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D1068-15 – Standard Test Method Technical Guide
🔬 Scope and Summary of Test Methods
ASTM D1068-15 provides standardized procedures for determining the concentration of iron in water. The standard covers methods for total recoverable iron, dissolved iron, and ferrous iron. By performing the total and dissolved iron determinations, the undissolved iron fraction can be calculated. The methods are designed to accommodate various concentration ranges and matrix complexities, ensuring the user can select the appropriate technique based on the specific analytical requirements.
| 🟦 Method | 📏 Technique | 📐 Range | 🎯 Reference Sections |
|---|---|---|---|
| A | Atomic Absorption, Direct | 0.1 to 5.0 mg/L | 7 to 16 |
| B | Atomic Absorption, Graphite Furnace | 5 to 100 µg/L | 17 to 26 |
| C | Photometric Bathophenanthroline | 40 to 1000 µg/L | 27 to 38 |
It is the user’s responsibility to ensure the validity of these test methods for waters of untested matrices. The values stated in SI units are to be regarded as standard.
⚙️ Significance, Procedure, and Quality Control
Iron is the second most abundant metallic element in the earth’s crust and is essential in the metabolism of plants and animals. However, excessive amounts form oxyhydroxide precipitates that stain laundry and porcelain. The recommended limit for iron in domestic water supplies is 0.3 mg/L, making these test methods critical for water quality management. The procedures involve acid digestion for total recoverable iron and direct analysis or colorimetric reduction for dissolved and ferrous iron species.
⚠️ Quality Control: Refer to Practice D5847 for writing quality control specifications and Practice D2777 for determination of precision and bias. Sample holding times must be established following Practice D4841 to ensure sample integrity.
💡 Key Terminology: Total recoverable iron is an analytical term describing the iron forms recovered following the specific acid-digestion procedure in these test methods. This differentiates the measurement from dissolved or ferrous iron fractions.
Proper sampling is critical. Standard practices for sampling water from closed conduits (D3370) and steam (D1066) provide specific guidance. Safety considerations include specific hazard statements given in Note 4, 11.7.1, and X1.1.2 of the standard.
📊 Analytical Scope and Related Standards
This standard is part of a comprehensive suite of ASTM methods for trace metals in water, including chromium (D1687), copper (D1688), lead (D3559), and zinc (D1691). It also references Guide D5810 for spiking into aqueous samples. Users should be aware that the chelation-extraction and two former photometric test methods have been discontinued. See Appendix X2 for historical information.
| 🟦 Analyte | 📐 ASTM Standard | ⚡ Common Technique |
|---|---|---|
| Chromium | D1687 | AA / Photometric |
| Copper | D1688 | AA / Photometric |
| Lead | D3559 | AA / Photometric |
| Zinc | D1691 | AA / Photometric |
| Trace Elements | D5673 | ICP-MS |
❓ Frequently Asked Questions
🔍 What is the difference between total iron and dissolved iron?
Total recoverable iron is determined by analyzing an unfiltered, acid-digested sample. Dissolved iron is determined on a sample that has been filtered prior to acidification and analysis. Undissolved iron is then calculated as the difference between the total and dissolved concentrations.
💡 What is the recommended limit for iron in drinking water?
According to the significance section of the standard, the recommended limit for iron in domestic water supplies is 0.3 mg/L. This limit primarily addresses aesthetic issues such as the staining of laundry and porcelain caused by oxyhydroxide precipitates.
⚡ Which ASTM method should I use for ultra-trace iron analysis?
For very low concentrations (5 to 100 µg/L), Test Method B (Graphite Furnace Atomic Absorption) is the most appropriate. For concentrations between 40 and 1000 µg/L, Test Method C (Photometric Bathophenanthroline) offers a highly sensitive colorimetric option.
📌 Have any methods in D1068 been discontinued?
Yes. The chelation-extraction and two former photometric test methods were discontinued. Historical information regarding these methods is provided in Appendix X2 of the standard.