How can Seliwanoff's test be used to distinguish fructose from sucrose?

Question

How can Seliwanoff's test be used to distinguish fructose from sucrose?

Answer 1

As stated in the previous post, both fructose and sucrose react.  However, the difference lies in the amount of time it takes for the reaction to occur.  A deep red colored precipitate within 5 minute indicates ketohexoses in the form of sucrose.  Sucrose gives a positive ketohexose test because of partial hydrolysis to glucose and fructose.  Other sugars (like fructose) give a red color upon prolonged heating -- 10 minutes or more.

Answer 2

it cannot since both react - see below.

Seliwanoff’s test is utilized to differentiate between aldose and ketose sugars.The reagents consist of resorcinol and conc. HCl, acid hydrolysis of polysaccharides and oligosaccharides thus yielding simpler sugars. A ketose is present if the mixture turns red. Ketoses are distinguished from aldoses via their ketone/aldehyde functionality. If the sugar contains a ketone group, it is a ketose and if it contains an aldehyde group, it is an aldose. When heated, ketoses are more rapidly dehydrated than aldoses. The dehydrated ketose then reacts with the resorcinol to produce a deep cherry red color. Aldoses may react slightly to produce a faint pink color. Fructose and sucrose are two common sugars which give a positive test. Sucrose, a disaccharide consisting of fructose and glucose, also gives a positive test. This is due to the presence of concentrated HCl, which hydrolyses sucrose into its monosaccharide subunits (glucose and fructose). Thus fructose, being a keto sugar, give positive Seliwanoff's test.