Choosing suitable indicators from titration graphs

https://imageshack.com/my/images
^^ I hope you can see that,I need to choose 2 indicators for this graph, is the idea to chose indicators for either end, or to choose any that fit on the equivalence point? I have three possible ones, I think it’s definitely bromothymol blue and then the other is either methyl red or cresolphthalein. (You can see from the table the indicators and their PH ranges)

Reply 1

Lpzxaz

https://imageshack.com/my/images
^^ I hope you can see that,I need to choose 2 indicators for this graph, is the idea to chose indicators for either end, or to choose any that fit on the equivalence point? The equivalence point is between 4ish and 10ish. I have three possible ones, I think it’s definitely bromothymol blue and then the other is either methyl red or cresolphthalein both of these hardly fit on the graph though so I’m stuck on what to choose. I’ve chose them just because it fits on the straight line, the equivalence point.(You can see from the table the indicators and their PH ranges)

(edited 2 years ago)

Reply 2

username5510846

Original post by Lpzxaz

https://imageshack.com/my/images
^^ I hope you can see that,I need to choose 2 indicators for this graph, is the idea to chose indicators for either end, or to choose any that fit on the equivalence point? The equivalence point is between 4ish and 10ish. I have three possible ones, I think it’s definitely bromothymol blue and then the other is either methyl red or cresolphthalein both of these hardly fit on the graph though so I’m stuck on what to choose. I’ve chose them just because it fits on the straight line, the equivalence point.(You can see from the table the indicators and their PH ranges)

I can't seem to access the photo you uploaded, but yes, the pH range of the indicator needs to match the steep part of the graph. I think the idea is that the pH range of the indicator needs to fall roughly within +/- 0.5 of the pH at the equivalence point (at least that's what I've seen in some textbooks).
Could you try reuploading the image?

Reply 3

UrFellowMedic

Original post by Lpzxaz

https://imageshack.com/my/images
^^ I hope you can see that,I need to choose 2 indicators for this graph, is the idea to chose indicators for either end, or to choose any that fit on the equivalence point? I have three possible ones, I think it’s definitely bromothymol blue and then the other is either methyl red or cresolphthalein. (You can see from the table the indicators and their PH ranges)

I can’t see the image but the idea is that long straight line up during the titration curve, the indicators range should fit within it! Hopefully that helps.

Reply 4

Lpzxaz

Original post by caviaporcellus

Thank you, you should be able to see it now hopefully and is the equivalence point the entire straight line of the graph or the middle of the straight line?

(edited 2 years ago)

Reply 5

Lpzxaz

Original post by UrFellowMedic

I can’t see the image but the idea is that long straight line up during the titration curve, the indicators range should fit within it! Hopefully that helps.

Thank you that helps! I hope you can see the image now, I have three possible indicators from the table I’m not sure what to choose

(edited 2 years ago)

Reply 6

charco

Study Forum Helper

Original post by Lpzxaz

Thank you that helps! I hope you can see the image now, I have three possible indicators from the table I’m not sure what to choose

To be honest, with strong base-strong acid titrations almost every indicator will do the business. In this case, your equivalence point is pH7 so choose the one with a pKa closest to pH7

Reply 7

UrFellowMedic

Original post by Lpzxaz

Thank you that helps! I hope you can see the image now, I have three possible indicators from the table I’m not sure what to choose

Imo I’d use methyl red as it’s still in range and the range includes equivalence point

Reply 8

username5510846

Original post by Lpzxaz

Thank you, you should be able to see it now hopefully and is the equivalence point the entire straight line of the graph or the middle of the straight line?

Yes, the equivalence point is the steepest point on the graph so it is in the middle of the straight line. So for a strong acid-strong base titration, the equivalence point will be at pH 7 (because strong acid + strong base produces a neutral salt), which is shown in your graph. This means that you'll want the pH range of your indicator to roughly match the range of 7 +/- 0.5 (an approximate range of 6.5 - 7.5) Then choose the two indicators closest to this range. I agree with your choice of bromothymol blue, so now you just need to choose the next most suitable indicator.
Hope this makes sense :smile:

Reply 9

Lpzxaz

Original post by charco

To be honest, with strong base-strong acid titrations almost every indicator will do the business. In this case, your equivalence point is pH7 so choose the one with a pKa closest to pH7

Thanks! I was taught the equivalence point is the full straight line, but it’s the middle of the straight line then is it? And you can see the table of indicators above and I need to choose two would you choose bromothymol blue since it’s exactly on seven and then I’m stuck between methyl red and cresolphthalein but methyl red is closer to 7 than cresolphthalein so would you choose that?

Reply 10

Lpzxaz

Original post by caviaporcellus

Thanks it does! The other two indicators that can fit is methyl red and cresolphthalein, methyl reds PH range is closer to the equivalence point 7 than cresolphthaleins PH range so would you choose methyl red? :smile:

(edited 2 years ago)

Reply 11

Lpzxaz

Original post by UrFellowMedic

Imo I’d use methyl red as it’s still in range and the range includes equivalence point

The equivalence point is 7 isn’t it? And methyl red only goes up to 6.2 which is what I don’t understand but it can still be used right?

Reply 12

UrFellowMedic

Original post by Lpzxaz

The equivalence point is 7 isn’t it? And methyl red only goes up to 6.2 which is what I don’t understand but it can still be used right?

Equivalence point is about around 6-7, definitely creso’s range doesn’t fit this. You said two indicators so it’ll make sense for it to be these two right?

Reply 13

username5510846

Original post by Lpzxaz

Yes, I'm pretty sure :smile:

Reply 14

Lpzxaz

Original post by caviaporcellus

I wanted to ask, the table you see in the photo it says the colours of the indicators in acid and base, for example bromothymol blue is purple in acid and yellow in base what colour is it in the end point this is what I’m confused about, is the colour in base the colour of the end point for all the indicators?

Reply 15

UrFellowMedic

Original post by Lpzxaz

Depends on your titration. Generally the base is in the burette and acid in the conical flask, but it can be the other way around. If your base is in the flask, and you are using bromothymol blue, your end point would be purple. If acid in flask, end point would be yellow.

Why this works is because at equivalence point, [H+] = [OH-], as soon as there’s more [H+] or [OH-], colour will change (that’s how phenolphthalein in titration). I’m also a student so what I could be saying is wrong but I’m pretty sure this is correct as this is what I would say in an exam!