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D2351-90 – Standard Test Method Technical Guide
📋 Overview and Scope
This test method, designated ASTM D2351‑90 (Reapproved 2015), provides a standardized procedure for the determination of sulfide sulfur in white pigment that has been separated from solvent‑reducible paints. The standard was originally adopted in 1965 under the jurisdiction of ASTM Committee D01 on Paint and Related Coatings, Materials, and Applications.
The values stated in SI units are regarded as the standard. This method is particularly significant for the analysis of paints containing sulfide‑based pigments such as lithopone, allowing formulators and users to monitor the concentration of this compound in whole paint systems. It is the responsibility of the user to establish appropriate safety and health practices prior to use.
🧪 Reagents and Materials
The accuracy of the test method relies heavily on the purity of the reagents used. All chemicals must conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where applicable. Reagent water must conform to Type II of ASTM Specification D1193. The following table summarizes the key reagents required for the analysis.
| 🟦 Reagent | ℹ️ Composition / Specification | 📏 Key Details |
|---|---|---|
| Ammoniacal Cadmium Chloride Solution | 8 g CdCl₂·2H₂O in 200 mL water + 200 mL NH₄OH (sp gr 0.90) | Alternative: 200 g ZnSO₄·7H₂O in 1080 mL H₂O + 920 mL NH₄OH (sp gr 0.90) |
| Hydrochloric Acid (HCl) | Concentrated, sp gr 1.19 | Generates H₂S gas from the pigment sample |
| Alkaline Lead Nitrate Solution | Saturated Pb(NO₃)₂ solution (250 g in 500 mL H₂O) added to KOH solution (56 g in 140 mL H₂O) until precipitate ceases to redissolve | Filtered through a glass filter and diluted with an equal volume of water |
| Mossy Zinc | Reagent grade | Placed in the flask with the pigment to facilitate H₂S generation |
⚠️ Safety Precautions: This standard involves the use of concentrated hydrochloric acid and alkaline lead nitrate solutions. Appropriate personal protective equipment (PPE) and chemical handling procedures must be followed. The standard emphasizes that it does not purport to address all safety concerns, and users must establish their own safety and health practices prior to use.
⚙️ Summary of Test Procedure
The method involves a chemical decomposition and quantification sequence. The extracted pigment is placed in a reaction flask with mossy zinc. Hydrochloric acid is added to generate hydrogen sulfide (H₂S) gas. This gas is bubbled through an absorption flask containing an alkaline lead nitrate solution, where it reacts to form a lead sulfide (PbS) precipitate.
The lead sulfide precipitate is then dissolved using nitric acid (HNO₃). Lead is subsequently determined as lead sulfate (PbSO₄) following the gravimetric procedures described in Test Methods D215. The standard also briefly mentions a “rapid method,” though the primary detailed quantification is gravimetric.
The following table outlines the procedural chain and the phase of each step.
| 🔬 Step | ⚡ Action | 🎯 Product / Observation |
|---|---|---|
| 1. Digestion | Pigment + Mossy Zinc + HCl | Generation of H₂S gas |
| 2. Absorption | H₂S gas bubbled through Alkaline Pb(NO₃)₂ solution | Formation of black PbS precipitate |
| 3. Dissolution | PbS dissolved in HNO₃ | Lead ions in solution |
| 4. Quantification | Lead determined as PbSO₄ per Test Methods D215 | Gravimetric determination of sulfide content |
💡 Technical Note on Reagent Grades: While ACS reagent grade chemicals are recommended, Section 5.1 permits the use of other grades provided the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination. Always validate the suitability of a substitute reagent against the specific test requirements.
❓ Frequently Asked Questions
🔍 What is the primary scope of ASTM D2351-90?
This standard specifically covers the determination of sulfide sulfur in white pigment separated from solvent-reducible paints.
💡 What is the principle behind the analytical method?
The extracted pigment is placed in a flask with mossy zinc. The hydrogen sulfide generated by addition of HCl reacts with lead nitrate in an absorption flask forming lead sulfide. This lead sulfide is dissolved with nitric acid (HNO₃) and the lead determined as lead sulfate in accordance with Test Methods D215.
⚡ Why is measuring sulfide content in paints important?
Sulfide containing pigments such as lithopone have been used in paints in varying degrees. This test method is useful to formulators and users to monitor the amount of this compound in whole paints, affecting product quality and consistency.
📌 What are the specific purity requirements for water and reagents?
Unless otherwise indicated, water must be reagent grade water conforming to Type II of ASTM Specification D1193. Reagents must be of reagent grade chemicals conforming to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available.