Sodium Citrate
Properties
| State | Solid (white crystalline powder or granules) |
| Color | White |
| Solubility | Very soluble in water (425 g/L at 25 °C); insoluble in ethanol |
| Melting Point | Decomposes at ~150 °C (dihydrate loses water then decomposes) |
| Boiling Point | Decomposes before boiling |
About Sodium Citrate
Sodium citrate is the trisodium salt of citric acid (Na3C6H5O7, MW 258.068, or the dihydrate at 294.10 g/mol) and one of the few additives that does four jobs at once: pH buffer in the 3-6 range, emulsifier, calcium chelator, and a mildly tart flavoring. The buffering capacity comes from citrate's three carboxylate groups, which absorb protons across a useful span without sharp transitions. Hospitals stock it as the anticoagulant in light-blue-cap Vacutainer tubes for PT, PTT, and INR coagulation panels because citrate sequesters Ca2+, the cofactor that drives the clotting cascade through Factors IV, VII, IX, and X. Continuous renal replacement therapy and apheresis circuits use regional citrate anticoagulation for the same reason, with calcium gluconate infused on the return line to restore systemic Ca2+. In molecular biology it shows up as the C in SSC buffer (saline-sodium citrate) for Southern blots, Northern blots, and FISH, where the citrate stabilizes nucleic acid hybridization at the chosen stringency. Modernist cooks use it with calcium chloride for reverse spherification (the technique that makes flavored 'caviar' burst on the tongue), and it is the secret behind smooth nacho cheese sauce.
Where you'll encounter it
If you've ever pulled a pale-blue-top tube for a coag panel, run a Southern blot, or made the three-ingredient nacho cheese sauce that goes viral every couple of years (cheese, beer, sodium citrate), you've used Na3C6H5O7. In a clinical lab the 3.2% citrate concentration matters precisely because the tube must give a 9:1 blood-to-anticoagulant ratio for INR to be valid; underfilled tubes get rejected by the hematology analyzer. In a molecular biology lab, 20x SSC (3 M NaCl, 0.3 M sodium citrate, pH 7.0) is the buffer you dilute down to control hybridization stringency. In a kitchen, sodium citrate keeps casein proteins from balling up when you melt aged cheddar over pasta, which is the same chemistry Kraft has used in American singles since 1916.
Common Uses
- Anticoagulant in 3.2% solution for blue-top coagulation tubes (PT, PTT, INR)
- Regional citrate anticoagulation in continuous renal replacement therapy and apheresis
- C component of SSC hybridization buffer for Southern, Northern, and FISH protocols
- Emulsifying salt in processed cheese and modernist cheese sauces (Kraft singles, mornay)
- Calcium-source counterion for reverse spherification in molecular gastronomy
- Acidity regulator (E331) in soft drinks, jams, gelatin, and effervescent tablets
- Urinary alkalinizer in oral therapy for kidney stone prevention and metabolic acidosis
- Sequestrant in dishwasher detergents that softens hard water by binding Ca2+ and Mg2+
Safety Information
Generally Recognized as Safe (GRAS) by the FDA for food use. Not classified as hazardous under GHS and carries no acute health pictograms. The oral LD50 in rats is around 1,548 mg/kg, so ingestion of culinary or pharmaceutical quantities is well-tolerated. High oral doses can cause GI upset, diarrhea, and electrolyte shifts (notably hypocalcemia and metabolic alkalosis at gram-scale dosing). The clinical risk to watch is citrate toxicity during massive transfusion or apheresis: rapid infusion of citrated blood faster than the liver can metabolize citrate causes acute hypocalcemia, presenting as perioral tingling, tetany, and QT prolongation; calcium gluconate is the antidote. No PEL is set. Standard lab PPE is sufficient.
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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.