[Music] hi and welcome back to free science lessons by the end of this two part video you should be able to describe how to use a colorimeter to determine the concentration of glucose in a solution now i need to tell you that this can appear quite tricky you might want to watch this a couple of times to get the idea in the last video we saw how to use benedict solution to determine the presence of a reducing sugar such as glucose and we saw the color produced gives us a very approximate idea of the level of reducing sugar present for example a brick red color shows a greater level of reducing sugar than a green color because the benedict's test only gives a very approximate idea of the level of reducing sugar scientists say that it's semi-quantitative so in this video we're going to look at how we can use the benefits test to determine the concentration of a reducing sugar more accurately we're going to start by recapping the science behind the binance test benedict's reagent is a blue solution and this blue color is caused by the presence of the copper ion cu2 plus reducing sugar such as glucose donate an electron reducing the cu2 plus ion to the cu1 plus ion and this forms a red precipitate the red precipitate plus the blue of the benedict solution leads to the color changes that we've already seen now if we left our test tube for several hours then the red precipitate would settle on the bottom and this would allow us to see the blue benedict solution above it now the key idea you need to understand is that this benedict solution will be less blue than it was before the test that's because some of the copper ii plus ions have reacted and are no longer in solution and if we did the test with a greater concentration of glucose then the benedict solution would appear even less blue at the end as even fewer copper ii plus ions would remain in solution so bear in mind that with the benefits test the greater the concentration of glucose the less blue the benedict solution will be at the end now these changes in the blueness of the solution may be too subtle to detect by eye so we quantify the blueness of the solution by using a machine called a color emitter before we use the colorimeter we first need to filter off the red precipitate leaving just the blue benedict solution coming up we look at how a colorimeter works [Music] okay let's look at how a colorimeter works i've got here a lamp which is emitting white light remember that white light consists of all of the different colors of the spectrum if we shine white light through a sample of benedict solution then all of the colors in white light will be absorbed apart from blue that's because benedict's solution allows blue light to pass through and that's why benedict's solution has a blue color now this absorption of light can be used to quantify the level of blueness but rather than using white light the best color to use is actually red that's because benedict's solution absorbs red the most out of all the different colours as red is on the opposite end of the spectrum to blue scientists say that red is the complementary colour to blue so now we place a red filter in front of our lamp and this red filter only allows red to pass through okay now in this diagram i'm using a benedict solution which is less blue than before this could be benedict solution which has reacted with glucose and has lost some of its blue color in this case because the solution is less blue than before less of the red light will now be absorbed meaning that some of the red light will be transmitted and this red light can be detected by the photoelectric cell a photoelectric cell is just a type of light detector here's a sample of benedict's solution which is very pale blue so this sample could have been reacted with a relatively large amount of glucose in this case even less red light is absorbed by the solution so as you can see the less red light is absorbed the greater the amount of glucose that must have reacted with our benedict solution so by using the benedict's test with a colorimeter we can more accurately determine the concentration of glucose in a sample and we look at how to do that in the next video [Music]