Determination of Water Content in Oil Samples Using Karl Fischer Titration Following DIN EN ISO 8534:2017

Introduction

The water content has an important role in the quality and the shelf life of most food products. But how can we determine specifically the water content, in all its forms, but without any other decomposition products? It can either be done with big and expensive instrumentation (for example GC, NMR, etc) or by the water specific Karl Fischer titration. The international norm ISO 8534:2017 specifies a method for the determination of the water content of animal and vegetable fats and oils using Karl Fischer apparatus and a reagent which is free of pyridine. Milk and milk products (or fat coming from such products) are however excluded.

Dissolved fat is titrated against an iodine solution and sulfur dioxide (SO2) is oxidized by iodine in the presence of water. The alcohol reacts with SO2 and a nitrogenous base (RN) to form an intermediate alkylsulfite salt, which is oxidized by iodine to an alkylsulfate salt. This oxidation reaction consumes water contained in the sample.

H2O + I2 + SO2 + CH3OH + 3RN -> [RNH]SO4CH3 + 2[RNH]I

Water and iodine are consumed in equimolar amounts in the reaction – it means that, if you know the amount of I2 consumed, you know the amount of water that was present in the sample. This application exemplify how you can comply with the norm, and effectively determine the water content in a commercially available oil sample using our CombiCoulomat frit one-component reagent, together with a high quality water standard, in an appropriate Karl Fischer titrator.

Procedure

(use an appropriate Karl Fischer apparatus)

The Karl-Fischer reagent is placed into the cathode and anode compartment of the titration cell with diaphragm. The coulometer is started and the solvent is titrated dry. After preliminary titration and stabilisation of drift the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the water determination is started.

  1. Check the apparatus by measuring a standard (inject 0.5 mL)
  2. If the result comes back within ±10%, it’s usable
  3. Inject the sample into the titration vessel (0.5 mL sample)
  4. Record the mass
  5. Enter the sample mass in the instrument
  6. The instrument will calculate the final result
  7. Measure the sample two times, then take the average
  8. The difference must be within 10% between the two measurements

Results

  Standard Sample 1 Sample 2
Total weight (g) 3.3128 2.9408 2.9561
Sample weight (g) 0.4561 0.4657 0.4786
Result (mg/kg) 9767.4 292.5 292.2

Conclusion

Foodstuffs encompass the most diverse products. Depending on whether carbohydrate-rich, fatty or protein-rich substances are under investigation, different working techniques are favorable. Complex matrices that dissolve slowly, or where the water can only be slowly extracted, necessitate the use of solubilizers. In addition, titration under heating and the application of a homogenizer to accelerate water release are expedient. No additional solubilizer was required for the samples in this test.

Materials

     
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