Learn about Senior Chemistry, Acids and Bases 10, in this comprehensive video by bannanaiscool.
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Rob Lederer: Acid base indicators or chemical species that have the ability to change color when the pH is altered within different types of ranges for each indicator. Here is methyl orange, here is bromothymol blue, and phenolphthalein; three of the big indicators and I'll show you how they work. By the way, phenolphthalein is also an active ingredient to laxative; don't get any bright ideas. Here is methyl orange indicator indicates within a pH range of around 3.2-4.4. So plink, plink, in goes the indicator into an acid and it gets that nice pink reddish color to it. But you put it over here into a solution that's greater than 4.4, it's a base in this case but it might not be, it's just a solution greater with 4.4, it still could be an acid, and the intermediate color in between if we had a pH that was in between 3.2 and 4.4, methyl orange would be orange. Bromothymol blue is a chemical indicator that has an indicator range between 6.0 and 7.6. So if we take a little bit of this and put it into a solution that's less than 6.0, we get a nice yellow color and that indicates an acid with bromothymol blue. An intermediate between 6.0, 7.6 will give you the green color, which is in this bottle right here, and any pH is higher than 7.6 and bromthymol blue is a lovely blue color. So, there is your range for bromthymol blue indicator. Here is phenolphthalein indicator. It has an active range of around 8.2-10. Any solution that's less than 8.2, on alert, nothing really down happening there; it's colorless. Any solution that has a pH that's greater than 10, it would turn pink. It is not turning anything right here, that's because this is sodium bicarbonate, which is not only a weak acid, but it's kind of diluted in this solution, pH is probably not going to be anything that's greater than 8, but if I put in some drain cleaner, stir that around a little bit, there you go. Drain cleaner, ladies and gentlemen, are highly corrosive substance. See that warning on the label there; danger. That corrosion comes from it being a very, very strong base or should I say a concentrated strong base, and so you get your pink color. This is the acid form of the indicator of the acid and the base form of the indicator pH negative would be in here, that's the base. So, when we write the formulas for these chemical indicators, which can be very complicated, they are very long times and complicated organic formulas. We just kind of like to scrunch it all down and say, okay, well, if we got bromothymol blue indicator, that's pretty cool name. Then moving just right to HBb for the form that has the proton in the acidic form, and then if it loses a proton, we'll call that Bb-negative, but there is no element Bb of course, it just stands for a lot of junk. Remember, indicators can be made from a lot of different types of substances. Many of you have actually boiled cabbage, a red cabbage back in elementary school or junior holidays and use that as a chemical indicator. Most of the indicators that we have come from plant extract and they are pH-sensitive in terms of the colors that they go from and to. So, bromothymol blue indicator is yellow in the form of HBb and it's blue in the form of Bb-negative, and it's pH range of change is between 6.0 and 7.6, where anything that when the indicator is less than 6.0, that's when it's yellow and greater than 7.6, that's when it's blue. So, what's the color of the indicator in between? Well, you saw in the demonstration that was indicated that is the in-between color between the yellow and blue. So, in-between 6 and 7.6, you get a nice healthy green color for bromothymol blue. That can be very significant in acid base titrations. If you want to get, say, at the titration what we call the end-point or the equivalence point of a reaction, I'll explain the difference between the two terms. If you want to get a pH of 7 at the end, then bromothymol blue indicator would be green in there and you could put an indicator an