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D168-94 – Standard Test Method Technical Guide
🔬 Apparatus and Sample Preparation
This test method determines the coke residue (fixed carbon) of creosote, coal tar, and creosote-coal tar solutions. Testing begins by subjecting the oil to a distillation method (per ASTM D 246) to obtain a weighed residue at 355°C. This distillation residue is thoroughly mixed to ensure homogeneity before a sample is taken for the coking test.
The critical apparatus is a specialized platinum crucible with a tightly fitting vented cover. The crucible holder, made from No. 20 Nichrome wire and a brass ring, provides stable support during the high-temperature operation.
| 🟦 Specification | 📐 Required Dimension / Value |
|---|---|
| Cover Depth | 10.0 – 10.2 mm |
| Center Hole Diameter (Cover) | 2.0 mm |
| Crucible Capacity (without cover) | 25 – 30 mL |
| Outside Diameter (Top) | 34 – 35 mm |
| Outside Diameter (Bottom) | 19 – 21 mm |
| Crucible Height | 40 – 45 mm |
| Total Weight (Crucible + Cover) | 25 – 35 g |
🔥 Coking Procedure and Critical Parameters
A one-gram sample of the thoroughly mixed distillation residue is transferred into the prepared platinum crucible. The crucible and vented cover are then placed into a furnace maintained strictly at 950 ± 20°C for exactly 7 minutes. After the coking operation, the crucible is cooled and weighed; the mass of the remaining material represents the coke residue of the sample.
⚠️ Critical Process Control: The furnace temperature tolerance (±20°C) and the coking time (exactly 7 minutes) are strict requirements for valid results. The unique vented cover design (2.0 mm hole) standardizes the escape of volatiles and controls the atmosphere during carbonization.
| ⚡ Parameter | 🎯 Specification / Tolerance |
|---|---|
| Furnace Temperature | 950 ± 20°C |
| Furnace Residence Time | 7 minutes (exact) |
| Sample Mass (from 355°C Residue) | 1 g |
| Distillation Residue Temperature | 355°C |
📊 Calculation and Practical Significance
The final coke residue of the original solution is calculated from three measured values: (a) the percentage of distillation residue retained at 355°C, (b) the weight of the sample of distillation residue transferred to the platinum crucible, and (c) the weight of coke remaining in the crucible.
The standard itself notes that the method “is therefore not a quantitative index of the percentage of coal tar” because different coal tars have varying inherent coke residue values. Despite this, it is critically important for specification compliance. By specifying maximum amounts of coke residue, as done in AWPA Standards P 2 and P 12, the method effectively limits the amount of coal tar in solutions. Specifying a minimum, as in Standard P 12, ensures the presence of some coal tar.
💡 Key Application in Standards: A low coke residue generally indicates a low concentration of coal tar pitch, often signifying a “cleaner” creosote (per AWPA P1). A specified minimum (per AWPA P12) guarantees the presence of coal tar for demanding marine applications, verifying that the preservative retains its intended material properties.
❓ Frequently Asked Questions
🔍 Why is the crucible cover so precisely specified with a 2.0 mm hole?
The vent hole standardizes the escape of volatile matter during carbonization while restricting the influx of oxygen. This creates a consistent, reproducible coking environment essential for an accurate fixed carbon determination.
⚡ What happens if the furnace temperature drops below 930°C or exceeds 970°C?
The coking reaction rate is highly temperature-dependent. Low temperatures may not fully carbonize the residue, resulting in a falsely high coke weight. High temperatures can cause excessive oxidation or loss of carbon, yielding a falsely low result. The strict tolerance of 950 ± 20°C is critical for reliable data.
📌 How does D168-94 interact with AWPA Standards P2 and P12?
AWPA Standards P2 and P12 define the material specifications for creosote-coal tar solutions. They mandate specific maximum and minimum coke residue values. ASTM D168-94 is the recognized test method cited to verify that these specification limits are met.
💡 Does a higher coke residue always mean more coal tar is present?
Generally yes, but cautiously. Different source coal tars have inherently different coke residue values. Therefore, D168 cautions that it is strictly a specification compliance tool rather than an exact quantitative indicator of the coal tar percentage.