volumetric KF titration Automated coulometric KF titratio Conclusions

3 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 Law Butter: max 16.5% water content

4 Karl Fischer Titration Reaction: SO RN + ROH + H 2 O + I 2 = (RNH)SO 4 R + 2 (RNH)I Methylsulfate Intermediate: (RNH)SO 3 R 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.

5 Volumetric / Coulometric Titration Volumetric Karl Fischer Analysis: Iodine is added by burette during titration. Water as a major component: 100 ppm % - + Coulometric Karl Fischer Analysis: Iodine is generated electrochemically during titration. Water in trace amounts: 5-10 ppm - 5 %

6 Karl Fischer Titration Important points: 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

7 Which water is determined? Entrapped Water Surface moisture Water Particle Particle Moisture/water Example: Food Example: Sugar Crystal water Capillary-bound water Water in the crystal lattice Example: Salt Water Example: Vegetables

8 Karl Fischer Titration: Sample Preparation Only free water: Dissolve Crush Extraction Evaporation

9 Analytical procedure Sampling Sample preparation Analytical method Measurement Analytical principle Evaluation

10 Analytical procedure Sampling Sample preparation Analytical method Measurement (Titration method) Evaluation

11 Manual KF titration From: G. Wieland, 1985

12 Manual versus automatic Titration Some Considerations Colour Indication End vs. Equivalence Point Titration Glassware (burettes) Weighing Uncertainty Assessment Calculations Reporting Handling of Chemicals

13 Manual versus automatic Titration Colour perception: Titrated? Titrated? Overtitrated? Glassware accuracy: ± 0.02 ml (Class A) at 25 C Different classes of burettes Calibrated at different temperatures Volume reading error: 1-3 drops ~ µl

14 Analytical procedure Sampling Sample preparation Analytical method Measurement (Titration method) Evaluation

15 Motorized piston burette DV/DK Titration System, 1972

16 Indication: Double pin Pt polarized electrode Excess of iodine after the endpoint: Ion conductivity 2 I - = I 2 + 2e - e e I 2 + 2e - = 2 I - +

17 KF dedicated titrators DL38 KF titrator, 1998 DL18 KF Titrator, 1984

18 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

19 Analytical procedure: sample series Sampling Sample preparation Analytical method Measurement (Titration method) Evaluation

20 Automation of volumetric KF titrations Optimum tightness: rubber seal rubber lips on top of seal magnetic stirrer (drift < µg / min) Beakers with foil and rubber seal Foil is pierced by electrode. Glass micro titration beaker ml solvent

21 Automation of volumetric KF titrations Sample n Sample size (g) Water content (%) srel (%) Titration time (s) Mix time (s) Titer determination: Sodium tartrate Titer: Test: Glycerine Honey (Guatemala) Honey (Switzerland) Honey (France) Gin Schnapps (Williams) srel : relative standard deviation Total analysis time = Titration time + Mix time

22 Analytical procedure Sampling Sample preparation Analytical method Measurement (Titration method) Evaluation

23 Stromboli is a volcano in Italy...

24 ...but also an oven sample changer for automated Karl Fischer Titration

25 Easy sample handling Large glass vials of nominal volume of 25 ml Large volume provided for samples with low moisture content or low density. Sealing of vials: an aluminium foil and a rubber seal Simple sealing of vials Vials and rubber seals can be cleaned and reused

26 Sample processing 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.

27 Drift and Blank Value Determination Drift determination: Compensate moisture introduced by the carrier gas. Waiting time for a stable drift: minutes Expected drift value: 4-10 µg/min Blank determination: Compensate the moisture from the vial. Expected blank value: µg

28 Automated KF coulometric titration Stromboli KF Oven Sample Changer result srel recovery temp time % % % C min Water Standard 5.55% 5,52 1, KF-Oven Set value: 5.53% Water Standard 1% 0,99 0, KF-Oven Set value: 1.00±02%

29 Automated KF coulometric titration Sample n Sample size (g) Water content (%) srel (%) Set T ( C) Titration time (min) Mix time (min) Garlic powder Soluble coffee Cinnamon (powder) Soluble tee (powder) Cocoa (powder) srel : relative standard deviation

30 Side reaction Test sample manual KF oven 300 µg Water 250 C 160 C 115 mg 660 ppm C min Comparison: External extraktion in methanol 610 ppm

31 Automated KF coulometric titration KF determination in soluble tea, sample no. 5

32 Summary What are the benefits in automated KF water analysis? Increased sample throughput and efficiency Saves time and money Identical and reproducible analysis conditions for all samples. Enhances precision and reliability of results Reduced reagent ans solvent consumption Environmental friendly, less expenses

33 Questions? Thank you for your attention!