Description

Chemical Identity

Name: Citric Acid Monohydrate

Chemical Formula: C₆H₈O₇·H₂O

Molar Mass: ~210.14 g/mol

CAS Number: 5949-29-1

Chemical Class: Organic acid (tricarboxylic acid)

Citric acid monohydrate is citric acid that contains one molecule of water of crystallization per molecule of citric acid.

2. Structure and Composition

Citric acid itself has:

Three carboxyl groups (–COOH)

One hydroxyl group (–OH)

It is a triprotic acid, meaning it can donate three hydrogen ions (H⁺) in solution.

The “monohydrate” part does not mean one acidic hydrogen; it means one water molecule is physically bound in the crystal lattice.

3. Physical Properties

Appearance: White or colorless crystalline powder or granules

Odor: Odorless

Taste: Strongly acidic (sour)

Solubility: Very soluble in water; soluble in ethanol

Melting Point: ~135 °C (decomposes)

Density: ~1.54 g/cm³

4. Chemical Properties

Acidity (pKa values):

pKa₁ ≈ 3.1

pKa₂ ≈ 4.8

pKa₃ ≈ 6.4

Acts as:

Chelating agent (binds metal ions like Ca²⁺, Fe³⁺)

Buffering agent

Acidulant

In aqueous solution, citric acid can exist as:

Citric acid (H₃Cit)

Dihydrogen citrate (H₂Cit⁻)

Monohydrogen citrate (HCit²⁻)

Citrate ion (Cit³⁻)

5. Production

Produced industrially by fermentation of sugars (glucose or sucrose) using Aspergillus niger.

The monohydrate form crystallizes naturally from aqueous solutions at room temperature.

6. Uses and Applications
Food Industry

Food additive E330

Flavor enhancer and preservative

Controls pH in beverages, candies, jams, and dairy products

Pharmaceutical & Medical

Excipient in tablets and syrups

Used in effervescent formulations

Prevents kidney stone formation (citrate therapy)

Laboratory & Chemical Industry

Buffer preparation

Chelating agent

Cleaning and descaling agent

Analytical chemistry reagent

Cosmetics & Personal Care

pH adjuster

Exfoliating agent (AHA)

7. Safety and Handling

Generally Recognized as Safe (GRAS)

Mildly irritating to eyes and skin in concentrated form

Inhalation of dust may cause respiratory irritation

Store in a cool, dry place in a tightly closed container

8. Difference Between Citric Acid Monohydrate and Anhydrous Citric Acid
Feature Monohydrate Anhydrous
Water content Contains 1 H₂O No water
Molecular weight Higher Lower
Stability More stable More hygroscopic
Common use Food & pharma Labs & dry formulations
9. Summary

Citric acid monohydrate is a widely used organic acid containing one molecule of water, known for its acidic, buffering, and chelating properties, and is essential in food, pharmaceutical, cosmetic, and industrial applications.