What is the Seliwanoff test for glucose and fructose?

December 8, 2025 •

4 min read

The Seliwanoff test is a qualitative chemical test used since the 19th century to differentiate ketose sugars from aldose sugars. It is particularly effective at distinguishing fructose, a ketose, from glucose, an aldose, based on their distinct reaction times and color changes with the Seliwanoff's reagent.

Quick Summary

This chemical test helps distinguish ketoses like fructose from aldoses such as glucose. It uses concentrated acid and resorcinol to produce a rapid, deep cherry-red color for ketoses, while aldoses react much slower to produce a faint pink color or none at all.

Key Points

Ketose vs. Aldose: The Seliwanoff test distinguishes ketose sugars (like fructose) from aldose sugars (like glucose) based on their different reaction speeds with Seliwanoff's reagent.
Positive Test (Fructose): Fructose, a ketose, rapidly reacts with the reagent when heated, producing a deep cherry-red color within a few minutes.
Negative/Weak Positive Test (Glucose): Glucose, an aldose, reacts very slowly and will either remain colorless or develop a faint pink color only after prolonged heating.
Key Reagents: The test uses Seliwanoff's reagent, a mixture of resorcinol and concentrated hydrochloric acid (HCl).
Dehydration Process: The concentrated HCl dehydrates the sugar. Ketoses dehydrate faster to form 5-hydroxymethylfurfural, which then reacts with resorcinol to produce the red color.
False-Positive Risk: The acid can convert glucose to fructose if the heating is too long, leading to a false-positive result for ketoses.
Sucrose Result: Sucrose, a disaccharide of glucose and fructose, will also give a positive result because the acid hydrolyzes it into its constituent monosaccharides.

Seliwanoff's Test Explained

The Seliwanoff test is a classic biochemical assay that utilizes a specific chemical reaction to categorize sugars based on their structural differences. The core principle lies in the differing reaction rates of ketoses and aldoses when heated with a strong acid. At the heart of this test is the Seliwanoff's reagent, a solution composed of resorcinol and concentrated hydrochloric acid (HCl).

The Chemical Principle

When a sugar solution is heated with the Seliwanoff's reagent, the concentrated HCl acts as a dehydrating agent. Sugars, being carbohydrates, are dehydrated to form specific derivatives:

Ketoses (like fructose): Due to the presence of an internal ketone group, ketoses undergo rapid dehydration to form 5-hydroxymethylfurfural. This derivative then condenses with the resorcinol in the reagent, producing a deep cherry-red complex. The reaction occurs quickly, typically within one to two minutes of heating.
Aldoses (like glucose): With a terminal aldehyde group, aldoses are dehydrated much more slowly under the same conditions. Any dehydration that does occur produces furfural, which also reacts with resorcinol, but the reaction is significantly slower. As a result, aldoses will either produce no color change or, if heating is prolonged, a faint pink color that is much less intense than the ketose reaction.

This key difference in reaction speed and color intensity is the basis for differentiating between the two sugar types.

Performing the Seliwanoff's Test

For a typical laboratory procedure, the following steps are followed:

Take two clean test tubes. Add 1 ml of the sample sugar solution to one and 1 ml of distilled water to the other as a control.
Add 2 ml of Seliwanoff's reagent (resorcinol and HCl) to both test tubes.
Place both tubes in a boiling water bath for a specified period, usually one to two minutes.
After the heating period, observe and compare the color changes in both tubes.

This simple procedure provides a clear qualitative result.

Result Interpretation for Glucose and Fructose

The Seliwanoff test produces distinct visual results for fructose and glucose, allowing for their differentiation:

Positive Result (Fructose): A rapid formation of a deep cherry-red color indicates the presence of a ketose like fructose.
Negative/Weak Positive Result (Glucose): A clear solution, or one that develops only a very faint pink color after prolonged heating, indicates the presence of an aldose like glucose.

Comparison: Seliwanoff vs. Other Sugar Tests

To understand the specificity of the Seliwanoff test, it is helpful to compare it to other common sugar tests, such as Benedict's test.


Feature	Seliwanoff's Test	Benedict's Test
Primary Function	Distinguishes between ketoses and aldoses.	Detects the presence of reducing sugars.
Key Reagent	Resorcinol and concentrated hydrochloric acid.	Copper sulfate in an alkaline solution (sodium citrate and sodium carbonate).
Result for Fructose	Positive (rapid, deep cherry-red color).	Positive (reducing sugar).
Result for Glucose	Negative (no color or faint pink over time).	Positive (reducing sugar).
Specificity	Specific to the functional group (ketone vs. aldehyde).	Non-specific, identifies any sugar with a free anomeric carbon.
False Positives	Can occur with prolonged heating of aldoses.	Some disaccharides react slower than monosaccharides.
Sucrose Result	Positive (as it hydrolyzes to fructose and glucose).	Negative (non-reducing sugar).

Limitations and Considerations

While the Seliwanoff test is a valuable tool, it is not without its limitations:

False-Positive Results: The most common issue is a false-positive result for aldoses like glucose. If the heating time is extended too long, the concentrated acid can catalyze the conversion of glucose into fructose, which will then react to produce the characteristic cherry-red color. This makes strict adherence to the one-to-two-minute heating time crucial for accurate results.
High Sugar Concentration: A high concentration of glucose can also interfere with the test, producing compounds that may cause a color change similar to a positive ketose result.
Non-Specificity for Ketoses: The test is not specific to just fructose; any ketose will produce a positive result. Therefore, it cannot be used to specifically identify fructose if other ketose sugars are present in the sample.
Qualitative, not Quantitative: The test provides a qualitative result (presence or absence of a ketose) but does not provide information on the concentration of the sugar in the sample.

Conclusion

The Seliwanoff test is a simple and effective qualitative method for distinguishing ketoses from aldoses, with fructose and glucose serving as classic examples. By leveraging the difference in reaction speed and color formation, the test reliably identifies the presence of fructose based on the rapid formation of a deep cherry-red color. However, accurate interpretation requires careful attention to the heating duration to avoid false positives caused by the conversion of aldoses to ketoses. For definitive identification of specific sugars, the Seliwanoff test is best used in conjunction with other carbohydrate identification tests.

Frequently Asked Questions

The key difference is their functional group: glucose is an aldose with a terminal aldehyde group, while fructose is a ketose with an internal ketone group. This structural difference causes fructose to dehydrate much faster than glucose when heated with the reagent.

Fructose reacts more quickly because its ketone functional group is more easily dehydrated by the concentrated hydrochloric acid than the aldehyde group of glucose. This rapid dehydration leads to the rapid formation of 5-hydroxymethylfurfural, which produces the characteristic cherry-red color.

Yes, sucrose can give a positive result. Sucrose is a disaccharide made of glucose and fructose. The concentrated acid in the reagent first hydrolyzes sucrose into its monosaccharide components, including fructose, which then reacts to produce the red color.

A false-positive can occur if the mixture is heated for too long. Extended heating can cause the acid to catalyze the isomerization of glucose (an aldose) into fructose (a ketose), which will then react to give a positive cherry-red color.

A negative result for ketoses is indicated by a clear or transparent solution, or the appearance of only a very faint pink color after the standard heating time. This suggests the presence of an aldose or a non-carbohydrate.

The reagents for the Seliwanoff test, collectively known as Seliwanoff's reagent, are resorcinol and concentrated hydrochloric acid. The resorcinol is typically dissolved in the HCl to create the working reagent.

The Seliwanoff test is a qualitative test. It provides a visual indication of the presence or absence of ketoses but does not measure the exact concentration of the sugar in the solution.