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D2350-90 – Standard Test Method Technical Guide
🧪 Scope and Summary of the Test Method
This test method, designated as ASTM D2350-90 (Reapproved 2015), covers the determination of total antimony oxide in white pigment separated from solvent-reducible paints. The values stated in SI units are to be regarded as the standard.
The method utilizes a differential titration technique to distinguish between the trivalent (Sb₂O₃) and pentavalent (Sb₂O₅) states. In the first procedure (Section 3.1 of the standard), the pigment is dissolved in hydrochloric acid (HCl), sulfuric acid (H₂SO₄) is added, and the mixture is titrated with potassium permanganate (KMnO₄) to quantify antimony in the “ous” condition. In the second procedure (Section 3.2), the pigment is dissolved in H₂SO₄ with potassium sulfate (K₂SO₄), reduced with sodium sulfite (Na₂SO₃) to expel sulfur dioxide gas (SO₂), diluted, and then titrated with KMnO₄ for total antimony. The residual antimony pentoxide (Sb₂O₅) is calculated by subtracting the Sb₂O₃ value from the total (Section 3.3). The standard also describes a variant for samples containing large amounts of iron (Section 3.4).
⚗️ Key Reagents and Standardization
All chemicals must be reagent grade, conforming to the specifications of the Committee on Analytical Reagents of the American Chemical Society. The water used must be Type II reagent grade water conforming to Specification D1193.
| 🧪 Reagent | ⚙️ Specification | 📐 Key Use |
|---|---|---|
| Hydrochloric Acid (HCl) | Specific Gravity 1.19 | Dissolution of pigment for Sb(III) titration |
| Sulfuric Acid (H₂SO₄) | Specific Gravity 1.84 | Digestion medium & KMnO₄ standardization |
| Potassium Permanganate (KMnO₄) | Standard Solution (0.1 N) | Primary titrant for antimony |
| Potassium Sulfate (K₂SO₄) | Reagent Grade | Raises boiling point in total Sb digestion |
| Sodium Sulfite (Na₂SO₃) | Reagent Grade | Reducing agent for total Sb procedure |
💡 Standardization Tip: The 0.1 N KMnO₄ solution is prepared by dissolving 3.2 g in 1 L of water and must be allowed to stand for 8 to 14 days before use. It is standardized against 0.25 g of pure metallic antimony dissolved in 12 to 15 mL of H₂SO₄ (sp gr 1.84) in a 500-mL resistant-glass Erlenmeyer flask.
📊 Key Measured Properties and Significance
Antimony trioxide is commonly used in fire-retardant paints, making this test method a crucial tool for formulators and quality assurance laboratories. The differential calculation provides insight into the oxidation state of the antimony in the pigment.
| 🟦 Property | 🎯 Calculation | 📌 Significance |
|---|---|---|
| Antimony Trioxide (Sb₂O₃) | Direct titration of HCl/H₂SO₄ dissolved pigment with KMnO₄ | Quantifies antimony in the trivalent (“ous”) condition |
| Total Antimony (as Sb₂O₃) | Titration of the fully reduced sample with KMnO₄ | Determines the overall antimony content in the sample |
| Antimony Pentoxide (Sb₂O₅) | Total Sb₂O₃ (calculated) — Sb₂O₃ (direct titration) | Represents the antimony in the pentavalent (“ic”) state |
⚠️ Safety Responsibility: This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Proper PPE is required when handling concentrated acids and potassium permanganate.
❓ Frequently Asked Questions
🔍 What is the scope of ASTM D2350-90?
It covers the determination of total antimony oxide in white pigment separated from solvent-reducible paints.
💡 How does the method differentiate between Sb₂O₃ and Sb₂O₅?
The method uses two titration procedures. The first measures antimony in the trivalent state. The second measures total antimony after a reduction step. Sb₂O₅ is calculated as the difference between the total antimony and the trivalent antimony (Section 3.3).
⚡ What are the critical specifications for the KMnO₄ solution?
The 0.1 N KMnO₄ standard solution must be prepared by dissolving 3.2 g in 1 L of water, allowed to stand for 8 to 14 days, and standardized against 0.25 g of pure metallic antimony.
📌 Why is antimony oxide content monitored in paints?
Antimony trioxide (Sb₂O₃) is a primary component used as a flame retardant synergist in fire-retardant paint formulations, making this measurement important for product compliance and performance.