Citric Acid
Properties
| State | Solid (white crystalline powder) |
| Color | White (colorless in solution) |
| Solubility | Highly soluble in water (730 g/L at 20°C) |
| Melting Point | 153°C |
| Boiling Point | Decomposes above 175°C |
About Citric Acid
Citric acid is a triprotic α-hydroxy carboxylic acid with three pKa values stepping from 3.13 to 4.76 to 6.40, which means a single compound can buffer effectively across more than three pH units — that's why it's the backbone of the McIlvaine buffer (citric acid + Na2HPO4) that biochemists reach for when they need a continuous pH 2.6–7.6 buffer series for enzyme kinetics. The structure has three carboxylate groups arranged on a propane backbone with a central tertiary hydroxyl, and that combination of α-hydroxy plus three COOH makes it an outstanding chelator: log K1 for Ca²⁺ is 4.7, for Fe³⁺ it's 11.5, which is exactly why citrate is the standard preservative in blood-collection tubes (binds Ca²⁺ and stops the coagulation cascade) and in dishwasher tablets (sequesters hard-water ions before they can scale glassware). The molecule sits at the intersection of metabolism and the entire bioenergetic logic of aerobic life: in the TCA cycle, citrate is the first product of acetyl-CoA condensing with oxaloacetate, and the eight enzymatic steps that follow generate three NADH, one FADH2, and one GTP per acetyl unit oxidized. Industrially, every gram of commercial citric acid (~2.7 million tonnes/year) comes from Aspergillus niger fermentation of corn-sugar or molasses, a process pioneered by Pfizer in 1919 that displaced the older Italian lemon-extraction industry within a decade. You'll find citric acid (E330) in roughly 80% of carbonated soft drinks, every gummy candy on a shelf, and in pH-adjusted hair-care products where it shifts cuticle alignment to make hair shinier.
Where you'll encounter it
If you've ever descaled a kettle by boiling 20 g of citric acid in a liter of water, you've watched its calcium-chelation chemistry happen in real time — the limescale fizzes off the heating element as Ca²⁺ leaves the carbonate matrix and binds citrate. In a biochem lab, citrate buffer is what you load into a Bio-Rex resin column for cation-exchange chromatography of amino acids on the old Spinco analyzers, and citrate phosphate dextrose (CPD) is what's in every blood-bag bag at the Red Cross — citrate to chelate calcium, phosphate to buffer, dextrose to keep red cells alive for 35 days at 4 °C.
Common Uses
- Acidulant and preservative (E330) in carbonated soft drinks, gummies, jams, and canned tomatoes
- Anticoagulant in CPD blood-collection bags by chelating Ca²⁺ to block the coagulation cascade
- McIlvaine buffer component (with Na2HPO4) for continuous pH 2.6–7.6 enzyme-kinetics buffers
- Calcium and iron chelator in dishwasher tablets and stainless-steel passivation rinses
- Effervescent disintegrant in pharmaceutical tablets paired with sodium bicarbonate
- Cuticle-flattening pH adjuster in shampoos and conditioners targeting pH 4.5–5.5
- Citrate-phosphate antigen-retrieval buffer for paraffin-embedded immunohistochemistry
- Substrate analog for ATP-citrate lyase inhibitor screening in metabolic disease research
Safety Information
FDA Generally Recognized as Safe (GRAS, 21 CFR 184.1033). Acute oral toxicity is very low (LD50 in rats ~3000 mg/kg). GHS H319 (causes serious eye irritation) for the solid powder. Concentrated solutions (>10%) can cause skin and respiratory tract irritation. The most common practical concern is dental erosion — citric acid solutions below pH 3 dissolve hydroxyapatite in tooth enamel, which is why frequent sipping of acidic sodas is associated with non-carious enamel loss. OSHA does not list a PEL. The fermentation-derived material can rarely contain trace allergens from corn or soy substrates, an issue for severe-allergy formulations.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.