How does DCPIP work? Please print this page before reading it. You need to read it while watching the animations. Indicators Indicators are substances that can be used to indicate the presence of certain substances. In the previous chapter we discussed iodine, Benedicts’ solution and the Biuret test (cupper sulphate and sodium hydroxide). In the chapter on respiration and breathing you’ll learn about another indicator called lime water, a substance used to indicate carbon dioxide. In the Eureka Lab assignment you’ll be using another one called DCPIP. In this story I’ll show you how DCPIP works. Instead of turning blue, red or purple, DCPIP looses its colour. Iodine Let’s take a look at a normal indicator, let’s say iodine. - Open the animation “iodine and starch”. Press play to start. The animation starts with a number of white circles. These circles represent starch particles of a solution in test tube. To this solution iodine is added. The yellow circles coming down represent the mixing of these two solutions. The iodine combines with starch and together they turn purple. The test tube on the left contained little starch and therefore turns light blue. The test tube on the right contained more starch and turned dark blue. DCPIP - Open the animation “how does DCPIP work?”. You can see two test tubes, filled with DCPIP. DCPIP is a blue coloured substance (the blue circles). One droplet of vitamin C is added to the test tube with DCPIP. - Press play. When a vitamin C particle comes into contact with a DCPIP particle, the DCPIP looses its colour. Not all particles in the solution loose their colour. Add another droplet. - Press play again. All DCPIP particles are decolourised now: the solution turns transparent. When indicator DCPIP looses its colour, vitamin C is present. Calculating the vitamin C concentration How can this test be used to determine the vitamin C concentration. - Open the animation “DCPIP and vitamin C”. There are two test tubes with DCPIP (A and B). - Press play. Droplets is added to both test tubes. Test tube A: a droplet of a known vitamin C solution. Test tube B: a droplet of an unknown solution. Some decolourisation takes place. - Press play. A second droplet is added. DCPIP solution A is decolourised entirely. Test tube B is not decolourised yet. - Press play. A third droplet is added. DCPIP solution B looses its colour as well. What does this mean? The vitamin C concentration of solution B is lower than that of solution A. The number of vitamin C particles needed for decolourisation is present in two droplet in solution A. The same number of particles is present in three droplets of solution B. That is less particles per droplet ... Therefore, the concentration in solution B is lower than in solution A. How much lower? Solution A: 2 droplets. Solution B: 3 droplets. The vitamin C concentration of solution B is 2/3 of that of solution A.
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