Overview
ISO 808:1973 is an international standard published by the International Organization for Standardization (ISO), detailing a precise spectrophotometric method for determining the silicon content in aluminium and aluminium alloys. The method uses the reduced silicomolybdic complex to quantify silicon levels ranging between 0.02% and 0.4% by mass. It provides comprehensive guidance on the principle, reagents, apparatus, sampling, analytical procedure, expression of results, and reporting requirements. Notably, the standard excludes applicability to aluminium alloys containing tin or bismuth.
Key Topics
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Scope and Application
The standard specifically targets the measurement of silicon in aluminium and aluminium alloys within the 0.02 - 0.4% m/m range. It excludes alloys with tin or bismuth, as these interfere with the method.
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Analytical Principle
Silicon is determined by attacking the test sample with sodium hydroxide followed by oxidation using hydrogen peroxide. Acidification is performed with nitric and hydrochloric acids, leading to the formation of a yellow silicomolybdic complex. This complex is then acidified further and reduced to a blue silicomolybdic complex, which is measured photometrically near 810 nm wavelength.
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Reagents and Preparation
The method specifies the preparation of key reagents including sodium hydroxide, molybdate solutions (sodium or ammonium molybdate), tartaric acid, sulphuric acid, as well as the reducing agents such as l-amino-2-naphthol-4-sulphonic acid or ascorbic acid. Standard silicon solutions are prepared to calibrate the method accurately.
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Apparatus
Essential apparatus includes platinum vessels for sample digestion, polyethylene containers for reagent preparation, and a spectrophotometer capable of measuring absorbance at approximately 810 nm.
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Sampling Guidelines
Samples must be taken as chips no thicker than 1 mm, prepared from milling or drilling. A test portion of 0.25 grams is accurately weighed for analysis.
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Analytical Procedure
The step-by-step procedure involves alkaline fusion, oxidation, acidification, formation of the silicomolybdic complex, reduction, and measurement of the blue complex’s absorbance. Calibration curves are constructed using silicon standard solutions to quantify sample silicon content accurately.
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Quality Checks
Blank tests are recommended to ensure reagent and water purity, and check tests verify that molybdenum (VI) is not over-reduced.
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Result Expression and Reporting
Silicon content is calculated as a percentage by mass using calibration data and reported with method references and any observations during testing.
Applications
The ISO 808:1973 method is critically important in industries where precise silicon content in aluminium and aluminium alloys affects material properties and performance, such as:
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Metallurgy and Alloy Production
Ensuring accurate silicon levels improves mechanical properties like strength, corrosion resistance, and castability.
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Quality Control Laboratories
Routine determination of silicon during production or before use in manufacturing can prevent defects and maintain product standards.
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Research and Development
The method supports material scientists in characterizing new aluminium alloys or refining existing compositions.
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Recycling and Material Authentication
Verifying silicon content contributes to sorting and ensuring recyclate quality.
Related Standards
While ISO 808:1973 focuses on silicon determination via a spectrophotometric method with the reduced silicomolybdic complex, practitioners may also refer to other ISO standards related to aluminium alloy analysis for comprehensive material evaluation, including:
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Standards for chemical analysis of aluminium and its alloys covering different elements or measurement techniques.
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Sampling standards for aluminium and aluminium alloys to ensure representativity.
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Other spectrophotometric or gravimetric methods for complementary elemental analysis.
Keywords
Aluminium alloys, silicon determination, spectrophotometric method, silicomolybdic complex, chemical analysis, ISO 808, reduced silicomolybdic complex, metallurgical testing, silicon content analysis, aluminium quality control, photometric measurement, chemical reagents for silicon, sample digestion, aluminium alloy testing methods.