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D5257-17 – Standard Test Method Technical Guide
🧪 Scope and Application
ASTM D5257-17 specifies a standard test method for the determination of dissolved hexavalent chromium (Cr(VI)) in wastewater, surface water, and drinking water using ion chromatography. This method is validated for concentrations ranging from 1 to 1000 µg/L. Higher concentrations can be determined following appropriate sample dilution.
| 🟦 Parameter | 📏 Specification |
|---|---|
| Matrix | Wastewater, Surface Water, Drinking Water |
| Analytes | Dissolved Hexavalent Chromium (Cr(VI)) |
| Working Range | 1 – 1000 µg/L |
| Analytical Technique | Ion Chromatography |
⚙️ Sample Handling, Preservation, and Interferences
Proper sample preservation is critical to prevent the reduction of Cr(VI) to Cr(III). Samples containing high levels of reducing species (e.g., sulfides, sulfites) are particularly susceptible. The standard mandates that samples must be buffered to a pH of 9 to 9.5, filtered, and stored at 6°C or below to maintain integrity.
⚠️ Critical Alert: Interferences
High concentrations of anionic species such as chloride and sulfate can cause ion-exchange column overload. Additionally, the presence of reducing agents can lead to a negative bias through the reduction of Cr(VI). Strict adherence to the preservation protocol is essential for accurate results.
High concentrations of anionic species such as chloride and sulfate can cause ion-exchange column overload. Additionally, the presence of reducing agents can lead to a negative bias through the reduction of Cr(VI). Strict adherence to the preservation protocol is essential for accurate results.
A maximum holding time of 28 days is permissible if the user can demonstrate that sample integrity is not compromised during this period, in accordance with U.S. EPA 40 CFR 136, Part II.
🔬 Key Terminology and Quality Control
The standard defines critical quality control terms and procedures to ensure data reliability. The Continuing Calibration Blank (CCB) verifies freedom from carryover, while the Continuing Calibration Verification (CCV) standard monitors instrumental drift. The eluent acts as the ionic mobile phase.
| 📐 Term | ⚡ Definition |
|---|---|
| Continuing Calibration Blank (CCB) | Analyte-free solution used to verify blank response and freedom from carryover. |
| Continuing Calibration Verification (CCV) | Solution of known concentration used to verify freedom from excessive instrumental drift. |
| Eluent | The ionic mobile phase used to transport the sample through the ion-exchange column. |
💡 Related Standards and Methods: This test method is closely harmonized with U.S. EPA Method 218.6. For guidance on precision and bias, consult ASTM D2777. Quality control specifications should follow the guidelines in ASTM D5847.
❓ Frequently Asked Questions
🔍 What types of water can be tested with this method?
This test method is suitable for the determination of dissolved hexavalent chromium in wastewater, surface water, and drinking water.
💡 What is the method’s working range for Cr(VI)?
The method is validated for concentrations ranging from approximately 1 µg/L to 1000 µg/L. Higher levels can be measured following an appropriate dilution.
⚡ How should samples be preserved to prevent Cr(VI) reduction?
Samples must be buffered to a pH of 9 to 9.5, filtered, and stored at a temperature of less than 6°C. A maximum holding time of 28 days may be used if integrity is demonstrated per 40 CFR 136.
📌 What are the main interferences in this analysis?
High levels of anionic species like chloride and sulfate can cause column overload. Reducing species such as sulfides and sulfites can reduce Cr(VI) to Cr(III), causing low bias.