(Prelims) BioChem Lab #3

Molisch Test

General test for carbohydrates where concentrated sulfuric acid dehydrates sugar to furfural, which reacts with α-naphthol to form a violet ring.

Reducing Sugar

Any sugar that is capable of acting as a reducing

agent because it has a free aldehyde group or a free ketone group.

All monosaccharides, along with

some disaccharides, oligosaccharides, and polysaccharides.

Example of Reducing Sugars

Moore's Test

Heating a reducing sugar with alkali produces enediols, leading to a brown color and caramel odor.

Fehling's Test

Detects reducing sugars by reduction of cupric (Cu²⁺) ions to red cuprous (Cu⁺) oxide precipitate - which indicates the presence of reducing sugars.

Fehling's Test

Detection method for monosaccharides,

specifically aldoses and ketoses.

Fehling's A Solution

Blue aqueous solution of Copper (II) sulfate

Fehling's B Solution

- Potassium sodium tartrate (Rochelle salt)

- Sodium hydroxide in water

Positive Fehling's Test

Reddish brown precipitate

Negative Fehling's Test

No red precipitate

Benedict's Test

Detects reducing sugars; color changes from blue → green → yellow → orange → brick-red depending on sugar concentration.

Benedict's Test

Used to test for simple carbohydrates that

identifies reducing sugars (monosaccharides and

some disaccharides), which have free ketone or

aldehyde functional groups.

Benedict's Test

Reducing sugars are heated in the presence of an alkali

they are converted to powerful reducing species

known as enediols.

Positive Benedict's Test

Formation of a reddish precipitate within three minutes.

Negative Benedict's Test

No color change (Remains Blue).

Barfoed's Test

Differentiates monosaccharides (react quickly, 1-2 minutes) from reducing disaccharides (react slowly, 7-12 minutes) using cupric acetate in acidic medium.

Barfoed's Test

Brick red color is obtained in this test which is due to

formation of cuprous oxide.

Nylander's Test

Reducing sugars reduce bismuth salts in alkaline solution to form black metallic bismuth.

Nylander's Test

A chemical test used for detecting the presence of reducing sugars.

Positive Nylander's Test

When Nylander's reagent is added to a solution with reducing sugars, a black precipitate of metallic bismuth is formed.

Picric Acid Test

Reducing sugars reduce yellow picric acid to red picramic acid in alkaline solution.

Picric Acid Test

The reducing sugars react with Picric Acid (toxic yellow

crystalline solid) also chemically known as 2,4,6-

trinitrophenol (TNP) to form a red coloured Picramic Acid.

Positive Tollens' Test

Aldehydes (like reducing sugars) reduce ammoniacal silver nitrate to metallic silver, producing a "silver mirror."

Tollens' Test

Determines the presence of aldehyde, aromatic aldehyde, and alpha-hydroxy ketone functional groups.

Tollens' Test

The reagent consists of a solution of silver nitrate, ammonia and some sodium hydroxide (to maintain a basic pH of the reagent solution).

False

It results in a VIOLET ring.

True or False

A positive Molisch Test results in a cyan ring.

True

True or False

All monosaccharides are reducing sugars.

False

A BLACK precipitate of metallic bismuth is formed.

True or False

When Nylander's reagent is added to a solution with reducing sugars, a gray precipitate of metallic bismuth is formed.

True

True or False

Reducing sugars reduce picric acid (yellow solution) to

picramic acid (mahogany red solution).

False

Formation of a REDDISH precipitate within three minutes.

True or False

A positive Benedict's test is the formation of a pink precipitate within three minutes.