Steel and iron - Determination of molybdenum content - Thiocyanate spectrophotometric method
1Key Takeaways
This document specifies a thiocyanate spectrophotometric method for the determination of molybdenum contents in steel and iron. The method is applicable to molybdenum mass fractions between 0,005 % and 0,125 %. Vanadium and tungsten interfere with the measurement if, because of their contents, the V/Mo ratio is greater…
2Expert Interpretation
An in-depth interpretation of the international standard ISO 4941:2024 "Thiocyanate spectrophotometric determination of molybdenum in steel", covering the method principle, reagent configuration, instrument requirements, calibration curve establishment and accuracy data. It is applicable to the detection of molybdenum content of 0.005%-0.125%, including interfering element treatment solutions and technology evolution analysis.
Analysis of the core content of the standard
| Technical parameters | 1994 edition | 2024 edition | Improvement points |
|---|---|---|---|
| Scope of application | 0.005-0.125% | 0.005-0.125% | Maintain the original detection range |
| Limit of interfering elements | V/Mo≤16 W/Mo≤8 | V/Mo≤16 W/Mo≤8 | Clearly define the measurement time limit |
| Calibration solution configuration | Unspecified acid ratio | Hydrochloric acid (1+1) quantitative control | Improve reproducibility |
Method principle and technical points
This method is based on the determination of the absorbance of molybdenum-thiocyanate complex at a wavelength of 470nm using a spectrophotometer. The key steps include:
- Acid hydrolysis system: Use hydrochloric acid-nitric acid mixture (3:1) to dissolve the sample, and perchloric acid/phosphoric acid mixture to decompose carbides
- Color reaction: Under the catalysis of copper (II) ions, molybdenum and thiocyanate form an orange complex
- Extraction and separation: Use a 100ml spherical separatory funnel to extract the complex with butyl acetic acid
Key reagents and instrument specifications
Special reagent requirements
- Pure iron standard: Molybdenum content <0.0005% and does not contain tungsten and vanadium
- Tin (II) copper (II) chloride solution: Need to be prepared and used immediately (SnCl₂·2H₂O 80g + HCl 155ml)
- Ammonium thiocyanate solution: 320g/L concentration, store in dark
Instrument precision control
When using 10mm/20mm cuvette, it must be ensured that the 20mm optical path is exactly twice that of 10mm. The separatory funnel must comply with ISO 4800 Type 3 standard.
Precision Data and Implementation Suggestions
| Molybdenum content (%) | Repeatability limit r | Reproducibility limit R | Allowance error |
|---|---|---|---|
| 0.005 | 0.0006 | 0.0019 | ±12% |
| 0.05 | 0.0021 | 0.0047 | ±9.4% |
| 0.125 | 0.0033 | 0.0067 | ±5.4% |
Operation Precautions
- Interference Control: When V/Mo＞16 or W/Mo＞8, the determination must be completed within 30 minutes after extraction
- Safety Warning: Perchloric acid vapor may explode when in contact with organic matter, so the operation should be carried out in a fume hood
- Quality Control: It is recommended to carry CRM 096-1 standard material (molybdenum 0.003%) with each batch of tests