Citric acid

 

Citric acid

Chemical formula	C6H8O7
OTP appearance	white solid
Index of refraction	1.49-1.51
Molar Mass(g/mol)	192
Density(g/cc)	1.665
Melting Point(°C)	156
Boiling Point(°C)	310
Solubility in water(g/L)	1477
Solubility in ethanol(g/L)	489
NFPA 704	1 2 0 OX

Citric acid is the tangy component of citrus fruit juice. It is an important acid medicinally because most of its conjugate bases can be digested. The pKa of the first three hydrogens are 3.13, 4.76, and 6.4

Uses

Primary

• Pharm: Feedstock for caffeine citrate on the WHO LEM.

Other

• Prevents coagulation of stored blood.

Natural Occurrence

• Occurs naturally in virtually all life on earth
• High concentrations in citrus fruit such as lemons
• Can be produced by aspergillus niger as well

Production

Synthesis

• Combine calcium citrate with Sulfuric acid

  Ca₃(C₆H₅O₇)₂ + 3 H₂SO₄ → 2 C₆H₈O₇ + 3 CaSO₄

• Would phosphoric acid work as well?

  3 Ca₃(C₆H₅O₇)₂ + 9 H₃PO₄ → 6 C₆H₆O₇ + 5 CaHPO₄ + 4 CaH₄PO₄

• Would carbon dioxide work as well?

  4 Ca₃(C₆H₅O₇)₂ + 18 CO₂ + 7 H₂O → 12 CaCO₃ + 9 C₆H₆O₇

Extraction

from citrus juice

1. Prepare filtered citrus juice
2. Add sodium hydroxide until pH is 9, producing sodium citrate along with some sodium ascorbate
3. Filter and discard residue
4. Add an appropriate amount of saturated calcium chloride solution to the filtrate
5. Heat the solution. This starts the reaction and gives sodium chloride and a precipitate
6. Filter and discard filtrate.
7. Wash residue with hot water. The washed residue is of calcium citrate. along with some calcium ascorbate
8. Get the mass the residue to estimate moles
9. Add dilute sulfuric acid to the solution precipitating calcium sulfate and leaving a solution of citric acid and ascorbic acid
10. Separate the acids by fractional crystallization.

See Also

• Preparation of citric acid from lemon juice
  courtesy www.erowid.org.
• US patent 2027264
  Link courtesy Google
• US patent 2362906
  Link courtesy Google