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D1696-95 – Standard Test Method Technical Guide
🔬 Scope and Significance
This test method (D1696-95, Reapproved 2019) is intended for application to dissolving-type cellulose pulps prepared from cotton or wood. It is not directly applicable to unrefined pulps for use in chemical conversion processes because solubility equilibrium may not be attained within the specified extraction time. The values stated in SI units are to be regarded as standard.
The measurement of soluble oxidizable components of cellulose in sodium hydroxide is indicative of the purity of the cellulose sample, since pure cellulose is insoluble in sodium hydroxide. The extracted components are typically hemicelluloses, which are naturally present in the wood.
⚙️ Extraction Procedure and Methodology
Pulp is steeped in a sodium hydroxide solution of a specified concentration for 1 hour at 20 °C. The soluble fraction is estimated by dichromate oxidation of the filtered steeping alkali. The concentration of sodium hydroxide used in the pulp extraction process must be reported as part of the analytical result. Sodium hydroxide concentrations of 10%, 18%, and 21.5% are most frequently used. Data are reported as percent of dry sample weight.
| 🟦 Parameter | 📏 Specification |
|---|---|
| Steeping Time & Temperature | 1 hour at 20 °C |
| NaOH Concentrations | 10 %, 18 %, or 21.5 % by mass |
| Oxidation Reagent | Potassium Dichromate (K₂Cr₂O₇) |
| Reporting Unit | Percent of dry sample weight |
⚡ Critical Distinction: This procedure avoids dilution sequences, and therefore, the results are not comparable to data obtained by the alpha, beta, gamma methods of pulp fractionation (see TAPPI Method T 429). The terms “alpha-“, “beta-“, or “gamma-” cellulose must not be applied to any test values obtained by this procedure since they are defined only by the method of their determination.
💡 Alternative Methods: The standard describes two alternative methods for estimating the dichromate consumed during the oxidation of the soluble material: titration with ferrous ammonium sulfate and titration with sodium thiosulfate.
📊 Key Measured Properties and Reporting
The essential feature of the method is to prepare sodium hydroxide extracts and oxidize the soluble material with dichromate as described. The soluble fraction of the cellulose pulp is reported exclusively as a percentage of the original dry sample weight. The specific sodium hydroxide concentration used (e.g., 10%, 18%, or 21.5%) must be clearly noted as part of the analytical result, as the solubility values are concentration-dependent.
❓ Frequently Asked Questions
🔍 What is the principle of this test method?
The method measures the fraction of a cellulose pulp sample that is soluble in sodium hydroxide. Since pure cellulose is insoluble in NaOH, the soluble fraction primarily consists of hemicelluloses, providing a measure of pulp purity.
💡 What concentrations of sodium hydroxide are specified?
The standard most frequently uses NaOH concentrations of 10%, 18%, and 21.5% by weight. The specific concentration used must be reported with the results.
⚡ Can results be compared to alpha, beta, gamma cellulose fractions?
No. This extraction procedure avoids the dilution sequences characteristic of fractionation methods. The results are not comparable to the TAPPI T 429 alpha-, beta-, gamma-cellulose fractions, and those terms must not be applied to this method.
📌 How long must the pulp sample be steeped?
The pulp is steeped in the alkali solution for exactly 1 hour at a controlled temperature of 20°C to ensure consistent extraction equilibrium.