Definition
Sodium citrate refers to any sodium salt derived from citric acid (C₆H₈O₇). The most common forms are trisodium citrate (Na₃C₆H₅O₇) and disodium citrate (Na₂HC₆H₅O₇). These compounds are widely employed as buffering agents, emulsifiers, anticoagulants, and food additives.
Overview
Sodium citrate is a white, crystalline solid that is highly soluble in water, forming an alkaline solution (pH ≈ 8.3 for a 0.1 M solution of trisodium citrate). It is odorless and non‑volatile. In the food industry it is designated as E 331 and is used to regulate acidity, improve stability, and act as a preservative. In medical and laboratory contexts, sodium citrate chelates calcium ions, thereby preventing blood coagulation; it is a standard component of plasma‑preserving anticoagulant tubes. Pharmaceutical formulations employ sodium citrate as an alkalinizing agent to neutralize gastric acidity and to adjust the pH of oral liquids.
Etymology / Origin
The term “citrate” originates from “citric acid,” which was first isolated from lemon juice in 1784; the name derives from the Latin citrus (the citrus fruit). “Sodium” comes from the Latin natrium, referring to the chemical element Na (atomic number 11). Sodium citrate is produced industrially by neutralizing citric acid with sodium hydroxide, sodium carbonate, or sodium bicarbonate, yielding the corresponding sodium salt(s).
Characteristics
- Chemical formula: Na₃C₆H₅O₇ (trisodium citrate) or Na₂HC₆H₅O₇ (disodium citrate).
- Molar mass: 258.07 g·mol⁻¹ (trisodium citrate).
- Solubility: ~70 g L⁻¹ in water at 20 °C; solubility increases with temperature.
- pH buffering: Effective in the pH range 3.0–6.2 for citric acid/citrate systems; maintains stable pH in alkaline range when used as a sodium salt.
- Chelating ability: Binds divalent cations, especially Ca²⁺ and Mg²⁺, which underlies its anticoagulant and metal‑sequestering properties.
- Stability: Chemically stable under normal storage conditions; hygroscopic, requiring sealed containers to prevent moisture uptake.
- Toxicity: Low acute toxicity; acceptable daily intake (ADI) set by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) at 0–10 mg kg⁻¹ body weight.
Related Topics
- Citric acid
- Trisodium citrate and disodium citrate (specific sodium citrate salts)
- Buffer solutions and pH control
- Anticoagulants (e.g., citrate‑based blood collection tubes)
- Food additive E 331
- Chelating agents
- Sodium salts of organic acids
- Pharmaceutical alkalinizing agents