What Is Seliwanoff’s Test?
Seliwanoff’s test is a chemical test which differentiates between Aldose and ketose sugars. If the sugar contains a ketone group, it is a ketose whereas if it contains an aldehyde group, it is an Aldose. This test is much like Bial’s test; it relies on the principle that, when heated, ketoses are more rapidly dehydrated than Aldoses. It is named after the Russian chemist Theodor Seliwanoff who devised the test in 1887.
- The main objective of Seliwanoff’s test is to distinguish between sugars that have a ketone group (ketose) and sugars that have an aldehyde group (Aldoses).
Principle Of Seliwanoff’s Test
Seliwanoff’s test is specific to ketonic saccharides (fructose or sucrose).To differentiate between aldehydic and ketonic saccharides, a ketose is heated with a strong mineral acid. When concentrated hydrochloric acid is added to a sample containing ketose sugar, ketoses undergo dehydration to yield furfural derivatives more rapidly than Aldoses. These derivatives (dehydrated ketose) then react with equivalents of resorcinol in a series of condensation reactions to produce complexes with a deep cherry red color. The test reagent causes the dehydration of ketohexoses to form 5-hydroxylmethylfurfural, which then reacts with resorcinol present in the test reagent to produce a red product within two minutes. Aldohexoses reacts so more slowly to produce a faint pink color. Fructose and sucrose are two common sugars which give a positive test. Sucrose gives a positive test as it is a disaccharide consisting of fructose and glucose.
Reagent And Material Required
- Seliwanoff’s reagent (0.05% resorcinol (m-hydroxybenzene) in 3 N Hydrochloric acid (HCl).
- Test solution (5% Glucose, 5% Sucrose, 5% Fructose).
- Distilled water.
- Test tubes
- Test tube stand
- Water bath
- Take two clean, dry test tubes and add 1 ml of the test sample in one test tube and 1 ml of distilled water in another as a control.
- Add 2 ml of Seliwanoff’s reagent to both test tubes.
- Keep both test tubes in a water bath for 5 minutes.
- Observe any color change and note it down.
Seliwanoff’s Test Result Interpretation
- Positive Test: The solution changes to cherry red-color within the first 2 minutes of heating. This indicates that the sample contains ketoses.
- Negative Result: The color of solution remains unchanged or absence of cherry red color after prolonged period of heating. This indicates that the test sample does not contain ketoses.
Limitation Of Seliwanoff’s Test
- High concentration of glucose or fructose in a test sample may corrupt the result by producing similar cherry red color on heating with Seliwanoff’s reagent.
- Seliwanoff’s test is a generalized test and cannot distinguish between specific types of ketoses. Usually a separate test is required to further classify the ketoses sugar.
- Long periods of boiling and the catalytic effect of the acid can transform glucose to fructose and form cherry red-color hence giving a discordant or false result.