[MUSIC] Well, if we have a stain on a suspect or at a crime scene that we suspect might be blood, how do we demonstrate that it is blood? Well, there's a number of quite simple chemical tests which we can use as presumptive tests for blood. One of the old ones is called the Benzidine test. This tends to be not used anymore because of the toxicity of the chemicals involved. An excellent test that gives a nice colour reaction is the Kastle-Meyer test using phenolphthalein, and another very good reagent for testing for blood is Luminol. All of these tests are based on the chemical property of blood, that it can catalyze the break down of hydrogen peroxide to water and oxidizing species. [BLANK_AUDIO] Let's take a look at the Kastle-Meyer test. What you need for doing the Kastle-Meyer test is reduced phenolphthalein. Now on the left, this is phenolphthalein in its acidic form, which is a colourless material. But in alkali, phenolphthalein loses a proton and turns into its alkali form, which is a very bright pink colour. Now if this alkaline form is reacted with zinc powder, it's reduced, and this reduced form is, once again, colourless. Now, in the presence of an oxidizing agent, that reduced form is converted back into the bright pink form. So if we give it hydrogen peroxide, this is an oxidizing agent, but hydrogen peroxide alone cannot turn the reduced form back into the pink alkali form. What it needs is a catalyst and one excellent catalyst for this reaction is blood. So if we have a suspected blood stain, we treat it with reduced phenolphthalein and hydrogen peroxide, and we see the colour change to pink, then it probably is blood. But it is still only a presumptive test for blood, it doesn't prove it's blood. It just says, it's most likely to be blood. This is because there are other materials, not just blood, that can catalyze this reaction. For instance, a potato, it turns out that there are very powerful enzymes in potatoes which are excellent catalysts for this reaction. So, potatoes are one example of something that can give what we call a false positive in the Kastle-Meyer test. There's a mysterious stain on this piece of tissue paper here. Could be a blood stain, but then, old blood stains just look like brown stains, so it could also be paint or chocolate or something like that. But what we are going to do is to use the Kastle-Meyer test to find out if that is a blood stain. This is just a little bit of ethanol. This is the Kastle-Meyer reagent from reducing alkaline phenolphthalein with zinc, and you can see a little bit of the zinc in the bottom of the vial. [BLANK_AUDIO] And you notice, when we add the Kastle-Meyer reagent, nothing happens. That's because we haven't added the oxidizing agent, which is hydrogen peroxide. And this isn't the dilute kind of hydrogen peroxide that some people might use to bleach their hair. This is 30% hydrogen peroxide, and this will burn holes in your epidermis. And you immediately see the pink colour, as the hydrogen peroxide re-oxidizes the reduced form of phenolphthalein, a reaction that is catalyzed by blood. So probably, that brown stain on that tissue is blood. Well, the Kastle-Meyer test is not perfect because a number of substances will also give positive results. That is so-called "false positive" and one of these substances that will do this is the humble potato. So, here is a potato. I am going to treat it in exactly the same way. [BLANK_AUDIO] That was the ethanol. This is the Kastle-Meyer reagent here. [BLANK_AUDIO] And finally, some hydrogen peroxide. [BLANK_AUDIO] So, potato's a little bit slow until it goes pink. [BLANK_AUDIO]