Citric Acid: Formula, Structure & chemical Identification

Citric acid identification scheme showing structure, physical tests, functional group tests, confirmatory tests like CaCl2 test, and derivatives. IUPAC name, common name, Molecular formula Molecular weight — Citric Acid Identification — Structure, Physical Constants, Functional Group Tests and Derivatives

Citric Acid Identification: Key Facts at a Glance

Physical Properties: Physical Appearance · Solubility · Flame Test · Melting Point (153–156 °C) · Citric Acid Triamide m.p. (210 °C) · Citric Acid Trianilide m.p. (199 °C) · Molecular Formula (C₆H₈O₇) · MW (192.12 g/mol) · pKa₁ (3.13) · pKa₂ (4.76) · pKa₃ (6.40) · CAS No. (77-92-9)
Chemical Tests: Litmus · NaHCO₃ Test · Ester Test · FeCl₃ Test · Bromine Water · Acidified KMnO₄ · CaCl₂ Test (on heating) · Naphthol Test (green colour) · Sodium Nitroprusside Test (red → violet) · Control Test · Derivatives · Mixed Melting Point · Chemical Structure · Chemical Reactions · Applications · Safety · FAQs · MCQs · Viva Questions

Citric acid physical constants chart showing the structure with three carboxyl groups and a central hydroxyl, molecular formula C₆H₈O₇, molecular weight 192.12 g/mol, CAS 77-92-9, melting point 153–156 °C, density 1.665 g/cm³, and the three pKa values 3.13, 4.76 and 6.40. — Physical constants of citric acid: the triprotic structure with three successive pKa values, alongside melting point, density and solubility data for both the anhydrous and monohydrate forms.

What is Citric Acid? — Definition, Common Name & IUPAC Name and physical properties

Citric acid is a white, odourless, crystalline solid with an intensely sour taste — the dominant acid in lemon and lime juice. It is a weak, triprotic, hydroxy tricarboxylic acid with molecular formula C₆H₈O₇, molar mass 192.12 g/mol, and three ionisable protons giving pKa1 = 3.13, pKa2 = 4.76, pKa3 = 6.40. A 1% aqueous solution has pH ≈ 2.2.
The citric acid IUPAC name is 2-hydroxypropane-1,2,3-tricarboxylic acid. Its common name, Citric acid, derives from the Latin citrus; in food regulation it is also known as E330. The chemical name used in pharmacopoeia monographs (BP, USP, EP) is identical to the IUPAC name. CAS: 77-92-9 (anhydrous), 5949-29-1 (monohydrate). The molecular weight is 192.12 g/mol (anhydrous) and 210.14 g/mol (monohydrate, C₆H₈O₇·H₂O).

Citric Acid — Chemical Formula, Structure and Properties

Citric acid has the molecular formula C₆H₈O₇ (anhydrous) with a molar mass of 192.12 g/mol. Its IUPAC name is 2-hydroxypropane-1,2,3-tricarboxylic acid, reflecting its three carboxyl (–COOH) groups and one hydroxyl (–OH) group on the central carbon. The condensed structural formula is HOOC–CH₂–C(OH)(COOH)–CH₂–COOH. Citric acid is a weak, triprotic acid with pKa values of 3.13, 4.76, and 6.40, and a melting point of 153–156 °C (anhydrous). The complete physical and chemical properties are summarised in the table below.

Property

Value / Description

1. CHEMICAL IDENTITY & NOMENCLATURE

IUPAC name	2-Hydroxypropane-1,2,3-tricarboxylic acid
Common name	Citric acid
Also known as	E330 (food additive) • 2-hydroxy-1,2,3-propanetricarboxylic acid
CAS No.	77-92-9 (anhydrous) • 5949-29-1 (monohydrate)
PubChem CID	311
Acid classification	Weak, triprotic, hydroxy tricarboxylic acid

2. MOLECULAR & STRUCTURAL DATA

Molecular formula	C₆H₈O₇ (anhydrous) • C₆H₈O₇·H₂O (monohydrate)
Structural formula (condensed)	HOOC–CH₂–C(OH)(COOH)–CH₂–COOH
Elements / atom count	Carbon, Hydrogen, Oxygen only (6 C + 8 H + 7 O per molecule)
Functional groups	3 × carboxyl (–COOH) + 1 × hydroxyl (–OH, tertiary)
Molar mass	192.12 g/mol (anhydrous) • 210.14 g/mol (monohydrate)
Chirality	Achiral — molecule possesses a plane of symmetry; C-2 bears two identical –CH₂COOH substituents and is therefore not a true stereocentre

3. ACIDITY & STOICHIOMETRY

Basicity / valency	C₆H₈O₇ (anhydrous) • C₆H₈O₇·H₂O (monohydrate)
pKa values	HOOC–CH₂–C(OH)(COOH)–CH₂–COOH
pH (1% aqueous solution)	Carbon, Hydrogen, Oxygen only (6 C + 8 H + 7 O per molecule)
Equivalent weight	3 × carboxyl (–COOH) + 1 × hydroxyl (–OH, tertiary)

4. APPEARANCE & SENSORY PROPERTIES

Appearance	White crystalline solid or colourless crystals; also available as powder
Crystal System	Orthorhombic (anhydrous) • Monoclinic (monohydrate)
Odour	Odourless — both solid and in solution (contrast: acetic acid is pungent)
Taste	Odourless — both solid and in solution (contrast: acetic acid is pungent)

5. THERMAL BEHAVIOUR

Melting point (anhydrous)	153 – 156 °C (use this value for melting-point identification)
Monohydrate on heating	Effloresces (loses H₂O) at 70 – 75 °C → converts to anhydrous form → melts at 153 – 156 °C. No independent melting point.
Thermal decomposition	Above ≈ 175 °C: dehydration → cis-aconitic acid; further decomposition → itaconic acid + CO₂
Boiling point	None — decomposes before boiling (no true boiling point)

6. DENSITY & SOLUBILITY

Density	1.665 g/cm³ (anhydrous) • 1.542 g/cm³ (monohydrate)
Solubility in water (20 °C)	133 g/100 mL — very freely soluble
Solubility in ethanol	Freely soluble
Solubility in diethyl ether	Slightly soluble
Solubility in chloroform	Insoluble

Natural Sources

Citric acid is found in highest concentrations in citrus fruits: lemon juice 5–8 g/100 mL, lime 5–6 g/100 mL, grapefruit ~2–3 g/100 mL. Secondary sources: strawberries (~0.6), pineapple (~0.5), tomatoes (~0.3 g/100 mL). Commercially, it is produced by fermentation of glucose or molasses using Aspergillus niger — not extracted from fruit.

Citric Acid Anhydrous vs Monohydrate — Key Differences

Citric acid exists in two forms: the anhydrous form (C₆H₈O₇, molar mass 192.12 g/mol) and the monohydrate form (C₆H₈O₇·H₂O, molar mass 210.14 g/mol). The anhydrous form melts cleanly at 153–156 °C and crystallises in the orthorhombic system, making it the preferred primary standard in alkalimetry. The monohydrate form, which crystallises in the monoclinic system, has no independent melting point — it effloresces at 70–75 °C, losing its water of crystallisation and converting to the anhydrous form before melting. Their equivalent weights differ accordingly: 64.04 g/eq (anhydrous) and 70.05 g/eq (monohydrate). Full comparison is in the table below.

Property	Anhydrous	Monohydrate
Formula	C₆H₈O₇	C₆H₈O₇·H₂O
CAS number	77-92-9	5949-29-1
Molar mass	192.12 g/mol	210.14 g/mol
Melting point	153–156 °C (clean melt)	No clean MP — dehydrates at 70–75 °C first
Density	1.665 g/cm³	.542 g/cm³
Crystal System	Orthorhombic	Monoclinic
Water content	0%	8.57% (1 mol H₂O / mol)
Equivalent weight	64.04 g/eq	70.05 g/eq (210.14 ÷ 3)
Lab use	Primary standard, anhydrous reactions	General reagent, food applications

Planning the Identification of Citric acid: Think Like a Chemical Detective 🔍

Before picking up a single test tube, every good chemist asks one question: “What clues does this molecule already give me?”

Look closely at the structure of citric acid (HOOC–CH₂–C(OH)(COOH)–CH₂–COOH) — it is like a map with three highlighted zones, and each one tells us exactly which tests to run.

Step 1: Identify the Functional Groups in Citric Acid

🔵 Three –COOH groups → tricarboxylic acid (polyprotic)
🟡 One –OH group (tertiary) → alcohol / hydroxyl character
✅ Aliphatic (no benzene ring) → saturated carbon skeleton

Step 2: Citric Acid Qualitative Tests — Matching Structural Clues to Chemical Tests

Structural Clue	Tests to Confirm It
🔵 Three –COOH groups (tricarboxylic acid)	Litmus Paper Test • NaHCO₃ Test (CO₂ evolution) • Ester Test
🟡 –OH group (tertiary alcohol)	FeCl₃ Test (positive) • Naphthol Test (green colour) • Sodium Nitroprusside Test (red → violet)
🟢 Aliphatic / saturated skeleton	Bromine Water Test — no decolourisation • No sooty flame • Freely soluble in water
⚪ Whole compound (citric acid fingerprint)	Calcium Chloride Test (white precipitate) • Melting Point (153 °C, anhydrous) • Mixed Melting Point

Step 3: Begin with Preliminary Observations — Do Not Skip

Before any chemical test is run, three preliminary observations narrow down the compound class: physical appearance, solubility in water, and the flame test.

Citric Acid — Preliminary Observations (Physical & Solubility Properties)

Property	Observation / Value
Appearance	White crystalline solid (monohydrate form common)
Solubility	Freely soluble in water; sparingly soluble in ethanol
Dehydration behaviour	Loses water of crystallisation at 130 °C to form the anhydrous acid
Melting point (anhydrous)	153 °C
Flame test	Clean blue flame; no soot (aliphatic compound, no benzene ring)

A colourless crystalline solid that is freely soluble in cold water, dehydrates at 130 °C, and burns with a clean non-sooty flame is already pointing clearly toward an aliphatic hydroxy-tricarboxylic acid. The chemical tests then confirm what the physical observations already suggest.

How to Identify Citric Acid — Qualitative Analysis Step by Step

Identifying citric acid follows a systematic four-stage sequence. Preliminary observations (Stage 1) establish it as a white, aliphatic, acidic solid through appearance, solubility, and flame test. Functional group tests (Stage 2) — NaHCO₃, ester, bromine water, KMnO₄, and FeCl₃ — confirm carboxyl groups and saturated skeleton. The three citric acid-specific tests (Stage 3) — calcium chloride (white precipitate), naphthol (green colour), and sodium nitroprusside (red → violet) — provide the whole-compound fingerprint. Stage 4 confirms identity through melting point (153 °C), derivative melting points (amide 210 °C, anilide 199 °C), and mixed melting point. The complete sequence is in the table below.

Note: The table summarises only what each test establishes and why it matters for the identification of citric acid. Full procedural details — quantities, conditions, safety notes, and interpretation of borderline results — are provided on the page dedicated to each test. Readers are strongly encouraged to study the relevant page before attempting any procedure in the laboratory.

Test

What It Establishes

STAGE 1 — Preliminary Tests

1	Physical appearance	White crystalline solid (monohydrate); confirms a pure organic acid
2	Solubility	Freely soluble in water; confirms –COOH ionisation; aliphatic profile
3	Elemental Detection	Clean blue non-sooty flame; confirms absence of benzene ring (no aromatic character)
4	Elemental Detection	To check the presence of N, S, Cl, Br, & I
5	Litmus Test	Compound is acidic; turns blue litmus red

STAGE 2 — Functional Group Tests

6	NaHCO₃ test	Specifically a carboxylic acid; brisk CO₂ effervescence (gas turns lime water milky)
7	Ester test	Final –COOH confirmation; fruity ester odour formed with ethanol / conc. H₂SO₄
8	Bromine Water Test	No decolourisation; confirms absence of C=C double bond (saturated aliphatic skeleton)
9	Acidified KMnO₄ Test	No decolourisation; confirms saturated aliphatic structure (Baeyer’s reagent stays purple)
10	FeCl₃ Test	Gives a positive result with FeCl₃ solution (characteristic colour change)

STAGE 3 — Specific Tests for Citric Acid

Calcium Chloride (CaCl₂) Test

White precipitate of calcium citrate confirms citrate ion; whole-compound fingerprint

Procedure: Neutralise a small quantity with NH₄OH, boil off excess NH₃, add CaCl₂ solution, and boil again → white precipitate forms. Adding a single drop of alkali facilitates precipitate formation.

Naphthol Test

Reacting with naphthol in conc. H₂SO₄ → green colour; colour persists on warming but becomes colourless on cooling; specific for citric acid

Sodium Nitroprusside Test

Add 2 drops of sodium nitroprusside solution to 1 mL neutral acid solution → red colour; addition of acetic acid changes colour to violet

STAGE 4 — Confirmation

14	Melting point	Purity check; anhydrous acid melts at 153 °C (monohydrate loses H₂O at 130 °C first)
15	Literature Survey	Cross-check physical and chemical constants
16	Control Test	Validates CaCl₂ / Naphthol results against authentic citric acid standard
17	Derivatives	Amide derivative m.p. 210 °C • Anilide derivative m.p. 199 °C — independent physical checkpoints
18	Mixed Melting Point	Gold-standard definitive proof of identity — no depression with authentic sample

This is the real skill of qualitative organic analysis: it is not about running every test in the textbook, but about reading the molecular structure and selecting tests that carry real diagnostic weight. The Naphthol and Sodium Nitroprusside tests are especially characteristic for citric acid and are rarely positive for other common aliphatic acids — making them powerful confirmation tools. Once that habit is established, identifying any unknown organic compound becomes a logical exercise rather than a guessing game.

Materials and Reagents Required for Citric Acid Identification Tests

Citric acid sample (unknown)
Distilled water
Blue and red litmus paper
NaHCO₃ solution (sodium bicarbonate, ~5% aqueous)
Freshly prepared lime water (Ca(OH)₂ solution) — for CO₂ confirmation
Absolute (anhydrous) ethanol — for ester test
Concentrated H₂SO₄ — for ester test and Naphthol test
Bromine water (dilute aqueous solution of Br₂)
Acidified KMnO₄ solution (dilute) — Baeyer’s reagent
Neutral FeCl₃ solution (~1% aqueous) — for FeCl₃ test
Calcium chloride solution (CaCl₂, ~10% aqueous) — for calcium citrate precipitate test
Ammonium hydroxide solution (NH₄OH) — for neutralisation in CaCl₂ test
β-Naphthol (dissolved in conc. H₂SO₄) — for Naphthol test
Sodium nitroprusside solution (Na₂[Fe(CN)₅NO], freshly prepared) — for Nitroprusside test
Glacial acetic acid — for second step of Nitroprusside test (colour change to violet)
Melting point apparatus with sealed capillary tubes
Delivery tube assembly — for passing CO₂ into lime water
Authentic (known) citric acid sample — for control test and mixed melting point

Result Summary — Citric Acid Identification

All physical and chemical observations are consistent with the identity of the compound as citric acid (C₆H₈O₇).

The sample was a white crystalline solid (monohydrate form), freely soluble in cold water, losing its water of crystallisation at 130 °C and melting sharply at 153 °C (anhydrous). The flame test confirmed an aliphatic structure (clean non-sooty blue flame), while the litmus, NaHCO₃, and ester tests together confirmed the presence of carboxylic acid (–COOH) groups. The bromine water and acidified KMnO₄ tests both gave negative results, confirming the absence of C=C unsaturation. The FeCl₃ test gave a positive result consistent with an α-hydroxy acid. The CaCl₂ test produced a white precipitate of calcium citrate under mildly alkaline conditions, providing a whole-compound fingerprint. The Naphthol test gave a characteristic green colour (reversible on cooling) and the Sodium Nitroprusside test gave a red colour changing to violet on addition of acetic acid — both highly specific for citric acid. The sharp melting point of 153 °C (anhydrous), together with the amide derivative (m.p. 210 °C) and anilide derivative (m.p. 199 °C), provided strong independent physical confirmation. Finally, the mixed melting point test showed no depression or elevation, conclusively confirming the identity of the compound.

Conclusion

Based on the combined physical and chemical evidence — aliphatic character (clean non-sooty flame), saturation (bromine water and KMnO₄ negative), polycarboxylic acid functionality (NaHCO₃, litmus, and ester tests), positive FeCl₃ behaviour (α-hydroxy acid), whole-compound fingerprints (CaCl₂ white precipitate; Naphthol green colour; Nitroprusside red-to-violet), matching literature melting point (153 °C anhydrous; 130 °C dehydration of monohydrate), confirmed derivative melting points (citric amide 210 °C; citranilide 199 °C), and absence of mixed melting point depression — the given unknown compound is conclusively identified as:

Citric Acid (C₆H₈O₇)

IUPAC name: (2R,3R)-2,3-dihydroxybutanedioic acid | MW: 150.09 g/mol | m.p. 170–172 °C (L-form) | dl-form m.p. 206 °C

Important note: Mixed melting point test — gold standard identity confirmation
The mixed melting point (or mixture melting point) test is the classical gold standard for confirming that an unknown compound is identical to a known authentic sample. It requires a separately sourced authentic sample of L-tartaric acid for comparison.

What is mixed	Expected result if SAME compound	Expected result if DIFFERENT compound
Unknown alone	MP = 170–172 °C	MP = 170–172 °C
Authentic L-tartaric acid alone	MP = 170–172 °C	MP = 170–172 °C
1:1 mixture of unknown + authentic	MP = 170–172 °C (NO depression) Conclusion: SAME compound	MP depressed 10-30 °C below both pure samples (eutectic formation) Conclusion: DIFFERENT compounds

Theory: why depression occurs
When two different compounds are mixed, each acts as an impurity in the other, lowering both melting points (eutectic depression). The degree of depression is related to the mole fraction of impurity by the van’t Hoff equation. If no depression is observed, the two samples are the same compound — because neither acts as an impurity in the other’s lattice.
For tartaric acid specifically: mixing L-tartaric acid with meso-tartaric acid would show significant depression, confirming they are different compounds despite both giving zero optical rotation in the meso case. This is one practical example where the mixed MP test distinguishes isomers that a single-compound melting point cannot resolve.

Safety Precautions

General Laboratory Safety

Wear a laboratory coat, safety goggles, and nitrile gloves throughout the entire experiment.
Work in a well-ventilated laboratory. Use a fume cupboard for all steps involving concentrated H₂SO₄, SOCl₂, aniline, and β-naphthol.
Never pipette by mouth. Use a pipette filler or dropper for all liquid reagents.
Dispose of all chemical waste in the designated waste containers. Do not pour FeCl₃, KMnO₄, or heavy-metal solutions down the sink.
Know the location of the eyewash station, emergency shower, fire extinguisher, and first-aid kit before starting.
In case of skin or eye contact with any corrosive reagent, wash immediately with large volumes of water for at least 15 minutes and seek medical attention.

Per-Reagent Hazard and Precaution Table

Reagent	Hazard Classification	Specific Risk	Precaution
Citric acid	Low hazard (irritant)	May cause mild skin, eye, and respiratory irritation on prolonged exposure. Not acutely toxic.	Avoid inhaling dust. Wear gloves. Handle normally on open bench.
Concentrated H₂SO₄	CORROSIVE • OXIDISING	Causes severe, potentially permanent burns to skin, eyes, and mucous membranes. Reacts violently with water, releasing heat.	Always add acid slowly and carefully. Use a fume cupboard. Wear acid-resistant gloves and goggles. In case of contact, flush with copious water immediately.
Acidified KMnO₄	OXIDISING • IRRITANT	Strong oxidising agent. Contact with combustible materials may cause fire. Manganese compounds are toxic by ingestion.	Keep away from flammable solvents. Wear gloves. Dispose of waste in the heavy-metal waste container.
NaHCO₃ solution	Low hazard	Mild irritant only. CO₂ evolved during the test is asphyxiant in enclosed spaces.	Ensure adequate ventilation. Use a delivery tube to direct CO₂ into lime water safely.
FeCl₃ solution	IRRITANT	Irritating to skin and eyes. Mildly corrosive. Iron(III) salts harmful if swallowed.	Wear gloves and goggles. Avoid contact with skin. Dispose of waste in heavy-metal waste container.
CaCl₂ solution	Low hazard (irritant)	Mild irritant to skin and eyes. Hygroscopic — do not leave open.	Wear gloves. Store sealed.
NH₄OH solution	CORROSIVE • TOXIC (vapour)	Pungent, irritating vapour causes respiratory tract and eye irritation. Corrosive at high concentration.	Use in a fume cupboard. Keep the bottle closed when not in use. Wear goggles and gloves.
β-Naphthol / conc. H₂SO₄	CORROSIVE • IRRITANT	Naphthol is a skin and eye irritant; conc. H₂SO₄ causes severe burns. Mixture is highly exothermic to prepare.	Prepare in fume cupboard. Add naphthol to acid slowly. Wear acid-resistant gloves and goggles.
Sodium nitroprusside	TOXIC	Toxic by ingestion and skin absorption. Releases cyanide under acidic conditions.	Use in a fume cupboard. Wear gloves. Prepare fresh solution only. Dispose as toxic waste.
Thionyl chloride (SOCl₂)	HIGHLY CORROSIVE • TOXIC	Reacts violently with water, releasing HCl and SO₂ — both toxic and corrosive. Severe burns.	Use ONLY in a dry fume cupboard. Ensure all glassware is completely dry. Wear acid-resistant gloves and goggles. Keep away from all water sources.
Aniline	TOXIC • HARMFUL	Readily absorbed through skin. Causes methaemoglobinaemia. Suspected carcinogen.	Use in a fume cupboard at all times. Wear double gloves (nitrile over latex). Seek medical advice immediately if skin contact occurs.

First Aid Summary

Exposure Route	Immediate First Aid Action
Eye contact with any chemical	Immediately irrigate with large volumes of flowing water for at least 15 minutes, holding eyelids open. Seek medical attention.
Skin contact — corrosive (H₂SO₄, SOCl₂)	Remove contaminated clothing. Wash affected area with copious water for at least 15 minutes. Seek medical attention.
Skin contact — aniline	Remove contaminated clothing immediately. Wash with soap and water for at least 15 minutes. Seek medical attention — risk of systemic toxicity even after decontamination.
Skin contact — sodium nitroprusside	Wash thoroughly with soap and water. Seek medical attention. Alert physician to cyanide risk.
Inhalation — SOCl₂ or NH₄OH vapour	Move to fresh air immediately. If breathing is difficult, administer oxygen. Seek medical attention urgently.
Ingestion	Do NOT induce vomiting. Rinse mouth with water. Seek medical attention immediately and bring the chemical’s safety data sheet (SDS).

Uses and Applications of Tartaric Acid

Industry / Field	Application	Specific Role
Food & Beverage Industry	Acidulant and flavour enhancer	The most widely used food acidulant globally. Added to soft drinks, candies, jams, preserves, and processed foods to impart tartness and improve flavour balance. Assigned E330 status (EU food additive).
Food Preservation	Preservative / antioxidant synergist	Chelates metal ions that catalyse oxidative rancidity, extending shelf life of oils and fats. Works synergistically with ascorbic acid and tocopherols.
Pharmaceuticals	Effervescent formulation base	Essential component of effervescent tablets and granules (e.g. vitamin C tablets, antacid sachets). Reacts with NaHCO₃ to produce CO₂, aiding dissolution and palatability.
Cosmetics & Personal Care	pH adjuster and chelating agent	Adjusts pH in skin creams, shampoos, and lotions. Chelates calcium and magnesium ions in hard water to improve lather and product stability.
Cleaning Products	Descaling and chelating agent	Effective limescale remover in household and industrial descalers, dishwasher rinse aids, and bathroom cleaners. Chelates Ca²⁺ and Mg²⁺ from mineral deposits.
Organic Synthesis	Versatile building block	Starting material for synthesis of aconitic acid, itaconic acid, and various citrate esters. Used as a ligand in coordination chemistry and as a green solvent additive.
Biochemistry	Krebs cycle intermediate	Citric acid is the key metabolite that names the Krebs (citric acid) cycle — the central energy-producing pathway in aerobic organisms. Important biochemical significance beyond industrial use.
Photography / Metal Treatment	Complexing and buffering agent	Used in photographic development solutions and as a mild metal etchant / depassivation agent for stainless steel and titanium.

FAQ’s

Citric acid is an aliphatic α-hydroxy tricarboxylic acid with the molecular formula C₆H₈O₇, molecular weight 192.12 g/mol, and IUPAC name 2-hydroxypropane-1,2,3-tricarboxylic acid. It occurs naturally in citrus fruits (lemons, limes, oranges) and is the most widely used acidulant in the food, pharmaceutical, and cosmetics industries

Citric acid (HOOC–CH₂–C(OH)(COOH)–CH₂–COOH) contains three structural zones: three –COOH carboxylic acid groups (making it triprotic), one tertiary –OH group on C-2 (giving it α-hydroxy acid character), and a saturated aliphatic carbon skeleton with no benzene ring or C=C double bond. See Figure 1 for the annotated structural formula.

Citric acid is saturated. It contains no C=C double bond, confirmed experimentally by the absence of decolourisation of both bromine water and acidified KMnO₄ (Baeyer’s reagent). This contrasts directly with unsaturated acids such as cinnamic acid, which decolourise both reagents.

The melting point of anhydrous citric acid is 153 °C. The commercially common monohydrate form first loses its water of crystallisation at 130 °C before melting at 153 °C. The sample must be fully dried before melting point determination to obtain a reproducible and meaningful value.

The IUPAC name of citric acid is 2-hydroxypropane-1,2,3-tricarboxylic acid. The name specifies a three-carbon chain (propane) bearing carboxylic acid groups at positions 1, 2, and 3, and a hydroxyl group at position 2. The common name ‘citric acid’ is universally used in practice.

The degree of unsaturation (DoU) of citric acid (C₆H₈O₇) is 3, contributed entirely by the three –COOH groups (each carboxyl C=O counts as one degree). There is no benzene ring and no C=C double bond. DoU = (2×6 + 2 – 8) / 2 = 3.

The Naphthol test and Sodium Nitroprusside test together serve as the most specific confirmatory tests for citric acid: the Naphthol test gives a green colour that persists on warming but disappears on cooling, and the Nitroprusside test gives a red colour that changes to violet on addition of acetic acid. The CaCl₂ test (white calcium citrate precipitate under alkaline conditions) provides additional whole-compound fingerprint confirmation.

Citric acid does not decolourise bromine water because it contains no C=C double bond — there is no site available for electrophilic addition of Br₂. The orange-brown colour of bromine water is retained. This is the opposite of unsaturated acids such as cinnamic acid, which rapidly decolourise bromine water via addition across the C=C bond.

The NaHCO₃ test proves that citric acid contains –COOH groups rather than weaker acidic groups such as phenol. CO₂ gas (confirmed by lime water turning milky) is released because citric acid (pKa₁ 3.13) is acidic enough to protonate bicarbonate ion; phenol (pKa ~10) is not. The particularly vigorous effervescence reflects the three carboxyl groups reacting.

Citric acid is freely soluble in cold water because the three –COOH groups and one –OH group make the molecule highly polar and capable of extensive hydrogen bonding with water. This contrasts sharply with cinnamic acid (sparingly soluble ~0.4 g/L), whose benzene ring creates a dominant hydrophobic region.

Citric acid is used as a food acidulant (E330; soft drinks, confectionery, preserves), a pharmaceutical excipient (effervescent tablets), a cosmetics pH adjuster and chelating agent, a household and industrial descaling agent, a biochemistry reagent (Krebs cycle metabolite), and a versatile building block in organic synthesis.

Citric acid itself is a low-hazard mild irritant with no significant acute toxicity at normal handling concentrations. However, several reagents used in its identification (concentrated H₂SO₄, SOCl₂, aniline, sodium nitroprusside, β-naphthol) are hazardous. Always wear a lab coat, safety goggles, and nitrile gloves, and work with these reagents in a fume cupboard.

Citric acid is produced industrially by fermentation of sucrose or glucose by the mould Aspergillus niger under controlled aerobic conditions — a process that accounts for over 99% of world production (~2 million tonnes per year). The citric acid is recovered by precipitation as calcium citrate, followed by treatment with sulfuric acid and purification.

Citric acid has three ionisation steps: pKa₁ 3.13, pKa₂ 4.76, and pKa₃ 6.40. The low pKa₁ (3.13) is significant for the NaHCO₃ test: citric acid is acidic enough to protonate bicarbonate (H₂CO₃ pKa₁ ~6.4) and release CO₂. Phenols (pKa ~10) cannot achieve this.

No. Citric acid does not show geometrical isomerism because it has no C=C double bond (which would be required for restricted rotation). The tertiary carbon bearing the –OH group is a chiral centre, so citric acid can show optical isomerism — however, the naturally occurring form is the achiral meso-like form that is optically inactive due to the plane of symmetry through C-2.

Multiple Choice Questions

MCQ 1

1. The correct molecular formula of citric acid is:

A. C₆H₆O₆

B. C₆H₈O₇

C. C₆H₇O₇

D. C₄H₆O₅

MCQ 2

2. The melting point of anhydrous citric acid is:

A. 130 °C

B. 133 °C

C. 153 °C

D. 210 °C

MCQ 3

3. The IUPAC name of citric acid is:

A. 2-hydroxypropan-1,2-dicarboxylic acid

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

C. 3-hydroxypentanedioic acid

D. propane-1,2,3-triol

MCQ 4

4. Which pair of tests BOTH confirm that citric acid is saturated (no C=C)?

A. Litmus test and FeCl₃ test

B. No decolourisation of bromine water AND no decolourisation of KMnO₄

C. Flame test and ester test

D. CaCl₂ test and Naphthol test

MCQ 5

5. Citric acid reacts vigorously with NaHCO₃ but phenol does not. The best explanation is:

A. Citric acid is a stronger base than phenol

B. Citric acid (pKa₁ 3.13) is acidic enough to protonate HCO₃⁻; phenol (pKa ~10) is not

C. Phenol does not dissolve in NaHCO₃ solution

D. Citric acid has a higher molecular weight than phenol

MCQ 6

6. When bromine water is added to citric acid, the result is:

A. Immediate decolourisation (Br₂ adds across C=C)

B. White precipitate forms (ring bromination)

C. Orange-brown colour is retained — no decolourisation

D. Purple colour develops

MCQ 7

7. Acidified KMnO₄ (Baeyer’s reagent) applied to citric acid gives:

A. Decolourisation and brown MnO₂ precipitate

B. Purple colour retained — no decolourisation

C. Green colour

D. Colourless precipitate

MCQ 8

8. The Naphthol test for citric acid gives:

A. Red colour changing to violet

B. White precipitate

C. Green colour that persists on warming but becomes colourless on cooling

D. No visible change

MCQ 9

9. The number of distinct functional group types in citric acid is:

A. One (–COOH only)

B. Two (–COOH and –OH)

C. Three (–COOH, –OH, and C=C)

D. Four (–COOH, –OH, C=C, and benzene ring)

MCQ 10

10. Citric acid burns with a clean blue non-sooty flame because:

A. It contains nitrogen

B. It has a high C:H ratio due to the benzene ring

C. It is a saturated aliphatic compound with no benzene ring, giving complete combustion

D. It contains sulfur

MCQ 11

11. Citric acid is freely soluble in cold water because:

A. It has a non-polar aliphatic chain that dissolves in water

B. Three –COOH groups and one –OH make the molecule highly polar and capable of extensive hydrogen bonding with water

C. It has an extremely low molecular weight

D. It ionises completely (strong acid) in water

MCQ 12

12. The degree of unsaturation (DoU) of citric acid (C₆H₈O₇) is:

A. 0

B. 3 (three C=O of –COOH groups only

C. 6 (as in cinnamic acid)

D. 5

MCQ 13

13. Which statement about citric acid and isomerism is CORRECT?

A. Citric acid shows geometrical isomerism due to its three –COOH groups

B. Citric acid shows no geometrical isomerism (no C=C bond); C-2 is a chiral centre so optical isomerism is possible

C. Citric acid shows both geometrical and optical isomerism

D. Citric acid has no stereocentres and no isomers

MCQ 14

14. The CaCl₂ test for citric acid requires prior neutralisation with NH₄OH because:

A. NH₄OH decomposes the citric acid

B. Calcium citrate precipitates selectively under near-neutral or mildly alkaline conditions, not in strongly acidic solution

C. NH₄OH acts as a catalyst for the reaction

D. CaCl₂ does not react with citric acid at all without NH₄OH

MCQ 15

15. The melting point of citric amide (amide derivative of citric acid) is:

A. 153 °C

B. 199 °C

C. 210 °C

D. 130 °C

MCQ 16

16. The unknown sample is mixed with authentic citric acid. The mixture melts sharply at 153 °C — no depression. This means:

A. Both samples are the same compound — identity is conclusively confirmed

B. Both samples are impure

C. The test has failed and must be repeated

D. The two compounds are different

MCQ 17

17. The Sodium Nitroprusside test for citric acid gives:

A. No colour change

B. Green colour that disappears on cooling

C. Red colour initially; changes to violet on addition of acetic acid

D. White precipitate

MCQ 18

18. Which of the following is the primary large-scale industrial application of citric acid?

A. As a UV-B filter in sunscreens

B. As a food acidulant and preservative (E330) in beverages, confectionery, and processed foods

C. As a starting material for chloroquine synthesis

D. As a photocrosslinkable monomer in polymer chemistry

MCQ 19

19. Citric acid is produced commercially by:

A. Oxidation of glucose with acidified KMnO₄

B. Perkin condensation of benzaldehyde

C. Fermentation of sucrose or glucose by Aspergillus niger under aerobic conditions

D. Hydrolysis of triethyl citrate with NaOH

MCQ 20

20. The molecular weight of citric acid (C₆H₈O₇) is:

A. 132.12 g/mol

B. 176.12 g/mol

C. 192.12 g/mol

D. 208.12 g/mol