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D2194-22 – Standard Test Method Technical Guide
📐 Scope and Summary of D2194-22
This test method, issued under the fixed designation D2194 – 22, specifically covers the determination of the formaldehyde content of commercially available solutions. It is validated for a concentration range of 36 to 55 weight percent. The standard requires that results be rounded in accordance with Practice E29, and all values are stated in SI units. The procedure is a classical wet chemical analysis: the specimen is reacted with an excess of sodium sulfite solution, and the resulting sodium hydroxide is titrated with standardized sulfuric acid using thymolphthalein as the indicator.
⚙️ Apparatus, Reagents, and Procedure
Specific apparatus is mandated in Section 5 of the standard. This includes a calibrated 100-mL buret with 0.1-mL divisions (ideally featuring a TFE-fluorocarbon resin stopcock), a 500-mL Erlenmeyer flask, and short-style specimen vials of 1 to 1½-dram (4 to 6 mL) capacity. All reagents must be reagent grade conforming to the specifications of the American Chemical Society, and the water used for reagent preparation must conform to Type IV of Specification D1193.
💡 Best Practice: The sample must be essentially neutral for accurate results. A correction must be applied if acidity is present; 0.1 % acidity (as formic acid) is equivalent to 0.065 % formaldehyde.
The analysis begins by accurately weighing the sample directly into the short-style vial. The vial is then introduced into the flask containing 50 mL of sodium sulfite solution and the indicator. The liberated sodium hydroxide is titrated with 0.5 N sulfuric acid until the blue color of the thymolphthalein permanently disappears. A reagent blank determination must be performed and subtracted from the sample titer.
📊 Key Measured Properties and Calculation
The formaldehyde concentration is calculated using the net volume of sulfuric acid consumed. The standard calculation is as follows:
Formaldehyde, weight % = (V × N × 3.003) / W
Where V is the gross titer minus the blank titer (mL), N is the normality of the sulfuric acid, W is the specimen weight (g), and 3.003 is the milliequivalent weight of formaldehyde × 100.
| 📐 Equipment / Reagent | 🔬 Specification | 🎯 Key Requirement |
|---|---|---|
| Buret | 100 mL calibrated | 0.1 mL divisions, TFE stopcock |
| Erlenmeyer Flask | 500 mL capacity | Standard borosilicate glass |
| Specimen Vial | Short style | 4 to 6 mL (1–1½ dram) capacity |
| Titrant | Sulfuric Acid (H₂SO₄) | 0.5 N standardized solution |
| Indicator | Thymolphthalein | Blue in alkaline solution, colorless at endpoint |
⚠️ Safety Note: The user must establish appropriate safety, health, and environmental practices. Consult the supplier’s Material Safety Data Sheet for hazard guidance. Formaldehyde is toxic and a suspected carcinogen; all work should be performed in a fume hood.
📊 Significance and Use for Specification Acceptance
This test method provides the definitive measurement of formaldehyde content (assay) for specification acceptance. It is under the jurisdiction of ASTM Subcommittee D01.35 on Solvents, Plasticizers, and Chemical Intermediates. Strict adherence to the specified apparatus—particularly the buret calibration and reagent purity—is critical for achieving the repeatability and reproducibility required for commercial transactions. The classical sodium sulfite reaction provides a highly specific and accurate determination within the defined concentration range.
❓ Frequently Asked Questions
🔍 What concentration range does D2194-22 cover?
The standard is validated for determining the formaldehyde content of commercially available solutions ranging from 36 to 55 weight percent.
💡 Why is the sodium sulfite method used in this standard?
The sodium sulfite method is a classic, highly specific titrimetric technique for aldehydes. Formaldehyde reacts quantitatively with sodium sulfite, releasing sodium hydroxide in stoichiometric amounts, which can be precisely titrated with acid.
⚠️ How is the acidity of the sample corrected in the calculation?
The sample should ideally be neutral. However, if acidity is present, a correction factor applies: 0.1 % acidity (as formic acid) is equivalent to 0.065 % formaldehyde, which must be subtracted from the calculated result.
📌 What type of water is specified for reagent preparation?
Unless otherwise indicated, references to water in the standard mean reagent water conforming to Type IV of ASTM Specification D1193.