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picric acid

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by

Gezer Gandeza

on 9 May 2014

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Transcript of picric acid

Picric Acid Test
what is picric acid test?
analysis
what is the principle behind picric acid test ?
procedures
what is a picric acid?
Picric acid is a chemical compound formally called 2,4,6-trinitrophenol (TNP).

*a toxic yellow crystalline solid and one of the most acidic phenols.
let's take a closer look
The picric acid test for carbohydrates is a very sensitive chemical test for the presence of carbohydrates, and is based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde. It is stated that it condenses with 2 molecules of phenol.
it is a test for reducing sugars
heat to boiling. do the same with all the different types of sugar soln.
write down the reactions involved
to 1 ml. of 5% glucose solution add 1 ml of saturated picric acid soln. and about 5 drops of 10% sodium carbonate
why saturated ?
picric acid in 1.3% H2O(saturated)
- used as a colorimetric reagent
- used as a general cytoplasmic stain
why add sodium carbonate? (Na2Co3)
to make the solution alkaline or basic.
All monosaccharides and disaccharides containing the potentially free aldehyde or ketone group possess reducing properties.

They reduce some organic acids when in alkaline solution.
Thus, reducing sugars reduce picric acid (yellow solution) to picramic acid (mahogany red solution).
Positive result for Picric acid test indicates the presence of
reducing sugars
picric acid
picramic acid
tabulated results
sugar soln.
theoretical result
observed results
glucose

galactose

maltose

lactose

fructose

sucrose
+ (brick red or mahogany red)
+ (red)

+(red)

+ (red)

+ (red)

+ (red)

- (yellow/orange)
BUT WHY??
why do sucrose does not have the reducing capacity when it is a disaccharide ?
Maltose is made up of two glucose units while sucrose is made up of glucose and fructose.
In the ring structures of sucrose and maltose, you have an anomeric carbon. this is the carbon which was hydrolyzed in the straight-chain structure.
Maltose's anomeric carbon is "free" and can therefore open up the ring and reduce the metal ion.

On the other hand, sucrose's anomeric carbon is not "free" since this carbon is used to link fructose and glucose together. therefore, this anomeric carbon can't open up the ring structure and react with the reagent.
maltose
sucrose
Full transcript