Citric acid

Citric acid

Food grade citric acid.
Names
IUPAC name 3-carboxy-3-hydroxypentane-1,5-dioic acid
Systematic IUPAC name Citric acid
Other names 3-carboxy-3-hydroxypentanedioic acid 2-hydroxy-1,2,3-propanetricarboxylic acid
Identifiers
Jmol-3D images	Image
SMILES OC(=O)CC(O)(C(=O)O)CC(=O)O
Properties
Chemical formula	C6H8O7
Molar mass	192.12 g/mol
Appearance	slightly hygroscopic colorless solid
Odor	Odorless
Density	1.665 g/cm3 (anhydrous) 1.542 g/cm3 (monohydrate, at 18 °C)
Melting point	156 °C (313 °F; 429 K) (decomposes)
Boiling point	310 °C (590 °F; 583 K) (decomposes from 175 °C)
Solubility in water	117.43 g/100 mL (10 °C) 147.76 g/100 mL (20 °C) 180.89 g/100 mL (30 °C) 220.19 g/100 mL (40 °C) 382.48 g/100 mL (80 °C) 547.79 g/100 mL (100 °C)
Solubility	Soluble in ethanol, diethyl ether, ethyl acetate, DMSO, methanol Insoluble in benzene, carbon disulfide, chloroform, toluene, xylene
Solubility in ethanol	62 g/100 g (25 °C)
Solubility in amyl acetate	4.41 g/100 g (25 °C)
Solubility in diethyl ether	1.05 g/100 g (25 °C)
Solubility in 1,4-Dioxane	35.9 g/100 g (25 °C)
Acidity (pKa)	pKa1 = 3.13 pKa2 = 4.76 pKa3 = 6.39, 6.40
Viscosity	6.5 cP (50% aq. sol.)
Thermochemistry
Std molar entropy (So298)	252.1 J/(mol·K)
Std enthalpy of formation (ΔfHo298)	-1548.8 kJ/mol
Hazards
Safety data sheet	ScienceLab
Flash point	155
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	3000 mg/kg (rats, oral)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Citric acid or 2-hydroxy-1,2,3-propanetricarboxylic acid (IUPAC name 2-hydroxypropane-1,2,3-tricarboxylic acid) is a weak organic acid, mostly used in the food industry, where it serves as a preservative and a food additive that gives a pleasant sour taste. It has the chemical formula C₆H₈O₇.

Properties

Chemical

Citric acid will react in solution with bases, carbonates and bicarbonates, as well as reactive metals, such as magnesium, to form their respective salts, citrates.

Mg + C₆H₈O₇ → C₆H₆O₇Mg + H₂

As can be seen with its use as a preservative for fruits, it often serves as a reducing agent, similar to ascorbic acid, both in nature and some syntheses.

Citric acid has a hydroxyl group, with which it can form some mineral acid esters. It also may be used in the production of some exotic polyesters.

Physical

Citric acid is at standard conditions a white hygroscopic crystalline powder. It exists either in an anhydrous (water-free) form or as a monohydrate. The monohydrate can be converted to the anhydrous form by heating above 78 °C. It is soluble in water, ethanol, diethyl ether, ethyl acetate, DMSO and insoluble in benzene, toluene, chloroform, carbon disulfide.

Availability

Citric acid is available in stores as lemon salt, either pure or mixed with other additives. It can be found in pickling and canning sections of grocery stores already in pure, food-grade form.

Williams-Sonoma sells citric acid in small glass jars.

Legal

While not controlled, there were some reports in the past years of law enforcement agencies monitoring the purchase of large amounts of citric acid, mainly due to its use in the production of HMTD. Some drug users use citric acid as cutting agent.

Preparation

Citric acid can be prepared by reacting a citrate salt with a stronger acid.

Industrially it is extracted from Aspergillus niger cultures. However, because it's dirt cheap, citric acid is easier to buy than to make it yourself.

Projects

• Alka-Seltzer rocket
• Anthocyanin extraction
• HMTD synthesis
• Propane-1,2,3-tricarboxylic acid synthesis
• Acetonedicarboxylic acid preparation
• Ferric ammonium citrate preparation
• Triethyl citrate synthesis
• Trimethyl citrate synthesis
• Benedict's reagent
• Citrazinic acid synthesis
• Preparation of Nitrocitric acid

Handling

Safety

Being a weak acid, it is not very toxic, but in high concentrations can irritate the skin and sensitive tissues. Ingesting large quantities of citric acid will upset the stomach and cause digestive problems, as well as metabolic acidosis.

Storage

Solid citric acid should be stored in closed bottles, in a dry place.

Storage of citrate-containing solutions often promotes the growth of bacteria that may metabolize it. Adding an antibacterial can prevents this.

Disposal

Citric acid can be neutralized before disposal, though this is not always necessary. It can be poured down the drain or dumped in the ground.

References

Relevant Sciencemadness threads

• how to reduce citric acid in solution
• A.niga