Unit 2: Qualitative Tests - Subjective Questions

Qualitative analysis is the identification of a substance based on characteristic chemical reactions rather than on the measurement of its exact amount.

Its importance includes:

• It helps determine whether a particular carbohydrate, amino acid, or protein is present in an unknown sample.
• It is useful for preliminary identification before quantitative analysis.
• It is based on observable changes such as colour formation, precipitate formation, ring formation, or gas evolution.
• It allows comparison of unknown samples with known standards.
• It is widely used in clinical diagnosis, food analysis, and biochemical research.

Qualitative tests must be performed with proper controls, clean glassware, and suitable reagents because interfering substances may produce false-positive or false-negative results.

Principle: Concentrated sulfuric acid dehydrates carbohydrates to form furfural or hydroxymethylfurfural derivatives. These compounds react with -naphthol to produce a violet or purple complex.

Procedure:

• Place about mL of the test solution in a test tube.
• Add a few drops of Molisch reagent, which contains -naphthol.
• Mix gently and carefully add concentrated sulfuric acid along the side of the inclined test tube to form a separate lower layer.

Observation: A violet or purple ring appears at the junction of the two liquid layers.

Inference: The formation of the violet ring indicates the presence of a carbohydrate. The test is general and may be positive for monosaccharides, disaccharides, and polysaccharides.

Principle: Benedict's reagent contains copper(II) sulfate, sodium carbonate, and sodium citrate. In an alkaline medium, reducing sugars reduce blue copper(II) ions to brick-red copper(I) oxide upon heating.

Procedure:

• Add approximately mL of Benedict's reagent to a test tube.
• Add about mL of the sample.
• Heat the mixture in a boiling water bath for a few minutes.

Interpretation:

• Blue solution: negative result.
• Green colour: trace amount of reducing sugar.
• Yellow or orange precipitate: moderate amount.
• Brick-red precipitate: high amount of reducing sugar.

The test is positive for glucose, fructose, lactose, and maltose, but generally negative for sucrose unless sucrose is first hydrolysed.

Both tests detect reducing sugars by reducing copper(II) ions to copper(I) oxide, but they differ in composition and preparation.

Both tests may give positive results with glucose, fructose, lactose, and maltose. Sucrose is normally negative unless hydrolysed.

Principle: Seliwanoff's reagent contains resorcinol and concentrated hydrochloric acid. Ketoses are dehydrated more rapidly than aldoses to form hydroxymethylfurfural, which reacts with resorcinol to produce a cherry-red complex.

Procedure:

• Add Seliwanoff's reagent to the carbohydrate solution.
• Heat the mixture gently in a water bath.
• Observe the colour produced and the time taken for its appearance.

Results:

• A rapid cherry-red colour indicates a ketose, such as fructose.
• Aldoses may produce a faint pink colour only after prolonged heating.
• Sucrose may give a positive result because it contains fructose, especially after acid hydrolysis.

The test should not be overheated because prolonged heating of aldoses can also produce furfural derivatives and cause misleading results.

Principle: Iodine molecules become trapped within the helical structure of amylose in starch, producing a characteristic blue or blue-black starch–iodine complex.

Procedure:

• Place a small quantity of the sample in a test tube or on a white tile.
• Add a few drops of dilute iodine solution.

Observation:

• Blue-black colour: starch is present.
• No blue-black colour: starch is absent.

When the solution is heated, the helical structure of amylose is disrupted and the iodine molecules are released, so the colour disappears. On cooling, the amylose helix reforms and iodine is again included within the helix, causing the blue-black colour to return. This test is mainly used for starch and does not generally detect simple sugars.

Principle: Barfoed's reagent contains copper(II) acetate in a weakly acidic medium. Monosaccharides reduce copper(II) ions to red copper(I) oxide more rapidly than disaccharides.

Procedure:

• Add the sample to Barfoed's reagent.
• Heat the mixture in a boiling water bath.
• Observe the formation of a precipitate within a short, specified time.

Interpretation:

• Rapid formation of a red precipitate indicates a monosaccharide.
• Delayed precipitation may indicate a disaccharide.
• Prolonged heating can cause some disaccharides to hydrolyse and produce a false-positive result.

Therefore, the timing of the reaction is essential. The test should be interpreted together with other carbohydrate tests, such as Benedict's or Seliwanoff's test.

Principle: Reducing sugars react with excess phenylhydrazine to form crystalline derivatives called osazones. The reaction involves the carbonyl group and the adjacent carbon atom of the sugar.

A simplified representation is:

Procedure:

• Mix the carbohydrate solution with phenylhydrazine hydrochloride, sodium acetate, and water.
• Heat the mixture in a water bath.
• Allow it to cool and examine the crystals microscopically.

Significance:

• The shape, size, and arrangement of crystals can help identify sugars.
• Glucose, fructose, and mannose form similar osazones because their configurations beyond carbon atom are identical.
• The test is historically important but is less commonly used today because phenylhydrazine is hazardous and modern analytical methods are more specific.

Reducing sugars possess a free aldehyde or ketone group, or can form one in solution through ring-chain tautomerism. They reduce alkaline copper(II) ions to copper(I) oxide.

Examples include:

• Glucose
• Fructose
• Lactose
• Maltose

Non-reducing sugars do not have a free reactive carbonyl group because their anomeric carbon atoms are involved in a glycosidic bond.

Example:

• Sucrose

Laboratory comparison:

• Reducing sugars give positive Benedict's or Fehling's tests.
• Sucrose generally gives a negative Benedict's test before hydrolysis.
• After acid hydrolysis, sucrose produces glucose and fructose, and the hydrolysate gives a positive Benedict's test.
• Molisch's test is positive for both reducing and non-reducing carbohydrates because it detects carbohydrates generally.

Thus, a negative Benedict's test does not prove that no carbohydrate is present; it may indicate a non-reducing carbohydrate.

Principle: Ninhydrin reacts with free amino groups of amino acids. Most amino acids undergo oxidative deamination and produce a blue-violet compound called Ruhemann's purple.

Procedure:

• Add a few drops of ninhydrin reagent to the amino acid solution.
• Heat the mixture gently in a water bath.
• Observe the colour produced after heating.

Observation: A blue, violet, or purple colour indicates the presence of an amino acid with a free primary amino group.

Exception: Proline and hydroxyproline contain a secondary amino group and generally produce a yellow or yellow-orange colour instead of Ruhemann's purple.

The test is highly useful for detecting amino acids in laboratory samples and is also used in chromatographic visualization.

Principle: Concentrated nitric acid nitrates the aromatic ring present in certain amino acid side chains, forming yellow nitro derivatives. Addition of alkali changes the colour to orange or deep yellow.

Procedure:

• Add concentrated nitric acid to the protein or amino acid solution.
• Heat carefully if required.
• Cool the mixture and add sodium hydroxide or ammonium hydroxide.

Observation:

• Yellow colour after treatment with nitric acid indicates nitration of an aromatic ring.
• Orange colour after alkalinisation supports a positive result.

Amino acids detected:

• Tyrosine gives a strong positive reaction.
• Tryptophan gives a positive reaction.
• Phenylalanine may give a weak positive reaction.

The test may be positive with proteins containing aromatic amino acid residues.

Principle: Millon's reagent contains mercuric ions in nitric acid. It reacts with the phenolic hydroxyl group of tyrosine to form a red or brick-red complex, particularly on heating.

Procedure:

• Add Millon's reagent to the sample solution.
• Heat the mixture carefully in a water bath.
• Observe the colour of the solution or precipitate.

Observation: A white precipitate that becomes red or brick-red on heating indicates a positive test.

Inference: The result suggests the presence of tyrosine or a protein containing tyrosine residues.

The test is not completely specific because other phenolic compounds may also react. Millon's reagent contains toxic mercury compounds, so it must be handled with appropriate safety precautions and disposed of as hazardous chemical waste.

Principle: The indole ring of tryptophan reacts with glyoxylic acid in the presence of concentrated sulfuric acid to form a violet-coloured condensation product.

Procedure:

• Add the sample solution to the test tube.
• Add glyoxylic acid reagent.
• Carefully layer concentrated sulfuric acid along the side of the tube to form two layers.
• Observe the interface between the layers.

Observation: A violet or purple ring at the junction indicates a positive test.

Inference: The result indicates the presence of tryptophan in a free amino acid or protein sample.

The test requires careful handling because concentrated sulfuric acid is highly corrosive. The acid should be added slowly, and the test tube should not be pointed toward anyone.

Principle: The guanidino group of arginine reacts with -naphthol and an oxidizing agent, commonly sodium hypobromite, to produce a red-coloured compound.

Procedure:

• Add the sample solution to a test tube.
• Add -naphthol reagent.
• Add the oxidizing reagent carefully and mix.
• Observe the colour immediately.

Observation: Formation of a red or cherry-red colour indicates a positive reaction.

Inference: The sample contains arginine or a protein containing arginine residues.

The reaction is based on the strongly basic guanidino group of arginine. Excess oxidizing reagent and prolonged standing may affect the colour, so the result should be observed promptly and compared with a suitable control.

Principle: On boiling with a strong alkali, the sulfur in cysteine and cystine is converted into sulfide. The sulfide reacts with lead acetate to form black or brown lead sulfide.

Procedure:

• Heat the sample with sodium hydroxide.
• Cool the mixture slightly and add lead acetate solution.
• Observe the colour of the precipitate.

Observation: A black or brown precipitate of lead sulfide indicates sulfur derived from sulfur-containing amino acids.

Distinction:

• Cysteine usually gives a positive test because its sulfur is readily released as sulfide.
• Cystine may give a weak or delayed positive reaction because its sulfur is involved in a disulfide bond.
• Methionine is generally negative under these conditions because its thioether sulfur does not readily form sulfide.

Lead compounds are toxic and must be handled and discarded safely.

Proteins are high-molecular-mass biological polymers composed of amino acids joined by peptide bonds. The Biuret test detects compounds containing two or more peptide bonds.

Principle: In an alkaline medium, peptide bonds coordinate with copper(II) ions to form a violet or purple copper–peptide complex.

Procedure:

• Add sodium hydroxide to the protein solution to make it alkaline.
• Add a few drops of dilute copper sulfate solution.
• Mix gently and observe the colour.

Observation: A violet or purple colour indicates a positive Biuret test.

Inference: The result indicates the presence of proteins or peptides with at least two peptide bonds. Free amino acids generally do not give a positive Biuret reaction because they lack sufficient peptide bonds.

Denaturation is the disruption of the native secondary, tertiary, or quaternary structure of a protein without breaking most of its peptide bonds. Heat increases molecular vibration and disrupts weak interactions such as hydrogen bonds, ionic interactions, and hydrophobic associations.

Procedure:

• Place the protein solution in a test tube.
• Heat it gradually in a water bath or over a flame.
• Observe turbidity or precipitate formation.

Observation: The solution may become cloudy and form a white coagulum or precipitate.

Factors affecting denaturation and precipitation:

• Temperature and duration of heating.
• Protein concentration.
• pH of the solution.
• Presence of salts or organic solvents.
• Nature and stability of the particular protein.

Some denatured proteins can be resolubilized, but heat coagulation is often irreversible, especially in albumin.

Proteins can be precipitated when their charges are neutralized or when they form insoluble complexes with certain reagents.

Precipitation by acids:

• Strong acids such as trichloroacetic acid or sulfosalicylic acid denature proteins and reduce their solubility.
• A visible white precipitate or turbidity is produced.
• At the isoelectric pH, the net charge of a protein is approximately zero, and solubility is usually minimal.

Precipitation by heavy metals:

• Heavy metal ions bind to negatively charged groups in proteins, including carboxylate and sulfhydryl groups.
• Insoluble metal–protein complexes are formed.

A simplified reaction is:

Examples include precipitation by mercuric chloride, lead acetate, and silver nitrate. Heavy metal reagents are toxic and require careful disposal.

The Biuret test is mainly a test for peptide bonds, whereas the ninhydrin test is mainly a test for free amino groups.

Interpretation of observations:

• The violet ring in Molisch's test indicates that the unknown is a carbohydrate.
• The brick-red precipitate in Benedict's test indicates that it is a reducing sugar.
• The rapid cherry-red colour in Seliwanoff's test indicates that it is a ketose rather than an aldose.

The most likely carbohydrate is fructose, which is a reducing ketohexose. Although fructose contains a ketone group, it can isomerize under alkaline conditions and reduce Benedict's reagent.

The results should be confirmed using a known fructose standard and appropriate negative and positive controls. Sucrose could also give a Seliwanoff-type reaction after hydrolysis, but it would normally be non-reducing before hydrolysis and therefore would not be expected to give the brick-red Benedict's result in the untreated sample.