Overview
ISO 1691:1976 specifies an international standard method for the determination of carbonate content in sodium and potassium silicates used in industrial applications. The method is a gas-volumetric technique that measures carbon dioxide evolved when silicate samples react with hydrochloric acid. This standard addresses three specific sample cases based on the presence or absence of interfering substances such as sulphides and chlorates, ensuring accurate carbonate quantification. It provides a reliable analytical procedure useful for quality control, product specification, and research in industries utilizing sodium and potassium silicates.
Key Topics
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Scope and Sample Classification
ISO 1691:1976 covers three cases distinguished by a preliminary test: silicates without sulphides or chlorates, those containing sulphides, and those containing chlorates. Proper identification of sample type is vital for applying the correct analytical procedure.
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Gas-Volumetric Methodology
The core principle involves acidifying the silicate sample with hydrochloric acid, then boiling with methyl orange indicator and a lead acetate paper strip. This setup detects sulphides (blackening of paper) or chlorates (methyl orange decolorization). For samples without sulphides or chlorates, CO₂ volume resulting from carbonate decomposition is measured volumetrically.
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Reagents and Apparatus
Key reagents include hydrochloric acid (approximately 6 N), methyl orange solution, lead acetate paper, sodium hydroxide, hydrogen peroxide (for sulphide oxidation), and ferrous sulfate solution (for chlorate reduction). The analytical apparatus consists of a conical flask, gas burette, absorbers, and thermostatic water bath to capture and quantify CO₂ volume accurately.
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Procedural Variations Based on Sample Type
- Silicates without sulphides/chlorates: Direct volumetric measurement of evolved CO₂.
- Silicates containing sulphides: Pre-oxidation with hydrogen peroxide to avoid interference from hydrogen sulfide gas.
- Silicates containing chlorates: Reduction with ferrous sulfate to convert chlorates to chlorides, preventing chlorine interference.
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Calculation and Expression of Results
Results are expressed as a percentage by mass of sodium carbonate (Na₂CO₃) or potassium carbonate (K₂CO₃) depending on the silicate type. The method demonstrates high repeatability with an accuracy of ±0.04% (m/m).
Applications
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Quality Control in Chemical Manufacturing
Ensures accurate carbonate content in sodium and potassium silicates, critical for consistent product performance in glass production, detergents, adhesives, and water treatment chemicals.
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Industrial Process Monitoring
Helps optimize manufacturing conditions by providing reliable carbonate measurement, preventing quality deviations caused by variable carbonate levels.
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Research and Development
Facilitates formulation improvements and comparative studies involving silicate raw materials through precise carbonate quantification.
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Regulatory Compliance
Supports adherence to product and industry standards requiring certified chemical composition testing.
Related Standards
ISO 1691 complements and is supported by a series of related ISO standards focused on sodium and potassium silicates, including:
- ISO 1686: Sampling and general test techniques for sodium and potassium silicates
- ISO 1687: Density determination methods for solutions
- ISO 1688: Gravimetric determination of dry matter content
- ISO 1689: Calculation of Na₂O or K₂O ratios in silicates
- ISO 1690: Gravimetric method for silica content determination
- ISO 1692: Total alkalinity determination by titration
- ISO 2122: Dissolution methods and insoluble content determination
- ISO 2123: Determination of dynamic viscosity
- ISO 2124: Silica content by titrimetric method
- ISO 3200: Sulfate content determination by gravimetry
- ISO 3201: Iron content determination via photometry
These standards collectively ensure comprehensive analytical capability for chemical properties essential to the industrial use of sodium and potassium silicates.
Keywords: ISO 1691, carbonate determination, sodium silicate analysis, potassium silicate, gas-volumetric method, industrial silicates, sulphide interference, chlorate interference, chemical analysis, quality control, carbonate content, hydrochloric acid reaction, methyl orange indicator.
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