Overview
ISO 5381:1983 specifies a modified Karl Fischer method for determining the water content of starch hydrolysis products. The standard adapts the general Karl Fischer titration (ISO 760) by directly using a methanol/formamide solvent to improve extraction and measurement of water from starch derivatives and by‑products. It covers reagents, apparatus, sample preparation, titration procedure, calculations and reporting.
Key topics and requirements
- Scope: Determination of water in starch hydrolysis products using a stabilized Karl Fischer reagent (iodine, sulphur dioxide, pyridine and 2‑methoxyethanol) with methanol/formamide solvent.
- Reagents:
- Stabilized Karl Fischer reagent (commercial or prepared per ISO 760).
- Methanol/formamide solvent: mix 700 ml anhydrous methanol with 300 ml anhydrous formamide.
- Sodium tartrate (Na2C4H4O6 · 2H2O) as standard for reagent standardization; check tartrate moisture by vacuum drying at 150 °C to constant mass (approx. 15.66% water).
- Apparatus:
- Karl Fischer titrator or equivalent electrometric end‑point detection.
- One‑mark pipettes, 10 ml syringe (special design for viscous samples), weighing tube for solids, analytical balance.
- Platinum electrodes, titration vessel, burette (25 ml typical) and stirrer.
- Procedure:
- Sample dispersion in methanol/formamide solvent to extract water.
- Standardize reagent daily using sodium tartrate; perform titration until stable end‑point.
- Carry out two determinations and use the mean if repeatability criteria are met.
- Calculations and quality criteria:
- Water content calculated from reagent volume, reagent water equivalent (T) and sample mass (formula given in the standard).
- Repeatability limits specified (e.g., difference ≤ 0.05 % for water