Citric Acid (C₆H₈O₇) — Properties, Uses & Safety

Overview

Citric acid (C6H8O7) is a weak organic acid with a molar mass of 192.12 g/mol, naturally found in high concentrations in citrus fruits — particularly lemons and limes, which can contain up to 8% citric acid by dry weight. Its IUPAC name, 2-hydroxypropane-1,2,3-tricarboxylic acid, describes its structure: a central carbon bearing a hydroxyl group flanked by three carboxylic acid groups. This triprotic acid can donate three protons in sequential ionization steps, with pKa values of 3.13, 4.76, and 6.40, making it an excellent example for studying polyprotic acid equilibria. At standard conditions, citric acid is a white crystalline powder with an intensely sour taste. It is one of the most widely used food additives in the world, serving as a flavoring agent providing tartness to soft drinks and candy, a preservative by chelating metal ions that catalyze spoilage, and a pH adjuster. The GHS hazard code H319 (causes serious eye irritation) reflects its relatively mild hazard profile. Industrially, citric acid is produced primarily by fermentation of sugars using the mold Aspergillus niger, with global production exceeding 2 million metric tons annually. Beyond food, citric acid is used in cleaning products as a natural descaler for removing limescale, in pharmaceuticals, and in cosmetics. In biochemistry, citric acid is the namesake of the citric acid cycle (Krebs cycle), the central metabolic pathway in aerobic organisms that generates energy by oxidizing acetyl-CoA. For chemistry students, citric acid connects to numerous curriculum topics: it demonstrates polyprotic acid behavior and buffer chemistry, chelation of metal ions, esterification reactions with alcohols, and the role of organic acids in biological metabolism. Its everyday presence in foods and household products makes it a relatable compound for exploring acid-base chemistry, equilibrium constants, and the intersection of organic chemistry with biochemistry.

IUPAC Name

2-hydroxypropane-1,2,3-tricarboxylic acid

CAS Number

77-92-9

PubChem CID

311

Molecular Formula

C₆H₈O₇

Molar Mass

192.124 g/mol

State at STP

solid

Appearance

White crystalline powder, intensely sour