Biology

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IODINE TEST FOR STARCH
The Iodine test is used to test for the presence of starch. Iodine solution — iodine dissolved in an aqueous solution of potassium iodide — reacts with the starch producing a purple black color. The colour can be detected visually with concentrations of iodine as low as 0.00002M at 20°C. However the intensity of the colour decreases with increasing temperature and with the presence of water-miscible, organic solvents such as ethanol. Also the test cannot be done at very low pHs due to the hydrolysis of the starch under these conditions. This reaction is the result of the formation of polyiodide chains from the reaction of starch and iodine. The amylose, or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark purple/black color. The amylopectin, or branched portion of starch, forms much shorter helices and iodine molecules are unable to assemble, leading the color to be of an orange/yellow hue. As starch is broken down or hydrolyzed into smaller carbohydrate units, the purple-black color is not produced. Therefore, this test can determine completion of hydrolysis when a color change does not occur. Iodine solution will also react with glycogen, although the color produced is browner and much less intense.It is rust colored normally. BENEDICT’S TEST FOR REDUCING SUGAR

The standard chemical test for sugar is Benedict's test. The reagent, a mixture of (mainly) copper sulfate and sodium hydroxide, is called Benedict's reagent. It can be purchased from many drug stores because it was once the standard test for sugar in the urine of diabetics.

Some of the solution to be tested (this could be the juice or extract of the fruit or vegetable in question) is mixed with Benedict's test reagent (by volume, usually about 4 solution to 1 reagent) and heated almost to boiling. A color change from the blue of the reagent to almost any other color -- green, yellow, orange, red, brown -- is an indication of the...
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