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# Titration curves and acid-base indicators

## Video transcript

now that we've looked at titration curves in great detail let's see how we could use an acid-base indicator to find the equivalence point for a titration so an indicator changes color and a specific pH range for example methyl red is an indicator that goes from red to yellow over a pH range of about four point four to six point two so we could say that's a pH range of approximately four to six bromothymol blue is an indicator that goes from yellow to blue over a pH range of about six to seven point six so we could say that's approximately six to eight and phenolphthalein is an indicator that goes from colorless to pink or magenta over a pH range of about eight point two to ten so we could say that's approximately eight to ten here let's look at our titration curves and let's see which acid-base indicators we could use all right we'll start at the top left here this was the titration curve for the titration of a strong acid with a strong base and at the equivalence point the pH is equal to seven so right here would be a pH equal to seven and so this was our equivalence point right so we drop down here for the example that we did we did the math it took twenty milliliters of our strong base to reach the equivalence point so if you're doing a titration of a strong s with a strong base the pH is equal to seven at the equivalence point alright so let's think about which acid-base indicator we could use for this titration you want to choose an indicator that changes color in a range close to your equivalence point for example bromothymol blue changes color from six to eight right so at a pH of six bromothymol blue is yellow so right here bromothymol blue be yellow and then it would change it would change to blue you might even see some green in there all right so you see a color change in this range so bromothymol blue would be a good acid-base indicator to use for your titration so when the blue color persists right you reach the end point of your titration and you've matched the end point of your titration with your equivalence point pretty well so this is a this is a way to approximate the equivalence point for a strong acid strong base we have a really steep titration curve right from a pH about 4 let me change colors there from a pH of about 4 to a pH of about 10 so we could have used the other two acid-base indicators - all right we could have used methyl red because methyl red changes from red to yellow and arranged about 4 to 6 and so that gives you that gives you a good approximation of the equivalence point or you could have used phenolphthalein because phenolphthalein changes to magenta and right somewhere between 8 to 10 so because you have this really steep titration curve like this right you could have used any of the three acid-base indicators to find the equivalence point for your titration alright next let's look at the titration curve for the titration of a weak acid with a strong base and that was this example over here so we started with a weak acid right in our example we used acetic acid and - the acetic acid we added a strong base sodium hydroxide the pH of the equivalence point is greater than 7 right so the pH was greater than 7 for our example it was was close to 9 right so it was right about there just under 9 was the pH for our equivalence point and it took about 200 milliliters of our strong base to reach our equivalence point all right the reason why the pH is greater than 7 at the equivalence point for this kind of titration is when you when you've neutralized all of your weak acids right you're left with the conjugate base - the weak acid and the conjugate base reacts with water to increase the concentration of hydroxide ions in solution and that's why your pH is greater than 7 so go back and watch the video for this titration if you want to see the exact calculation so for the titration of a weak acid with a strong base the pH is greater than 7 you want to choose an acid-base indicator that changes color in a range greater than 7 and so for our example phenolphthalein would work really well because it changes in a range of eight to ten all right so right on this range right in here we would see a color change from colorless to pink or magenta right and we would stop our titration at that point and that's a good approximation of the equivalence point you wouldn't want to use something like methyl red here because methyl red changes in a range of four to six and you can see four to six would be right in here on our titration curve so methyl red would be red and then it would change to yellow somewhere somewhere in here right so you get a color change somewhere in here and so you would you would miss the equivalence point this would not be a good acid-base indicator to use so you couldn't use methyl red here all right finally let's move on to the titration curve for the titration of a weak base with a strong acid so we're starting with a weak base and so that's why our pH is in the basic range before we'd added any of our acid here and in this case the pH at the equivalence point is less than seven the pH of the equivalence point is less than seven and that's because right if you if you've reacted all of your weak base with your strong acid right your your protonating your weak base so in our example we used NH three we used ammonia right if your pronating your weak base then you're going to be left with NH four plus right which is acidic and so at the equivalence point right you have ammonium which can donate a proton to water and so you increase the concentration of hydronium ions in solution and so at the equivalence point the pH is less than 7 once again watch these videos if you want to see the exact calculation for that so your pH is going to be less than seven in our example we got a little bit over five so right about there would be the equivalence point so if you're thinking about which acid-base indicator do you want to use you want to use one that changes color at a pH less than seven so methyl red would be a really good one to use here all right so methyl red would be yellow at a pH of about six and then it would change it would change color to red here alright and that would be the endpoint of a titration where you see the color change for your indicator and so that would be a good way to find the equivalence point you wouldn't want to use something like phenolphthalein because phenolphthalein changes color would change color up here right so at 10 it'd be pink or magenta right and then it would change color right about there then you get colorless and so if a changed color here right you've missed the equivalence point so phenolphthalein would not work for this titration methyl red would be the best choice