Understanding pH Indicators: Solving Sample Questions

[kirsten_2009]

pH INDICATORS...I DONT GET IT :(

1. Homework Statement

Hello!

I don't really need homework help...I'm actually studying for my final exam and I have a couple of sample questions that I have no idea how I would go about solving. What I would appreciate is if someone could explain to me how to solve them rather than giving me the answer. Thank you so much for all your help and time in advance! :)

Q#1: Phenol red indicator changes from yellow to red in the pH range from 6.6 to 8.0. State what color the indicator will assume in the following solution: 0.1 M NaCl

a) yellow
b) red-yellow mixture
c) red
d) The indicator is its original color
e) There is not enough information to answer this question

Answer: C Red

Q#2: Phenol red indicator changes from yellow to red in the pH range from 6.6 to 8.0. State what color the indicator will assume in the following solution: 0.10 M NH4NO3

a) Red
b) Red-Yellow Mixture
c) Yellow
d) The indicator is its original color
e) There is not enough information to answer this question

Answer: C Yellow

Homework Equations

I would think that a relevant equation would be pH = -log[H_3_O+] but not sure...

The Attempt at a Solution

Clueless...

 

[epenguin]

Do you know what the pH of a neutral solution is? I.e. pure water? And why? (There is an equation in that).

That said I cannot understand the answer given to Q1 myself.

 

[kirsten_2009]

Hi!

Yes the PH of a neutral solution is 7.00 because the [H3O+] = [OH-]. I believe that in pure water at 25 degrees Celsius the concentration of H3O+ is 1 x 10^-7 as is the concentration of OH- because of the self-ionization of water...but I don't understand how that can be used to solve the question.

I am very confused with Q#1 because I thought that NaCl was a salt of a strong acid and a strong base and thus did not hydrolyze, thus it did not affect the pH of a solution but...I dunno... :S ...How about question # 2 though? Thanks for replying!

 

[chemisttree]

Yes, so what color would phenol red be at pH = 7? Remember it changes from yellow to red at pH 6.6 to 8. Where is pH = 7 in that range? Yellow?

Salt doesn't change pH at all.

In question 2, you have a salt of a strong acid and a weak base. What happens in that case? Where's the pH? Less than or greater than 7?

 

[epenguin]

Hi!

Yes the PH of a neutral solution is 7.00 because the [H3O+] = [OH-]. I believe that in pure water at 25 degrees Celsius the concentration of H3O+ is 1 x 10^-7 as is the concentration of OH- because of the self-ionization of water...but I don't understand how that can be used to solve the question.

I am very confused with Q#1 because I thought that NaCl was a salt of a strong acid and a strong base and thus did not hydrolyze, thus it did not affect the pH of a solution but...I dunno... :S ...How about question # 2 though? Thanks for replying!

OK you do know the pH of pure water is 7 because "the concentration of H3O+ is 1 x 10^-7 as is the concentration of OH- because of the self-ionization of water"

I asked do you understand why it is that in terms of an equation; the equation is [H₃O⁺][OH^-] = 10^-14: you seem to know how that gives [H₃O⁺] = 10^-7. You are not required to know beyond that e.g. why this equilibrium constant is what it is which cannot be calculated from elementary first principles. The main thing to note is that it is low, i.e. considering that water is about 55M, 10^-7M of [H₃O⁺] is quite a small proportion!

You are right to say the salt should not make any difference.

So the simple-minded answer looks to me like the solution remains a bit on the acid side of where it changes colour so it should be yellow or b) mixture.

Maybe they are looking for some more sophisticated answer.
E.g. the indicator could well be the most concentrated acid/base species there and it that case it should be an equal mix of its two forms for similar reasons water is. So still at most a mix red/yellow, C is still the wrong answer.
As I found this perplexing I have checked the facts and you can see here http://antoine.frostburg.edu/chem/senese/101/acidbase/indicators.shtml the colour at neutral pH is pretty yellow but maybe if there is nothing there but indicator the pH would not be quite neutral for reason just given.

Q2 is more straightforward. You seem to know about strong/weak acids and bases - so what sort of acid and base make NH₄NO₃?

 

[chemisttree]

So the simple-minded answer looks to me like the solution remains a bit on the acid side of where it changes colour so it should be yellow or b) mixture.

I assure you that pH7 is not on the acid side of 6.6. It is on the basic side of that value.

Maybe they are looking for some more sophisticated answer.
E.g. the indicator could well be the most concentrated acid/base species there and it that case it should be an equal mix of its two forms for similar reasons water is. So still at most a mix red/yellow, C is still the wrong answer.

Indicators are used at the point at which the color change first becomes visible. That is how the endpoint is defined. That defines it as red even if it looks red/orange-like.

C is correct.

 

[kirsten_2009]

Well thank you very much for your replies. It's been very helpful and I get it. Thanks.

 

[Borek]

I don't have a picture of phenol red at pH 7, but just for a comparison with the picture linked to by epenguin:

http://www.titrations.info/acid-base-titration-indicators-preparation#phenol-red

that's how it looks like at 4 and 10.

If the color changes between 6.6 and 8.0 we can assume pKa for the indicator lies exactly in the middle (it doesn't have to be true, but that's quite good assumption). Knowing pH and pKa you can calculate ratio of concentrations of both red and yellow form - see

http://www.titrations.info/acid-base-titration-indicators

That's a little bit more systematic approach.

--

 

[epenguin]

I assure you that pH7 is not on the acid side of 6.6. It is on the basic side of that value.
Indicators are used at the point at which the color change first becomes visible. That is how the endpoint is defined. That defines it as red even if it looks red/orange-like.

C is correct.

I meant that as the question says "changes from yellow to red in the pH range from 6.6 to 8.0" and pH 7 is on the acid side of the middle of that range.

Actually there is more of the yellow form there than the red at neutral pH, but as you bring out the red form is more light-absorbent than the yellow. That point is rarely mentioned in texts (at least I have never seen) and resolves for me some puzzles I seem to remember I had when I was at Kirsten's stage!

Your reply was about the perception of change in a titration but the question was just the colour and it looks half way to me in the link I gave.

I hope I have clarified more than confused Kirsten, at least this explains why it is confusing.

 

[chemisttree]

If the color changes between 6.6 and 8.0 we can assume pKa for the indicator lies exactly in the middle (it doesn't have to be true, but that's quite good assumption). Knowing pH and pKa you can calculate ratio of concentrations of both red and yellow form - see...

My source has it at 7.7.

 

[kirsten_2009]

Thank you all for your help, much appreciated. I understand now what you're saying...I guess then that "C -red" must be a mistake from my chemistry textbook if a salt does not change the pH of a solution, the pH should be 7 and thus a mixture of the two colors. For the second question I see that it's made up of a weak base and a strong acid so the pH should be acidic. Thanks again!

 

[kirsten_2009]

Actually...and sorry to bother you all again...if possible could you help me with another question I posted under the title: "Weak/Strong Acid w/ Strong Base Titrations and pH Indicator Selection Help"? Thank you.