Overview
ISO 3566:1976 specifies a turbidimetric method for determining chloride content in sodium fluoride primarily used for aluminium production. The procedure is intended for samples with chloride levels equal to or above 0.005 % (m/m). The method relies on precipitation of chloride ions with silver nitrate in the presence of boric acid and evaluation of the resulting turbidity by visual comparison or photometric measurement.
Key Topics and Requirements
- Scope: Applicable to industrial sodium fluoride intended for aluminium manufacture when chloride ≥ 0.005% (m/m).
- Analytical principle: Chloride (Cl−) is precipitated as silver chloride (AgCl) using silver nitrate; boric acid is used to control the reaction medium; turbidity of the precipitate is compared with prepared standards.
- Reagents and materials:
- Boric acid solution (40 g/L).
- Silver nitrate solution, approximately 0.1 N.
- Nitric acid (approx. 68% w/w) for acidification.
- Chloride standard solution corresponding to 0.050 g Cl per litre (1 ml = 0.050 mg Cl).
- Distilled or equivalent purity water and analytical-grade reagents.
- Apparatus: Common laboratory glassware and Nessler tubes (50 ml capacity) for visual turbidimetric comparison; optional photometer for measuring opalescence.
- Procedure highlights:
- Dry and weigh a representative sample (example: 1 g).
- Dissolve in water with boric acid, acidify, and prepare test and a set of standard matching solutions.
- Add silver nitrate rapidly, allow to stand 5 minutes in the dark, then compare opalescence against standards on a black background under diffuse lateral lighting.
- Result expression: Chloride content is reported as percentage by mass (Cl), calculated by comparison with the standards per the formula in the standard.
Applications and Users
ISO 3566:1976 is practical for:
- Sodium fluoride manufacturers and suppliers ensuring product compliance for aluminium electrolysis feedstock.
- Quality control (QC) and analytical laboratories performing routine impurity testing.
- Procurement and materials engineers specifying raw-material acceptance criteria.
- Regulatory and standards bodies verifying conformity to industrial chemical specifications.
Benefits include a standardized, reproducible test for chloride contamination that can impact aluminium production processes and product quality.
Related Standards
Relevant ISO documents referenced in the original standard include:
- ISO 3428 - Sampling and sample preparation for sodium fluoride.
- ISO 2831–2833 - Tests for industrial sodium fluoride (insolubles, moisture, fluorine).
- ISO 3429, 3430, 3431, 4278 - Methods for iron, silica, sulfates, carbonates in fluoride materials.
Keywords: ISO 3566:1976, sodium fluoride, chlorides determination, turbidimetric method, silver nitrate precipitation, Nessler tubes, aluminium production, quality control.
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