# Gen Chem indicator problem... help!

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#### brood910

##### Full Member

Which of the following indicators would be BEST for the titration of p-nitrophenol (pka 7.2) by NaOH?

A: Penolphthalein (pH range of color change is 8 and 9.6

I know that pH of the soln has to be +-1 of the Pka of the indicator, but I have no idea how to apply this knowledge to this problem. Please help!

#### Dapik

##### Full Member
5+ Year Member
You didn't provide the entire problem, so I'm not sure what your confusion is exactly. But here's what you can tell without using numbers at all.

You need an indicator near the ballpark of the equivalence point. So the first thing you need to do is think about where the equivalence point is for this particular titration.

The equivalence point is where you have added just enough titrant to neutralize the acid/base in the solution. In this case, it's when you have added just enough NaOH so that you no longer have p-nitrophenol in solution (and also do not have excess NaOH).

p-nitrophenol is a weak acid by virtue of it not being one of the few strong acids. It has an -OH group, which loses a proton H+ when it reacts with the strong base NaOH, forming a conjugate weak base.

When the titration reaches its equivalence point, you have 0 moles of the titrant/strong base NaOH (you actually still have the Na+ ions hanging around interacting with the conjugate base, but this information is not necessary), you have 0 moles of the weak acid p-nitrophenol that you were titrating, and you have some moles of the weak base p-nitrophenol.

A neutral equivalence point is 7. Weak bases will increase this equivalence point, usually to the 8-9 range. If you want to rationalize the increase, you can think about the weak base reacting with water, which acts as an acid, to form some -OH ions. Thus you need an indicator in the 8-9 range.

If you have a lot of choices and need to be more exact, you can use the pKa to find the Ka, and then find the exact pH that way.

#### brood910

##### Full Member
You didn't provide the entire problem, so I'm not sure what your confusion is exactly. But here's what you can tell without using numbers at all.

You need an indicator near the ballpark of the equivalence point. So the first thing you need to do is think about where the equivalence point is for this particular titration.

The equivalence point is where you have added just enough titrant to neutralize the acid/base in the solution. In this case, it's when you have added just enough NaOH so that you no longer have p-nitrophenol in solution (and also do not have excess NaOH).

p-nitrophenol is a weak acid by virtue of it not being one of the few strong acids. It has an -OH group, which loses a proton H+ when it reacts with the strong base NaOH, forming a conjugate weak base.

When the titration reaches its equivalence point, you have 0 moles of the titrant/strong base NaOH (you actually still have the Na+ ions hanging around interacting with the conjugate base, but this information is not necessary), you have 0 moles of the weak acid p-nitrophenol that you were titrating, and you have some moles of the weak base p-nitrophenol.

A neutral equivalence point is 7. Weak bases will increase this equivalence point, usually to the 8-9 range. If you want to rationalize the increase, you can think about the weak base reacting with water, which acts as an acid, to form some -OH ions. Thus you need an indicator in the 8-9 range.

If you have a lot of choices and need to be more exact, you can use the pKa to find the Ka, and then find the exact pH that way.

I provided all the info that was present on the practice exam itself.
In the answer, they said that the exact pH is 9.2. I have no idea how they got this number.

#### Dapik

##### Full Member
5+ Year Member
If they didn't give you any moles/volumes, then I have no idea how they got the exact 9.2 number. But then again, if you read what I said, who cares how they got it. It's not relevant to solving the problem if Penolphthalein is the only indicator in the ballpark of 8-10.

EDIT: And what you didn't provide was what the other indicators were.

Last edited:

#### SKaminski

##### Full Member
7+ Year Member
If they didn't give you any moles/volumes, then I have no idea how they got the exact 9.2 number. But then again, if you read what I said, who cares how they got it. It's not relevant to solving the problem if Penolphthalein is the only indicator in the ballpark of 8-10.

EDIT: And what you didn't provide was what the other indicators were.

I too would like to see what the other answer choices were. It may have been easier to use process - of - elimination to get to the right answer here.

#### brood910

##### Full Member
Other choices were:

Thymol blue - pH 1.2 ~ 2.8
Methyl Red - pH 4.6 ~ 5.8
Bromthymol Blue pH 6 ~ 7.6

I guess I understand this now.
I am confused by the answer key tho.. Do you understand this?:

"To predict the pH near equivalence point, which is what you are doing when you choose an indicator, you should use the Henderson-Hasselbalch equation. At 2 pH units beyond the Pka of the acid, the ratio of conjugate base is 100:1, which means that the rxn is almost at equivalence. This is the point at which the indicator should start to show some color change. In this question, the pka for the weak acid is 7.2. The best indicator is therefore around pH = 9.2, which is between 8 ~ 9.6"

bump

#### sdm33

##### Full Member
Can you think of NaOH as making it Basic so the pH is 9.2, and the other options are all acidic

#### brood910

##### Full Member
Can you think of NaOH as making it Basic so the pH is 9.2, and the other options are all acidic

That's obvious.

#### sdm33

##### Full Member
A common end point for acid-base titrations is the color change associated with
an acid-base indicator. An acid-base indicator is usually an organic weak acid or base
that has a different color in solution than its conjugate form. These substances strongly
absorb light so that even a very small concentration in solution produces an obvious
color. If the weak acid form of the indicator is taken as HIn
so do the relative amounts of the different colored
conjugate species in solution. Thus, the indicator in solution will take on a specific color
depending upon the solution pH.
As an example, consider the acid-base indicator methyl orange indicator. The
weak acid form is red in solution while the conjugate base form is yellow (that is, HIn =
red and In- = yellow). In order to anticipate the pH range in which this indicator changes
from yellow to red, or vice versa, assume that a 10-fold excess of one colored form over
the other is needed to establish the color of one of the conjugate pairs. Thus, to see
predominately yellow in solution,
In a similar way it can be shown that the red color dominates when the solution pH is
- 1. Therefore, an indicator color change is expected in a pH range
equal to pKa
± 1. The pKa
for methyl orange is 3.46, so it should change from red to
yellow as the pH increases from about 2.5 to 4.5.

.
Consequently, knowledge of the pH at the equivalence point allows one to select
an indicator that will undergo a color change in a pH range that brackets the
equivalence point pH.
Polyprotic Acid and Base Titrations
The titration of a polyprotic acid or base is very similar to that for a monoprotic
weak acid or base except that more than one equivalence point is observed. The
maleic acid experiment in lab involves such a titration. The titration curve for the
titration of 50.0 mL of 0.100 M Na2
CO3
with 0.100 M HCl is shown in Figure 9.5. Since
the analyte and titrant concentrations are the same, the first equivalence point occurs at
50 mL and the second one occurs at 100 mL.
Figure 9.5 Curve for the titration of Na2
CO3
with a HCl(aq)Acid-Base Titrations 10/11/12
page 4
Check for Understanding 9.4 Solutions
1. Estimate the pH at the second equivalence point in the carbonate titration.
2. Which indicators are suitable for use at the first and second equivalence
points in the carbonate titration?
The titration reaction to the first equivalence point involves neutralization of the
weak base carbonate.
CO3
2- + H3
O
+
6 HCO3
-
+ H2
O
As this process occurs, a CO3
2-/HCO3
-
buffer is formed. The region before the first
equivalence point where the pH is changing slowly with added titrant corresponds to this
first buffer region. Throughout this portion of the titration pH ~ pKa
and at the halfway
point (25 mL), pH = pKa
= 10.33. Note that the Ka
is that for the weak acid in the buffer,
that is, HCO3
-
, which corresponds to Ka2 for H2
CO3
.
At the first equivalence point, all the CO3
2- has been converted to HCO3
-
and
additional titrant starts neutralizing the bicarbonate.
HCO3
-
+ H3
O
+
6 H2
CO3
+ H2
O
As this process occurs, a HCO3
-
/H2
CO3
buffer is formed. The region between the first
and second equivalence points where the pH is changing slowly with added titrant
corresponds to this second buffer region. Throughout this portion of the titration pH ~
pKa
and halfway between the first and second equivalence points (75 mL), pH = pKa
=
6.35. Here Ka
is that for the weak acid H2
CO3
and is thus Ka1 for carbonic acid.
An estimate of each equivalence point pH can be made as before by taking the
average of the pH plateau values before and after the equivalence point. For the first
equivalence point pH ~ (10.33 + 6.35)/2 = 8.3.
Because the pH jump at either equivalence point in the carbonate titration is not
very large, an indicator color change is not a very sharp end point. An indicator color
match can be used instead as the end point. For this to be a valid end point, the
indicator must have the same color in the comparison solution as it has in the titrationAcid-Base Titrations 10/11/12
page 5
Check for Understanding 9.5 Solution
1. What is the pH at the second equivalence point in the titration of Na2
CO3
with
0.100 M HCl? Use activities and assume a 0.200-g sample of Na2
CO3
is
dissolved in 60.0 mL of deionized water and titrated.
solution at the second equivalence point. This is true only if the pH of the comparison
solution is the same as the pH at the second equivalence point. Previously you have
calculated, using activities, the pH of a CO3
2-/HCO3
-
buffer solution that is suitable for
use as a comparison solution for the carbonate titration, and above you have estimated
the pH at the second equivalence point. To make a more careful comparison of pH,
make the following calculation of the second equivalence point pH.
Gran Plots
For the maleic acid experiment, you will use a graphical method known as a
Gran plot to determine the second equivalence point in your titration. In this approach,
the pH of your maleic acid solution is recorded as a function of the volume of added
base (Vb
) and a graph of Vb
@10-pH versus Vb
is made using data just before the
equivalence point. Assuming that the ionic strength of the solution is constant, the xintercept of this linear plot is the equivalence point volume (Ve
). An example of a Gran
plot is shown below. Derivation of relationship used for Gran plot
Figure 9.6 Gran plot for a weak acid-strong
base titration
© 2011 W. H. Freeman and CompanyAcid-Base Titrations 10/11/12
page 6
The ionic strength of the maleic acid solution is fixed by adding a large amount of
KCl to the titration solution. The key advantage of using a Gran plot is that the
equivalence point can be determined by using only data before the equivalence point
(typically the data between 0.9Ve
and Ve
). The slope of the Gran plot also allows one to
determine the Ka
of the weak acid (for a weak acid-strong base titration, slope = - Ka
&#947;
HA /&#947;
A-).
Exercises for Acid-Base Titrations

#### brood910

##### Full Member
A common end point for acid-base titrations is the color change associated with
an acid-base indicator. An acid-base indicator is usually an organic weak acid or base
that has a different color in solution than its conjugate form. These substances strongly
absorb light so that even a very small concentration in solution produces an obvious
color. If the weak acid form of the indicator is taken as HIn
so do the relative amounts of the different colored
conjugate species in solution. Thus, the indicator in solution will take on a specific color
depending upon the solution pH.
As an example, consider the acid-base indicator methyl orange indicator. The
weak acid form is red in solution while the conjugate base form is yellow (that is, HIn =
red and In- = yellow). In order to anticipate the pH range in which this indicator changes
from yellow to red, or vice versa, assume that a 10-fold excess of one colored form over
the other is needed to establish the color of one of the conjugate pairs. Thus, to see
predominately yellow in solution,
In a similar way it can be shown that the red color dominates when the solution pH is
- 1. Therefore, an indicator color change is expected in a pH range
equal to pKa
± 1. The pKa
for methyl orange is 3.46, so it should change from red to
yellow as the pH increases from about 2.5 to 4.5.

.
Consequently, knowledge of the pH at the equivalence point allows one to select
an indicator that will undergo a color change in a pH range that brackets the
equivalence point pH.
Polyprotic Acid and Base Titrations
The titration of a polyprotic acid or base is very similar to that for a monoprotic
weak acid or base except that more than one equivalence point is observed. The
maleic acid experiment in lab involves such a titration. The titration curve for the
titration of 50.0 mL of 0.100 M Na2
CO3
with 0.100 M HCl is shown in Figure 9.5. Since
the analyte and titrant concentrations are the same, the first equivalence point occurs at
50 mL and the second one occurs at 100 mL.
Figure 9.5 Curve for the titration of Na2
CO3
with a HCl(aq)Acid-Base Titrations 10/11/12
page 4
Check for Understanding 9.4 Solutions
1. Estimate the pH at the second equivalence point in the carbonate titration.
2. Which indicators are suitable for use at the first and second equivalence
points in the carbonate titration?
The titration reaction to the first equivalence point involves neutralization of the
weak base carbonate.
CO3
2- + H3
O
+
6 HCO3
-
+ H2
O
As this process occurs, a CO3
2-/HCO3
-
buffer is formed. The region before the first
equivalence point where the pH is changing slowly with added titrant corresponds to this
first buffer region. Throughout this portion of the titration pH ~ pKa
and at the halfway
point (25 mL), pH = pKa
= 10.33. Note that the Ka
is that for the weak acid in the buffer,
that is, HCO3
-
, which corresponds to Ka2 for H2
CO3
.
At the first equivalence point, all the CO3
2- has been converted to HCO3
-
and
additional titrant starts neutralizing the bicarbonate.
HCO3
-
+ H3
O
+
6 H2
CO3
+ H2
O
As this process occurs, a HCO3
-
/H2
CO3
buffer is formed. The region between the first
and second equivalence points where the pH is changing slowly with added titrant
corresponds to this second buffer region. Throughout this portion of the titration pH ~
pKa
and halfway between the first and second equivalence points (75 mL), pH = pKa
=
6.35. Here Ka
is that for the weak acid H2
CO3
and is thus Ka1 for carbonic acid.
An estimate of each equivalence point pH can be made as before by taking the
average of the pH plateau values before and after the equivalence point. For the first
equivalence point pH ~ (10.33 + 6.35)/2 = 8.3.
Because the pH jump at either equivalence point in the carbonate titration is not
very large, an indicator color change is not a very sharp end point. An indicator color
match can be used instead as the end point. For this to be a valid end point, the
indicator must have the same color in the comparison solution as it has in the titrationAcid-Base Titrations 10/11/12
page 5
Check for Understanding 9.5 Solution
1. What is the pH at the second equivalence point in the titration of Na2
CO3
with
0.100 M HCl? Use activities and assume a 0.200-g sample of Na2
CO3
is
dissolved in 60.0 mL of deionized water and titrated.
solution at the second equivalence point. This is true only if the pH of the comparison
solution is the same as the pH at the second equivalence point. Previously you have
calculated, using activities, the pH of a CO3
2-/HCO3
-
buffer solution that is suitable for
use as a comparison solution for the carbonate titration, and above you have estimated
the pH at the second equivalence point. To make a more careful comparison of pH,
make the following calculation of the second equivalence point pH.
Gran Plots
For the maleic acid experiment, you will use a graphical method known as a
Gran plot to determine the second equivalence point in your titration. In this approach,
the pH of your maleic acid solution is recorded as a function of the volume of added
base (Vb
) and a graph of Vb
@10-pH versus Vb
is made using data just before the
equivalence point. Assuming that the ionic strength of the solution is constant, the xintercept of this linear plot is the equivalence point volume (Ve
). An example of a Gran
plot is shown below. Derivation of relationship used for Gran plot
Figure 9.6 Gran plot for a weak acid-strong
base titration
© 2011 W. H. Freeman and CompanyAcid-Base Titrations 10/11/12
page 6
The ionic strength of the maleic acid solution is fixed by adding a large amount of
KCl to the titration solution. The key advantage of using a Gran plot is that the
equivalence point can be determined by using only data before the equivalence point
(typically the data between 0.9Ve
and Ve
). The slope of the Gran plot also allows one to
determine the Ka
of the weak acid (for a weak acid-strong base titration, slope = - Ka
&#947;
HA /&#947;
A-).
Exercises for Acid-Base Titrations

Dont copy and paste things that are not even relevant to the question.

#### Oh_Gee

##### Full Member
7+ Year Member
here is the question
passage gives the pka of p-nitrophenol(02NC6H40H) as 7.2
Which of the following indicators would be BEST for the titration of p-nitrophenol(02NC6H40H) by NaOH?
A. Thymol blue (pH range of color change is 1.2 to 2.8)
B. Methyl red (pH range of color change is 4.6 to 5.8)
C. Bromthymol Blue (pH range of color change is 6.0 to 7.6)
D. Phenolphthalein (pH range of color change is 8.0 to 9.6)

i thought it was Choice c because i thought that when you picked indicators, the pH range of the indicator should be around the pKa of the acid that you're titrating

#### theyellowking

##### Full Member
I believe this is how I solved it:

pH of indicator = (pKa + pH of whatever you're adding (In this case, NaOH))/2 = (7.2 + 10ish)/2 = roughly 8.5ish +/- 1 = (7.5ish, 9.5ish). I believe pH of 10 for NaOH is a very low estimate, but even so, the other answers don't necessarily make sense. D is by far the best one to include a broader of range.

As for the answer, I'm not really quite sure why they would use the HH equation. Which edition of the book are you using? Mine is 2010, and they simply gave the equation I put up there to solve for this type of problem.

#### Oh_Gee

##### Full Member
7+ Year Member
I believe this is how I solved it:

pH of indicator = (pKa + pH of whatever you're adding (In this case, NaOH))/2 = (7.2 + 10ish)/2 = roughly 8.5ish +/- 1 = (7.5ish, 9.5ish). I believe pH of 10 for NaOH is a very low estimate, but even so, the other answers don't necessarily make sense. D is by far the best one to include a broader of range.

As for the answer, I'm not really quite sure why they would use the HH equation. Which edition of the book are you using? Mine is 2010, and they simply gave the equation I put up there to solve for this type of problem.

i don't think you can use that equation because you would need to know the pH of NaOH. I ended up reasoning that if the pKa of the acid is 7.2, then at pH 7.2, that is the halfway point to equivalence so the equivalence point pH is a little higher than 7.2 which is why you should pick Phenolphthalein

#### IslandStyle808

##### Akuma residency or bust!
7+ Year Member
Which of the following indicators would be BEST for the titration of p-nitrophenol (pka 7.2) by NaOH?

A: Penolphthalein (pH range of color change is 8 and 9.6

I know that pH of the soln has to be +-1 of the Pka of the indicator, but I have no idea how to apply this knowledge to this problem. Please help!

The way it works is that the pKa of the indicator is the point where the color changes occur, and this is the point where you want to measure the equivalence point of the acid or base being titrated.

It is a deduction type problem, don't look for the answer, look for the wrong choices. The first three are wrong because A, B, and C happen when the pH=pKa or when the pH<pKa of the acid. You don't want this to happen because you don't want the indicator to interfere with the reaction (remember that the indicator is either an acid or base, in this case it acts like an acid). Which is why D is correct, because deprotonation of the indicator happens after the pkA=pH point of the acid being titrated.

EDIT: made some corrections to my response from initial post

#### Chrisz

##### Full Member
5+ Year Member
Which of the following indicators would be BEST for the titration of p-nitrophenol (pka 7.2) by NaOH?

A: Penolphthalein (pH range of color change is 8 and 9.6

I know that pH of the soln has to be +-1 of the Pka of the indicator, but I have no idea how to apply this knowledge to this problem. Please help!
bro, this question is supposed to be designed to be answered intuitively. 1. you are titrating a weak acid with a strong base. so the equivalence will be above 7, so anything below 7 are eliminated. Since the pKa is 7.2, there will be an abrupt surge at the equivalence point, any indicator around 7 will be elminated. So the best answer would be Phenolphthalein (pH range of color change is 8.0 to 9.6). If you want mathematical proof, you can do the following. pH at eq=0.5(pKa+pHtirant)
Since the question does not indicate what con of acid we use, we can make an assumption based on experience. Usually, the titrant we use are between 0.01 and 1M. so the corresponding pH of titrant should be between 12 and 14. 0,5(12+7.2)= 9.6 abd 0.5(7.2+14)=10.6. this would confirm that 8.0 to 9.6 is closer to the calculated value than the other answers.

• 1 users

#### Czarcasm

##### Hakuna matata, no worries.
5+ Year Member
bro, this question is supposed to be designed to be answered intuitively. 1. you are titrating a weak acid with a strong base. so the equivalence will be above 7, so anything below 7 are eliminated. Since the pKa is 7.2, there will be an abrupt surge at the equivalence point, any indicator around 7 will be elminated. So the best answer would be Phenolphthalein (pH range of color change is 8.0 to 9.6). If you want mathematical proof, you can do the following. pH at eq=0.5(pKa+pHtirant)
Since the question does not indicate what con of acid we use, we can make an assumption based on experience. Usually, the titrant we use are between 0.01 and 1M. so the corresponding pH of titrant should be between 12 and 14. 0,5(12+7.2)= 9.6 abd 0.5(7.2+14)=10.6. this would confirm that 8.0 to 9.6 is closer to the calculated value than the other answers.
Great explanation. I was surprised by the length discussions above. Absolutely unnecessary if you just realized the equivalent point would be above the pKa for the analyte. You'd want a an indicator with a pKa +/- 1 around the equivalence point pH.

• 1 user

#### Czarcasm

##### Hakuna matata, no worries.
5+ Year Member
here is the question
passage gives the pka of p-nitrophenol(02NC6H40H) as 7.2
Which of the following indicators would be BEST for the titration of p-nitrophenol(02NC6H40H) by NaOH?
A. Thymol blue (pH range of color change is 1.2 to 2.8)
B. Methyl red (pH range of color change is 4.6 to 5.8)
C. Bromthymol Blue (pH range of color change is 6.0 to 7.6)
D. Phenolphthalein (pH range of color change is 8.0 to 9.6)

i thought it was Choice c because i thought that when you picked indicators, the pH range of the indicator should be around the pKa of the acid that you're titrating
The purpose of an indicator is to determine when we reached equivalence point, so you'd want an indicator with a pKa around that pH value (at equivalence point).

#### Chrisz

##### Full Member
5+ Year Member
Great explanation. I was surprised by the length discussions above. Absolutely unnecessary if you just realized the equivalent point would be above the pKa for the analyte. You'd want a an indicator with a pKa +/- 1 around the equivalence point pH.
Yes, absolutely agree with you. I think the purpose of having MCAT questions designed this way that they do not want you to plug in numbers and calculate results. They want you to think outside of box and do it quickly.