Overview
ISO 6796:1981 specifies a standardized method for determining the hydroxyl number in polyglycols intended for industrial use. This method employs the phthalic anhydride esterification technique to quantify the hydroxyl groups present in a test sample. The standard outlines a procedure where hydroxyl groups are esterified with phthalic anhydride, the excess anhydride is hydrolyzed, and the resulting phthalic acid is titrated with a standardized sodium hydroxide solution. Corrections for any free acidity or alkalinity in the sample are applied to ensure accurate results. The corrected hydroxyl number can then be used to calculate the apparent relative molecular mass of the polyglycol.
This international standard is essential for quality control, research, and product development involving poly(ethylene glycol) and poly(propylene glycol) used in a variety of industrial applications.
Key Topics
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Scope and Application
- Applies specifically to polyglycols with hydroxyl functional groups, including poly(ethylene glycol) and poly(propylene glycol) with a degree of polymerization exceeding 3.
- Not recommended for samples containing more than 0.2% water unless the sample is thoroughly dried beforehand.
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Analytical Principle
- Hydroxyl groups are esterified with phthalic anhydride.
- Excess phthalic anhydride is hydrolyzed to phthalic acid, which is neutralized by titration using potassium hydroxide (KOH) standard solution.
- Corrections are made for samples with inherent acidic or alkaline properties.
- The hydroxyl number is expressed as milligrams of potassium hydroxide equivalent per gram of sample.
- The method enables calculation of the apparent relative molecular mass.
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Reagents and Apparatus
- High purity phthalic anhydride (≥ 99%).
- Pyridine, serving as solvent and esterification medium; toxic and highly flammable with irritant vapors, requires careful handling.
- Standardized sodium hydroxide and hydrochloric acid solutions for titration and correction.
- Phenolphthalein indicator solution in pyridine for endpoint detection.
- Laboratory equipment including reflux condensers with drying tubes, conical flasks, and precise analytical balances.
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Procedure Summary
- Preparation of phthalic anhydride solution in pyridine.
- Reflux of sample with phthalic anhydride solution to accomplish esterification.
- Hydrolysis and titration of excess anhydride using sodium hydroxide.
- Performing blank tests to ensure accuracy.
- Applying corrections for acidity or alkalinity in the sample.
- Calculating hydroxyl number and apparent molecular weight.
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Safety Considerations
- Pyridine and phthalic anhydride are irritants and toxic – strict lab safety protocols must be followed.
- Avoid direct contact and inhalation, use fume hoods and personal protective equipment.
Applications
The ISO 6796:1981 standard is primarily used in industrial chemical laboratories tasked with:
Related Standards
- ISO 1389-6: Industrial phthalic anhydride - Methods for purity testing.
- ISO 2211: Chemical liquids - Determination of color using Hazen (Platinum-Cobalt) scale, important for checking reagent quality.
- ISO 6796 complements analytical procedures for the characterization of polyether glycols in terms of molecular weight and hydroxyl content, supporting broader chemical testing frameworks.
Keywords: ISO 6796, hydroxyl number determination, polyglycols industrial use, phthalic anhydride esterification, poly(ethylene glycol), poly(propylene glycol), titration method, molecular weight calculation, industrial chemical standards, quality control polyglycols, pyridine safety, analytical chemistry standard.
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