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D4744-89 – Standard Test Method Technical Guide
This standard (Designation: D 4744 – 89, Reapproved 1995) covers the determination of organic halides (chlorine, bromine, and iodine) in water. It is an operational definition based on carbon adsorption and microcoulometric detection.
🧪 Scope and Key Limitations
The test method is applicable for organic halide concentrations ranging from 5 to 1000 µg/L. Higher concentrations may be analyzed by appropriate dilution. The measurement depends entirely on the adsorption of organics by Granular Activated Carbon (GAC).
⚠️ Inorganic Halide Interference: This method is only valid when the inorganic halide concentration does not exceed the organic halide concentration by more than 20,000 times. For samples with high background salinity, chloride ions must be removed or accounted for per Test Methods D512.
This standard does not address all safety concerns. It is the user’s responsibility to establish appropriate laboratory safety practices and validate the method for untested matrices.
⚙️ Test Procedure and Measured Fractions
The method relies on a specific three-step workflow: (1) Adsorption of organic halides from the water sample onto a packed GAC microcolumn; (2) Desorption of inorganic halides by thoroughly washing the GAC with a nitrate solution; (3) Combustion of the sorbed organics alongside the GAC, followed by microcoulometric detection of the resultant titratable species. The standard further defines several operationally distinct fractions:
| 🟦 Fraction | 📏 Acronym | 🎯 Definition |
|---|---|---|
| Total Organic Halides | TOX | Analysis of an unpurged, unfiltered sample. |
| Nonpurgeable Organic Halides | NPOX | Analysis of a purged sample. |
| Purgeable Organic Halides | POX | Mathematical difference (TOX − NPOX). |
| Dissolved Organic Halides | DOX | Analysis of the liquid portion after filtration or centrifugation. |
| Suspended Halides | SOX / SX | Analysis of the solid material resuspended in TOX-free water. |
💡 Operational Definition: The results for DOX and SX are highly dependent on the specific filtration or centrifugation process used. The standard defines these fractions strictly according to the procedural steps taken.
📊 Key Specifications and Performance
| ⚡ Parameter | 📐 Specification |
|---|---|
| Applicable Range | 5 to 1000 µg/L (as halide) |
| Target Analytes | Organic Chlorine, Bromine, Iodine |
| Max. Inorganic/Organic Ratio | 20,000 : 1 |
| Adsorption Medium | Granular Activated Carbon (GAC) |
| Inorganic Wash Solution | Nitrate solution |
| Detection Principle | Microcoulometric titration |
The method references ASTM Standards D512 (Chloride Ion), D1129 (Terminology), D1193 (Reagent Water), and D2777 (Precision and Bias). It was validated using several natural water matrices, though the user bears responsibility for validating untested sample types.
❓ Frequently Asked Questions
🔍 What is the purpose of the nitrate wash step?
The nitrate wash step is critical for desorbing inorganic halides from the GAC surface. This ensures that only the organic halides adsorbed on the carbon proceed to the combustion and detection stage, preventing significant positive bias from background salts.
⚡ What is the maximum concentration of inorganic halides allowed?
The method is valid only when the inorganic halide concentration (e.g., chloride as NaCl) does not exceed the organic halide concentration by more than 20,000 times. If the organic halide level is 10 µg/L, the chloride level must be below 200 mg/L for a valid analysis.
💡 How does D4744-89 define “Purgeable Organic Halides” (POX)?
POX is an operational definition. It is calculated as the difference between the Total Organic Halides (TOX) of an untreated sample and the Nonpurgeable Organic Halides (NPOX) of a purged sample. It represents the volatile fraction of the total organohalogens.
📌 What types of samples is D4744-89 typically used for?
This method is designed for water matrices where organic halides are a concern, such as wastewater effluent, drinking water, and surface water monitoring. The user must validate the method for any untested matrix, particularly those with high turbidity or dissolved organic carbon content.