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D3913-03 – Standard Test Method Technical Guide
🔬 Overview and Scope of ASTM D3913-03
ASTM D3913 — Standard Test Method for Acidity in Basic Chromium Tanning Liquors provides a standardized procedure for determining the titratable acidity of chrome tanning liquors. The term “titratable acidity” specifically includes not only free acidity, which is rarely present, but also the acidity of anions combined with weakly basic cations such as Chromium (III). These combined anions are measurable by titration with a base at the phenolphthalein endpoint. The values stated in SI units are regarded as the standard. Users of this standard are responsible for establishing appropriate safety, health, and environmental practices.
⚙️ Test Procedure and Chemical Principles
The test method requires a representative sample of suitable size, which is diluted and aliquoted as necessary. The sample is then heated to boiling and titrated hot with a standard base. The visual endpoint is reached upon the first appearance of a pink color that remains stable to boiling in the supernatant solution. The standard specifies two viewing techniques: observing the settled solution against the rim of a porcelain dish, or viewing it against the light in a tall beaker or Erlenmeyer flask.
The reversible titration reaction is governed by the complex equilibria of the chromium coordination sphere, involving hydroxo groups (OH) and other weak or strong complexing anions (X):
(CrOHxXy(H2O)6-x-y)(x+y-3)+ + (3 – x) OH– ⇌ Cr(OH)3 + y X + (6 – x – y) H2O
| 🟦 Parameter | 📏 Specification |
|---|---|
| 📐 Designation | D3913 − 03 (Reapproved 2020) ε1 |
| 🔬 Analyte | Titratable Acidity in Basic Chromium Tanning Liquors |
| 🎯 Endpoint Indication | First pink color stable to boiling in the supernatant solution |
| ⚡ Viewing Technique | Against the rim of a white porcelain dish / Against the light in a tall beaker or flask |
| 📉 Titration Condition | Hot (sample heated to boiling) |