Overview
ISO 15355:1999 specifies a potentiometric indirect titration method for the determination of chromium content in steel and iron. The method covers chromium concentrations from 1 % to 35 % (m/m) and defines limits on vanadium content to avoid interference. It is a laboratory standard describing sample preparation, reagents, apparatus, titration procedure and result calculation for accurate chromium analysis.
Key topics and technical requirements
- Scope and applicability: Chromium determination in steel and iron (1–35 % m/m). Vanadium restrictions apply: lower maximum V for low-Cr steels to limit interference.
- Principle: Sodium-peroxide fusion of a test portion, acidification, oxidation of Cr(III) to dichromate (ammonium peroxydisulfate with silver catalyst), reduction of dichromate by a known excess of iron(II) salt, and potentiometric back-titration of the excess with potassium dichromate.
- Sample & prep: Typical test portion 0.25 g; fusion at 650 °C in an argon atmosphere using zirconium crucibles; careful dissolution and handling to avoid loss or contamination.
- Reagents & standards: Includes sulfuric acid, silver nitrate, ammonium peroxydisulfate, 0.1 N potassium dichromate (standardized), and ferroammonium disulfate (iron(II) salt) with a required reduction-grade check.
- Apparatus: Class A volumetric glassware; muffle furnace with controlled temperature/argon; clean zirconium crucibles/lids; potentiometric titration assembly (Pt/Ag-AgCl redox electrode, high-impedance voltmeter or automatic titrator).
- Quality & precision: The standard includes interlaboratory precision data. Repeatability limits increase with chromium level; laboratories should follow ISO 5725 procedures for accuracy assessment.
- Vanadium interference: If V > 0.03 % (but ≤ 1 %), a mathematical correction is required; vanadium must be measured by validated methods before correction.
Applications and users
- Who uses it: Metallurgical and materials testing laboratories, steel manufacturers, quality control and inspection bodies, certification labs, and R&D departments involved in alloy composition analysis.
- Practical uses: Routine QC of stainless and alloy steels, verification of composition for heat-treatment or corrosion-resistance specifications, incoming material inspection, and research where accurate chromium quantification is required.
- Advantages: Well-defined procedure for high-Cr alloys, suited to samples with Cr well above trace levels where spectrometric methods may be less convenient.
Related standards
Keywords: ISO 15355, chromium determination, potentiometric titration, steel analysis, sodium peroxide fusion, potassium dichromate, vanadium interference, metallurgical laboratory.