Tests for substances

BIURET TEST FOR PROTEINS. BIURET TEST FOR PROTEINS.

Two stages to the test:

1. The test solution needs to be alkaline, by adding sodium hydroxide solution.

2. Then add copper sulphate solution

• If protein is present a purple layer forms
• If there is no protein, the solution will remain blue.

Reducing sugars, all include monosaccharides and some disaccharides, e.g maltose.

• Add benedicts reagent (which is blue), to a sample and heat it. If the sample contains reducing sugars it gradually turns brick red due to the formation of red precipitate.

Non reducing sugars, like sucrose have to be broken down in order to test them.

• Boil the test solution with dilute hydrochloric acid - neutralise with sodium hydrogencarbonate
• add benedicts solution
• if the result is positive, do the "reducing sugars" test.

IODINE TEST FOR STARCH. IODINE TEST FOR STARCH.

• Add iodine dissolved in potassium iodide solution to the test sample.
• If starch is present, the sample changes from browny-orange to blue-black

ALWAYS REMEMBER IODINE IN POTASSIUM IODIDE SOLUTION -  NOT JUST POTASSIUM!

Shake the sample with ethanol for about a minute.

Then pour the solution into water

If lipids are present a milkly emulsion will be produced.

The greater the presence of lipds, the more milky the substance will appear.