A UV-VIS spectrophotometer determines hexavalent chromium levels by performing colorimetric analysis on a specific chemical reaction. The instrument measures the intensity of the color formed when hexavalent chromium reacts with a reagent known as 1,5-diphenylcarbazide (DPC). By targeting a wavelength of 540 nm, the device converts optical absorbance signals into precise concentration data.
The core of this process is transforming a chemical reaction into a quantifiable number. By measuring the absorbance of the chromium-DPC complex, manufacturers can verify that footwear leather remains below the international safety threshold of 3 mg/kg.
The Mechanism of Detection
The Role of Colorimetric Analysis
The spectrophotometer does not "see" the chromium directly in its raw state. Instead, it relies on a chemical transformation.
When hexavalent chromium is extracted from leather and mixed with 1,5-diphenylcarbazide (DPC), a reaction occurs. This reaction produces a distinct colored complex.
The intensity of this color is directly proportional to the concentration of chromium. A darker or more intense color indicates a higher level of contamination.
Precision Measurement at 540 nm
To quantify this color, the UV-VIS spectrophotometer is set to a specific optical setting.
The instrument beams light through the sample at a wavelength of 540 nm. This wavelength is chosen because it is where the chromium-DPC complex exhibits maximum light absorbance.
The machine measures exactly how much light is absorbed by the liquid and how much passes through.
Converting Light to Data
The raw absorbance value is mathematically converted into a concentration figure.
This calculation allows the technician to determine the exact amount of hexavalent chromium per unit of leather.
This step is critical for determining if the material meets safety standards or exceeds the 3 mg/kg limit.
Understanding the Technical Constraints
Dependency on Chemical Reaction
The accuracy of the reading is entirely dependent on the DPC reaction.
If the chemical reaction is inhibited by other substances in the leather extract, the spectrophotometer may yield a false negative. The machine measures the color complex, not the element itself.
The "Non-Detect" Threshold
It is important to understand the concept of the detection limit.
The standard reference point for safety is 3 mg/kg.
Readings below this threshold are often treated as "non-detectable" for regulatory purposes, meaning the leather is considered safe, even if trace amounts exist below the instrument's sensitivity or the regulatory cap.
Ensuring Compliance in Leather Production
To effectively manage safety risks in footwear manufacturing, you must align your testing with specific goals.
- If your primary focus is safety certification: Verify that your analysis confirms levels definitively stay below the 3 mg/kg detection limit.
- If your primary focus is process monitoring: Track absorbance trends at 540 nm to catch rising chromium levels before they breach compliance thresholds.
Reliable spectrophotometric analysis is the industry standard for ensuring leather goods are safe for consumer use.
Summary Table:
| Aspect | Detail |
|---|---|
| Detection Method | Colorimetric Analysis |
| Key Reagent | 1,5-Diphenylcarbazide (DPC) |
| Target Wavelength | 540 nm |
| Safety Threshold | 3 mg/kg |
| Primary Goal | Ensure Leather Safety & Compliance |
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References
- Kallen Mulilo Nalyanya, Paul Kamau. Mitigating the Formation of Hexavalent Chromium in Leather by Using Aloe Barbadensis Miller Mixed with Carrageenan. DOI: 10.31881/tlr.2021.20
This article is also based on technical information from 3515 Knowledge Base .
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