[Voiceover] 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. An indicator changes color
in a specific pH range. For example, methyl red is an indicator that goes from red to
yellow over a pH range of about 4.4 to 6.2. We could say that's a pH range of approximately 4 to 6. Bromthymol blue is an indicator that goes from yellow to blue over a pH range of about 6 to 7.6. We could say that's approximately 6 to 8. Phenolphthalein is an indicator that goes from colorless
to pink or magenta over a pH range of about 8.2 to 10. We could say that's
approximately 8 to 10 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 that was strong acid with a strong base. At the equivalence point,
the pH is equal to 7. So right here would be a pH equal to 7. This was our equivalence point, right? So we drop down here. For the example that we
did, we did the math. It took 20 milliliters of our strong base to reach the equivalence point. If you're doing a
titration of a strong acid with a strong base, the pH is equal to 7 at the equivalence point. 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, bromthymol blue,
it changes color from 6 to 8. At a pH of 6, bromthymol blue is yellow. So right here, bromthymol
blue would be yellow, and then it would change to blue. You might even see some green in there. You see a color change in this range. So bromthymol blue would be
a good acid-base indicator to use for your titration. When the blue color persists, you've reached the
endpoint of your titration, and you've matched the
endpoint of your titration with your equivalence point pretty well. This is a good way to approximate the equivalence point. For a strong acid, strong base, we have a really steep titration curve from a pH of about 4, let
me change colors there, from a pH of about 4 to a pH of about 10. We could have used the other two acid-base indicators too. We could have used methyl red because methyl red
changes from red to yellow in a range of about 4 to 6. That gives you a good approximation of the equivalence point. Or you could have used phenolphthalein because phenolphthalein changes to magenta somewhere between 8 to 10. Because you have this really steep titration curve like this, you could have used any of the three acid-base indicators to
find the equivalence point for your titration. All right, next, let's
look at the titration curve for the titration of a weak
acid with a strong base. That was this example over here. We started with a weak acid. In our example, we used acetic acid, and to the acetic acid we added a strong base, sodium hydroxide. The pH at the equivalence
point is greater than 7. The pH was greater than 7. For our example, it was close to 9. It was right about there, just under 9 was the pH for our equivalence point. It took about 200 milliliters
of our strong base to reach our equivalence point. The reason why the pH is greater than 7 at the equivalence point
for this kind of titration, is when you've neutralized
all of your weak acid, you're left with a conjugate
base to the weak acid. The conjugate base reacts with water to increase the concentration of hydroxide ions in solution. That's why your pH is greater than 7. Go back and watch the
video for this titration if you want to see the exact calculation. 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. For our example, phenolphthalein
would work really well because it changes in a range of 8 to 10. Right in this range right in here, we would see a color change from colorless to pink or magenta, 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 4 to 6. You could see 4 to 6
would be right in here on our titration curve. So methyl red would be red, and then it would change to yellow somewhere in here. You'd get a color change
somewhere in here, so 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. Finally, let's move on
to the titration curve, for the titration of a weak
base with a strong acid. We're starting with a weak base. That's why our pH is in the basic range before we've added any of our acid here. In this case, the pH at
the equivalence point is less than 7. The pH of the equivalence
point is less than 7. That's because if you've reacted all of your weak base
with your strong acids, you're protonating your weak base. In our example, we used
NH3, we used ammonia. If you're protonating your weak base, then you're gonna be left
with NH4+ which is acidic. At the equivalence
points you have ammonium which can donote a proton to water, and so you increase the concentration of hydronium ions in solution. 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. Your pH is gonna be less than 7. In our example, we got
a little bit over 5. Right about there would
be the equivalence point. 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 7. Methyl red would be a
really good one to use here. Methyl red would be
yellow at a pH of about 6. Then it would change color to red here. That would be the
endpoint of your titration where you see the color
change for your indicator. That would be a good way to find the equivalence point. You wouldn't want to use something like phenolphthalein because phenolphthalein
would change color up here. At 10 it'd pink or magenta, and then it would change
color right about there, and then you would get colorless. If it changed color here, you've missed the equivalence point. Phenolphthalein would not
work for this titration. Methyl red would be the best choice.