Seliwanoff’s test is a type of chemical test that used to differentiate between ketose and aldose sugars. If the sugar has a ketone group, then it’s ketose. If a sugar has an aldehyde molecule, it is an aldose. This test is based on the idea that in the presence of heat, ketoses become more quickly dried than aldoses. The test is named in honor of Theodor Seliwanoff, the chemist who invented the test. If added to a mixture with ketoses, the color red appears quickly, indicating that the test is positive. When added to a mixture that contains aldoses, a slower developing light pink color is seen instead.
Objectives of Seliwanoff’s test
- To determine the presence of ketohexoses within the sample.
- To differentiate between ketoses and aldoses.
Principle of Seliwanoff’s test
This test can be used to identify ketose and aldose sugars. This test utilizes hydrochloric acid as a dehydrating agent, and the agent resorcinol to condense. When treated with HCL ketoses are dehydrated more quickly, giving a the furfural variant (5-hydroxymethyl fufural) that is then condensed by the chemical resorcinol, resulting in a cherry-red color compound. Aldoses react to create the identical product but with a slower rate producing a faint pink to yellow color complex.
Requirements for Seliwanoff’s test
- Reagent: Seliwanoff’s reagent, add 0.05% resorcinol (m-hydroxybenzene) in 3 N HCl. Dissolve 50 mg of resorcinol into 33 milliliters of concentrated HCl and then make it into 100ml by adding water.
- Materials: Test tubes, Test tube stand, Pipettes
- Equipment: Water bath
Procedure of Seliwanoff’s test
- In a clean, dry test tube, add 1ml of sucrose solution at 5% (ketose).
- In the second test tube, add 1 Ml of 5% glucose solution (aldose).
- For each tube, add 3ml of Seliwanoff’s Reagent and mix thoroughly.
- Make sure both tubes are in the hot water bath for 2 minutes.
- Examine the appearance of red-colored cherries for the sucrose (positive outcome).
Results of Seliwanoff’s test
- Positive Test: If the color changes to red, the Seliwanoff’s test is positive which means ketosugar (Fructose along with Sucrose) exists in the solution.
- Negative Test: If you find that no red color or even a slight pink tint is visible the result is negative and indicates that Aldose sugar (Glucose) is present in the solution.
Applications of Seliwanoff’s test
- The method employed for the determination of the colorimetric value of fructose concentration in fermentation media uses Seliwanoff’s colour reaction.
- The ketose content in a sample can be measured by using a modified version of Seliwanoff’s test.
Limitations of Seliwanoff’s test
- A high level of sugars such as glucose could interfere with Seliwanoff’s test reaction by forming similar color molecules.
- Long-term boiling is able to transform glucose into fructose through the catalytic effect of acid, leading to the formation of a cherry red-complex, and false-positive results.
- It is a broad test that doesn’t differentiate among different ketoses. A second test is needed to determine the exact ketose sugar.