Qualitative Tests, Structure and Uses of Formaldehyde, Paraldehyde, Acetone : Pharmaguideline

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Qualitative Tests, Structure and Uses of Formaldehyde, Paraldehyde, Acetone

The molecular weight of aldehydes and ketones are low, making them volatile compounds.

Qualitative tests

The molecular weight of aldehydes and ketones are low, making them volatile compounds. Aldehydes and ketones are identified by two different types of reactions: the double bond addition and the oxidation reactions. In aldehydes, a hydrogen atom is attached to the carbonyl group along with aliphatic or aromatic radicals. The carbonyl atom that is present in formaldehyde is attached to two hydrogen atoms. Ketones are organic compounds containing two aliphatic or aromatic groups attached to the carbonyl group.

Schiff’s test

Often, the Schiff reagent is the product of dye formulation reactions, including those involving sodium bisulphite and fuchsine. Aldehydes can be detected using the Schiff reagent. It is called fuchsine or rosaniline hydrochloride because it has a chemical formula of C20H20N3•HCl, which is decoloured by sodium bisulphate. An aldehyde group is indicated by the presence of pink colour.

Fehling's test

The Fehling test is performed with a fresh solution, which is usually prepared in laboratories. A blue copper (II) sulphate solution is Fehling's A. Fehling's B is a white copper (II) sulphate solution. In Fehling's B, potassium sodium tartrate (Rochelle salt) combines with an alkali, usually sodium hydroxide, to form a clear liquid. Solutions A and B are prepared separately. Cu2+ bis(tartrate) complex is the deep blue ingredient. The reduction of Cu2+ to Cu+ occurs when Fehling's solution is added to an aldehyde compound, resulting in the reduction of the aldehyde to acids. An orange precipitate forms in this reaction.

Tollen's test

Sugar reducers and non-sugar reducers are distinguished using the Tollens test. Silver mirror tests are a variation of this test. Tollen's reagent is precipitated into a grey-black precipitate or a silver mirror by aldehydes. It is always best to use freshly prepared reagent from Tollens. The Tollens reagent involves the oxidation of aldehydes to their corresponding acids, and the reduction of silver to the elemental state from the +1 oxidation state. This test usually does not detect ketones.

RCHO + 2[Ag(NH3)2]OH → R-COONH4+ 3NH3 + H2O + 2Ag↓(silver mirror)

Test with chromic acid

As aldehydes react with chromic acid, the result is a green or blue precipitate. Neither ketones nor chromic acid reacts with ketones. Alcohols of some form do not react with chromic acid.

R-CHO + 2CrO3 + 3H2SO4 → 3R-C(O)-OH + 3H2O + Cr2(SO4)3(green colour)

Sodium nitroprusside test

Aldehydes do not give this test, only ketones. A ketone reacts with an alkali and forms an anion, this anion then reacts with sodium nitroprusside to produce a coloured complex. This complex indicates that ketones are present.

CH3 + OH–→ CH3COCH2-+ H2O

[Fe(CN)5NO]2- + CH3COCH2- → [Fe(CN)5NO.CH3COCH2]3-



The compound formaldehyde belongs to the family of volatile organic compounds, which evaporate at room temperature and become gaseous. In the series of aliphatic aldehydes, formaldehyde is the first reactive molecule. Formaldehyde is an important chemical in industry. In addition to existing as a gas, formaldehyde can be dissolved in water and other solvents. Among the many applications of formaldehyde, there are a variety of natural forms.

The structure of an organic compound is just composed of the three simplest elements, namely carbon, hydrogen, and oxygen. The formula for this compound is HCHO. When condensed, this formula becomes CH2O.


  • The resins produced by the process are urea formaldehyde and phenol formaldehyde, which are used to create foundry cores and moulds.
  • Farming and medicine use it for disinfectants, fungicides, fumigants and preservatives.
  • A wide range of products are manufactured with formaldehyde, including building materials.

Qualitative test

Weigh 2g of each sample and added 2 mL of Schiff's reagent and 1 mol L-1 sulfuric acid to determine the composition of the formaldehyde in the hair straightening samples. After shaking, a purple complex was immediately formed proving the identification of the compound.

The 13 samples are analyzed qualitatively in triplicate according to the method for formaldehyde identification. Each sample tested will develop a pink / purple color in the presence of the reagent, therefore, indicating the presence of formaldehyde. Different samples will produce different colors, probably due to different levels of formaldehyde. However, despite the aforementioned results, this qualitative analysis does not indicate that commercial formulations may be irregular, as formaldehyde can be used in low concentrations as an antimicrobial preservative. To obtain quantitative information about them, a quantitative analysis was conducted.



Paraldehyde are cyclic trimers of acetaldehydes. The methyl group replaces the hydrogen atoms at each carbon in this 1,3,5-trioxane derivative. The tetramer corresponding to this compound is metaldehyde. Colorless liquid with a low solubility in water and a high solubility in ethanol! The oxidation of paraldehyde in the air turns it brown and gives it an acetic acid smell. When it comes into contact with rubber and plastics, it quickly reacts.


Certain convulsive disorders can be treated with paraaldehyde. Besides its use in alcoholism treatment and in treating nerve and mental disorders, it is also used to calm or relax nervous or tense patients, and to promote sleep.

Qualitative test

All aldehydes and ketones readily form 2,4-dinitrophenylhydrazones, which are bright yellow to deep red in color. In the case of aldehydes or ketones that have been conjugated with double bonds or aromatic rings, yellow derivatives are formed. Add 0.5mL of ethanol to 10 mg of paraaldehyde, and then add 0,75mL of 2,4-dinitrophenylhydrazine reagent. Pour into a bowl and mix thoroughly. If a yellow to red precipitate forms, the test is positive.

Reagent - 2,4-Ditrophenylhydrazine (1,5 g) in concentrated sulfuric acid 7.5 mL is dissolved in the acid. Stir this solution into the mixture of ethanol and water, 10 mL each.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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