Ferric Chloride Test Explained for Phenol Identification

ByDr. Shammas Din Chemical Tests

The Ferric Chloride Test is a simple and widely used qualitative test in organic chemistry, primarily employed to detect phenolic compounds and certain carboxylic acids. This test is based on the ability of ferric ions (Fe³⁺) to form colored complexes when they react with hydroxyl (-OH) groups of phenols or the carboxyl (-COOH) group of some acids.

Phenols react with ferric chloride to give different colors. Aromatic carboxylic acids also react and show various colors. The color depends on the strength of the ligands that form complexes with the Fe³⁺ ion.

Colour wheel representing the colours of different compounds colour observed when react with ferric chloride.

The Ferric Chloride Test is given by

  • Phenols (cresols, phloroglucinol, resorcinol, catechol, α-naphthol, β-naphthol and p-amino phenol).
  • Carboxylic acids (salicylic acid, formic acid, acetic acid, succinic acid, benzoic acid, adipic acid, tartaric acid and citric acid).
  • Another category of p-anisidine (amine).

Principle

The test is based on the coordination reaction between ferric ions (Fe³⁺) and phenolic hydroxyl groups. Phenols and certain carboxylic acids can react with Fe³⁺ ions to form colored complexes, with different compounds producing different colors. These colors, which may be violet, blue, green, or red depending on the structure of the phenol or carboxylic acid, are used for the identification of various phenols and carboxylic acids.

Historical background

The test is a classical qualitative method dating back to the 19th century. It has been widely used in organic laboratories for rapid phenol detection before the advent of modern spectroscopic techniques.

Chemical Reaction

The general reaction of the ferric chloride test can be represented as:

Fe³⁺ + 6 ArOH → [Fe(ArO)₆]³⁻ + 3 H⁺

Where ArOH represents a phenolic compound or certain carboxylic acids. One ferric ion reacts with six molecules of phenol or carboxylic acid to form a colored coordination complex, which is responsible for the color observed during the test.

Chemical equation representing FeCl₃ reaction with phenol forming coloured complex.

Materials Required

  • Freshly prepared neutral ferric chloride solution (1–5%)
  • Test tubes
  • Distilled water (as solvent)
  • Sample of phenol or certain carboxylic acid

Quick insight

Why must ferric chloride solution be freshly prepared before use in laboratory tests?
Freshly prepared ferric chloride solution is required because Fe³⁺ ions slowly hydrolyze and precipitate on standing, reducing reagent effectiveness and causing unreliable test results.

Procedure

  1. Dissolve the test sample in distilled water.
  2. Add neutral ferric chloride solution dropwise to the sample in the test tube.
  3. Observe any color change immediately.
Test tubes representing FeCl₃ test with phenol, final violet colour observed.

Ferric Chloride test with Phenols

Phenols react with freshly prepared ferric chloride solution to form colored complexes, typically violet, blue, or green. This color change occurs due to complex formation between Fe³⁺ ions and the phenoxide ion, confirming the presence of a phenolic –OH group.

Colour wheel representing colours observed by phenols with FeCl₃.

Ferric Chloride test with Carboxylic acids

Most carboxylic acids such as acetic, propionic, and benzoic acid do not give a characteristic color with neutral FeCl₃, showing only a pale yellow solution or a brown iron(III) carboxylate precipitate, which is not diagnostic. Compounds containing a phenolic –OH or an enolizable α-hydroxy structure, such as salicylic acid, give a positive violet FeCl₃ test. Any red color observed with simple acids like acetic or formic acid results from ferric carboxylate formation, not true phenolic complexation.

Colour wheel representing colour observed by carboxylic acids with FeCl₃

Inference table

Colour of Fe³⁺ complex

Colour Observed

Inference

Buff precipitate

Adipic, benzoic and succinic acids

Blue violet

Resorcinol

Green

Catechol

Purple

Salicylic acid, p-amino phenol

Purple in ethanolic solution

β- naphthol

Red

Acetic and Formic acids

Violet

Cresols, phenol, phloroglucinol, p-anisidine(amine), resorcinol, vanillin.

Violet in ethanolic solution

α- naphthol

White ppt, if solution prepared was in H2O

α- and β- Naphthols

Yellow colouration

Citric and Tartaric acids

Applications

  • The Ferric Chloride Test is used to quickly detect phenolic and aromatic carboxylic groups in organic compounds.
  • It helps in monitoring reactions and identifying functional groups in synthetic and natural samples.
  • The test provides a visual confirmation of metal-ligand complex formation, making it a valuable tool for understanding structure-reactivity relationships.
  • It is also a useful preliminary check before performing detailed spectroscopic analyses.

Limitations

  • FeCl₃ solution must be neutral; acidic solutions may interfere.
  • False positives possible with enolic or other reactive compounds.
  • Not a quantitative test—only indicates presence.

Precautions

Handle FeCl₃ carefully: it is corrosive; use gloves and avoid inhalation.

Conclusion

The ferric chloride (FeCl₃) test is a reliable qualitative method for identifying phenols through the formation of characteristic colored complexes, typically violet, blue, or green. In contrast, most carboxylic acids do not give a true positive FeCl₃ test, producing only pale yellow solutions or nonspecific precipitates. This clear difference makes the FeCl₃ test an effective tool for distinguishing phenols from carboxylic acids in organic chemistry analysis.

Viva questions

  1. What is the Ferric Chloride Test used for?
  2. Explain the principle behind the Ferric Chloride Test.
  3. Which functional groups give a positive Ferric Chloride Test?
  4. Why must FeCl₃ solution be neutral?
  5. What colors indicate a positive test for phenols?
  6. Can any non-phenolic compounds give a color change? Explain.
  7. Write the chemical reaction for the Ferric Chloride Test.
  8. What precautions should be taken while performing this test?

Multiple Choice Questions

MCQ 1

1. The Ferric Chloride Test is used to detect:

A. Ketones

B. Phenols

C. Ketones

D. Aldehydes

MCQ 2

2. Which color indicates a positive test for simple phenols?

A. Red

B. Violet / Blue

C. Green

D. Yellow

MCQ 3

3. Catechol gives which color in the Ferric Chloride Test?

A. Violet

B. Blue

C. Green / Blue-Green

D. Red-Brown

MCQ 4

4. Which of the following functional groups can give a false positive?

A. Alcohols

B. Enols

C. Alkanes

D. Ethers

MCQ 5

5. What is the ain reason for using a neutral FeCl₃ solution?

A. To speed up the reaction

B. To avoid hydrolysis and precipitation

C. To enhance color intensity

D. No effect

MCQ 6

6. Which form of ferric chloride is used in this test?

A. Solid FeCl₃

B. Alcoholic FeCl₃

C. Aqueous neutral FeCl₃

D. Concentrated FeCl₃

FAQ’s

It detects most phenols, but the color may vary depending on the number and position of −OH groups.

No, it is a qualitative test; it only indicates the presence or absence of phenolic compounds or certain carboxylic acids.

The adjacent −OH groups form a specific complex with Fe³⁺, producing a green-blue color.

Distilled water is commonly used to dissolve the sample.

Historically yes, but modern spectroscopic methods are preferred for more accurate detection.

References of FeCl3Test

  1. Vogel, A. I. Vogel’s Textbook of Practical Organic Chemistry, 5th Edition.
  2. Furniss, B. S., Hannaford, A. J., Smith, P. W. G., & Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 4th Edition.
  3. Mann, F. G., & Saunders, B. C. Practical Organic Chemistry.
  4. Nichols, D. E. Organic Chemistry Lab Techniques.

Similar Posts