1.2.2(f) use of qualitative reagents to identify biological molecules Flashcards
what should you to to food samples before caereying at the food tests?
Food samples should be ground down or finely broken up and made into a solution or fine suspension with water before being tested. (The test for starch can also be performed by adding the iodine solution/potassium iodide directly to the solid specimen.)
positive reducing sugars test
Monosaccharides e.g. fructose, glucose and galactose and some disaccharides e.g. maltose are known as reducing sugars. When heated with Benedict’s reagent they will reduce the blue copper sulphate which is soluble to an insoluble copper oxide (red-brown) precipitate. produces a precipitate which, depending on quantity, will vary in colour from green to yellow to orange to brick red/brown
when testing for a reducing sugar, why not Do not put the boiling tube directly in a flame to heat
as the solution is likely to spit and bump violently.
other ways for testing for reducing sugars that does not include benedits egent?
Test strips are also available for testing for reducing sugars (Clinistix being one example). Instructions in or on the packets must be carefully followed. these can be very convenient in some cases,
what about disaacararides?
Disaccharides such as sucrose will give a negative result when tested with Benedict’s reagent, therefore they must be hydrolysed first, neutralised and then re-tested with Benedict’s reagent.
For this the substance e.g. sucrose solution must first be brought to the boil in a water bath with a few drops of dilute hydrochloric acid.
The solution is then neutralised with sodium hydrogen carbonate,
tested with pH paper,
and then re-tested with Benedict’s reagent as for a reducing sugar.
test for starch
Starch is relatively insoluble in water and forms a colloidal suspension. Add dilute iodine solution/potassium iodide to the substance to be tested or to the colloidal suspension. A blue-black colouration is a positive result, indicating the presence of starch.
test for lipids
The substance to be tested should be shaken well in absolute ethanol for approximately a minute, (When shaking a bung should be used.)
ethanol drained off into a test tube containing water, (leaving the substance being tested behind).
A white cloudy emulsion will appear if the test is positive.
test for proteins
The biuret solution contains potassium hydroxide solution and copper sulphate.
When added to a solution of soluble protein a mauve or purple colouration will be observed. The mauve/purple colouration indicating the presence of protein appears slowly and is not instant.
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what happens If the potassium hydroxide solution is added to the copper sulphate solution before testing
then it must be used immediately and not stored
how Better results will be achieved
if the potassium hydroxide is added to the protein solution first, and then a couple of drops of copper sulphate solution are gentle added down the side of the test tube. If a protein is present a blue ring will first appear which when gently shaken will gradually turn the solution a mauve/purple colour
