1. Basic Physical & Chemical Properties
EDTA (Ethylenediaminetetraacetic acid) is a hexadentate chelating agent that forms 1:1 stable complexes with most metal ions. The three common EDTA salts differ greatly in sodium content and pre-complexed calcium, leading to distinct solution pH, solubility and chelating performance.
| Item | EDTA Disodium | EDTA Tetrasodium | EDTA Calcium Disodium |
|---|---|---|---|
| Chemical Formula | Na2H2C10H12N2O8 | Na4C10H12N2O8 | CaNa2C10H12N2O8 |
| pH of 1% Aqueous Solution | 4.5–6.0 (Weakly acidic) | 10.0–11.0 (Strongly alkaline) | 7.0–8.0 (Near neutral) |
| Solubility (25℃) | Moderate (~100 g/L) | Excellent (~250 g/L) | Moderate (~100 g/L) |
| Molecular Weight | 372.24 (Dihydrate) | 380.17 | 374.27 |
2. Optimal pH Range & Chelating Performance
EDTA's chelating efficiency is highly pH-dependent. More fully deprotonated \(\boldsymbol{Y^{4-}}\) ions exist at higher pH, resulting in stronger chelating ability.
2.1 EDTA Disodium
Optimal pH range: 4.0–7.0 (Weak acid to neutral)
Main existing form in solution: \(\boldsymbol{H_2Y^{2-}}\). It gradually converts to \(\boldsymbol{HY^{3-}}\) and \(\boldsymbol{Y^{4-}}\) when pH exceeds 6.2.
Performance: Highest chelating efficiency for \(\boldsymbol{Ca^{2+}, Mg^{2+}, Fe^{2+}, Cu^{2+}}\) at pH 5–7. Efficiency drops sharply below pH 4.
Typical applications: Food preservation, pharmaceuticals, cosmetics, water treatment and laboratory titration.
2.2 EDTA Tetrasodium
Optimal pH range: 8.0–11.0 (Weak to strong alkali)
Main existing form in solution: Fully deprotonated \(\boldsymbol{Y^{4-}}\) after dissolution.
Performance: Outstanding chelating power for high-valent metals like \(\boldsymbol{Fe^{3+}, Al^{3+}}\) under alkaline conditions. Poor performance below pH 7 due to protonation.
Typical applications: Industrial detergents, textile & paper bleaching, oilfield water treatment and alkaline cleaning formulations.
2.3 EDTA Calcium Disodium
Optimal pH range: 6.0–9.0 (Neutral to weak alkali)
Characteristic: Pre-complexed with calcium ions. Calcium can be displaced by heavy metals such as lead and mercury.
Performance: Selectively binds toxic heavy metals without chelating calcium and magnesium, which avoids calcium deficiency risks.
Typical applications: First aid for lead/heavy metal poisoning and medical chelation therapy.
3. Comparison of Chelating Capacity
Weight-based chelating value (mg CaCO₃/g):
EDTA Disodium: ≥ 265 mg/g (Highest)
EDTA Tetrasodium: ≥ 220 mg/g
EDTA Calcium Disodium: ≈ 180 mg/g (Lowest, occupied chelating sites by pre-bound calcium)
Molar ratio: All three products combine with metal ions at a 1:1 molar ratio.
4. Selection Guidelines
Match with system pH
Weakly acidic & neutral systems (pH 4–7): Choose EDTA Disodium (universal grade)
Alkaline systems (pH 8–11): Choose EDTA Tetrasodium (high solubility & strong alkalinity resistance)
Medical & biological systems: Choose EDTA Calcium Disodium (safe for calcium balance)
Chelating capacity ranking (by equal weight)EDTA Disodium > EDTA Tetrasodium > EDTA Calcium Disodium
Solubility consideration
High-concentration & fast-dissolving formulas: Prefer EDTA Tetrasodium
Regular formulations: Prefer EDTA Disodium
5. Common Misconceptions
Myth: EDTA Tetrasodium has the strongest chelating ability.Fact: EDTA Disodium owns higher chelating value per unit weight. Tetrasodium excels only in alkaline environments and high solubility.
Myth: EDTA Calcium Disodium cannot chelate metals.Fact: It effectively displaces and binds heavy metals like lead and mercury, while leaving calcium and magnesium untouched.
6. Summary
EDTA Disodium: pH 4–7, highest chelating capacity, universal chelator for acid & neutral formulations.
EDTA Tetrasodium: pH 8–11, superior solubility, specially designed for alkaline industrial systems.
EDTA Calcium Disodium: pH 6–9, selective chelation for heavy metals, exclusive for medical use.