Why do amino acids, which I'm assuming are weak acids, have equivalence points below 7, as opposed to above 7 seen above, when titrated with a strong base like NaOH?

Amino acids behave as polyprotic acids that have more than one dissociable proton because of the amide and carboxylate functionality. They essentially act as diprotic acids with two different equivalence points. One equivalence point below 7 and one above seven for the amino acid alanine for example. Typically, the conjugate base that is formed from the FIRST dissociation of a polyprotic acid is still considered a weak acid itself and will undergo acid hydrolysis with water. Therefore producing a pH at the first equivalence point that is less than 7.

What is the half-way to the equivalence point?

The half equivalence point is the point where half of the titrant necessary to neutralize the analyte has been added. So the amount of protonated [HA] and deprotonated [A-] species of the analyte is equal. When [A-] and [HA] are equal, the henderson hasselbalch equation, pH = pKa + log([A-]/[HA]) simplifies to pH = pKa because [A-]/[HA] = 1 and log(1) = 0.

Why does it take the same volume of base to reach the equivalence point regardless of whether it is a strong or weak acid?

Assuming the weak base and the strong base both have the same concentration, then saying that they have the same volume also means the have the same number of moles. Ultimately it is the number of moles that matters because an equivalence point is reached when the number of moles of base reacting is the same as the number of moles of acid reacting (so the two are neutralized). The reaction between a weak acid and a strong base goes essentially to completion, so for problems we say that it will. So a weak acid reacting with a strong base reacts just as much as a strong acid reacting with a strong base. Since the weak and strong acids have the same molarities, it will require the same volume (and thus same number of moles) to reach the equivalence point when they react with a strong base.

Can u show where show where the end point is sin a graph

The endpoint is where the indicator changes color, which is around the top (end) of the steep curve.

Why does a WB + WA titration curve look like that? I thought a buffer can't be made from a SA or SB. It has to be a Wa and Cb. But what is going on here? The titration curve looks like it is behaving as a buffer

it depends on the ka/kb of the wA and weak base, if you are using a weak acid and its conjugate base you can use the pKa and it will give you the equivalence point.

why the normality of NaOH solutions may change if the solution is exposed ti air for an extend period of time?

CO₂(g) from the air will dissolve in any aqueous (water based) solution. It reacts with water molecules to form carbonic acid (a weak acid) H₂CO₃(aq). This acid releases protons that will partially neutralize the basic NaOH.

In the "Titration of a strong acid with a weak base" section, HCl is used as the titrant, and NH3 serves as the titrand/ analyte solution. However, in the previous 2 videos, NH3 is used as the titrant, and HCl serves as the titrand/ analyte solution. Therefore, the resultant titration curves for the videos and this article are different even though they represent the same neutralization reaction. Does it matter whether the acid versus the base is used as the titrant versus the titrand/ analyte? Or are both titration curves accurate representations of this neutralization reaction? Thank you!

You are correct that both curves are representations of the same neutralization reaction, but the difference lies in what happens before and after the equivalence point. Thus, you end up with different graphs Before the equivalence point, you have an excess of the titrand/analyte solution in comparison to the titrant. In this lecture, the strong acid is the analyte so the pH up until the equivalence point will be quite acidic. As you can see in the videos, with the weak base as the analyte, the pH is initially much higher (until the equivalence point). You will see a similar relationship in terms of the pH after the equivalence point However, there should be no difference in the pH at the actual equivalence point as the neutralization reaction is the same: NH3 + H3O+ = NH4+ + H20 Also note that the pH of the half-equivalence point will differ in these two examples for the same reason

Can you give intuitive sight of what is meant by steepness of a slope?

The size of the angle between the slope and horizontal. A larger angle (closer to vertical) is steeper.

Why the part of the curve beyond the equivalence point is similar in the graphs from strong and weak acid even though you titrated a strong acid and a weak acid??

After titration, there is no acid left over. There is only base. Therefore, the right side of the graph will approach the pH of the base.

Main content

Acid-base titration curves

Google Classroom

Before we start discussing about titration and titration curves, we should quickly refresh the concept of a weak/strong acid and weak/strong base.

A strong acid dissociates (or ionizes) completely in aqueous solution to form hydronium ions (H

_{3}

^{+}

)

A weak acid does not dissociate completely in aqueous solution to form hydronium ions (H

_{3}

^{+}

)

A strong base dissociates completely in aqueous solution to form hydroxide ions (OH

^{-}

)

A weak base does not dissociate completely in aqueous solution to form hydroxide ions (OH

^{-}

)

Examples of weak/strong acids and bases

Type	Examples
Strong Acids	hydrochloric acid (HCl), sulfuric acid (H $_{2}$ ‍SO $_{4}$ ‍), nitric acid (HNO $_{3}$ ‍)
Weak Acids	acetic acid (CH $_{3}$ ‍COOH), hydrofluoric acid (HF), oxalic acid (COOH) $_{2}$ ‍
Strong Bases	sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH)
Weak Bases	ammonium hydroxide (NH $_{4}$ ‍OH), ammonia (NH $_{3}$ ‍)

Weak acids and weak bases always exist as conjugate acid-base pairs in an aqueous solution as represented below

Here, HA is the acid and A

^{-}

is termed as the conjugate base of HA

In the above reaction, A

^{-}

is a base and HA is the conjugate acid of A

^{-}

Rule of thumb is: Weak acids have strong conjugate bases, while weak bases have strong conjugate acids. As shown in the above two reactions, if HA is a weak acid, then its conjugate base A $^{-}$ ‍ will be a strong base. Similarly, if A $^{-}$ ‍ is a weak base, then its conjugate acid HA will be a strong acid.

How do we define ‘titration’?

Titration is a technique to determine the concentration of an unknown solution. As illustrated in the titration setup above, a solution of known concentration (titrant) is used to determine the concentration of an unknown solution (titrand or analyte).

Typically, the titrant (the solution of known concentration) is added through a burette to a known volume of the analyte (the solution of unknown concentration) until the reaction is complete. Knowing the volume of titrant added allows us to determine the concentration of the unknown analyte. Often, an indicator is used to signal the end of the reaction, the endpoint. Titrant and analyte is a pair of acid and base. Acid-base titrations are monitored by the change of pH as titration progresses.

Let us be clear about some terminologies before we get into the discussion of titration curves.

Titrant: solution of a known concentration, which is added to another solution whose concentration has to be determined.
Titrand or analyte: the solution whose concentration has to be determined.
Equivalence point: point in titration at which the amount of titrant added is just enough to completely neutralize the analyte solution. At the equivalence point in an acid-base titration, moles of base = moles of acid and the solution only contains salt and water.
Diagram of equivalence point

Acid-base titrations are monitored by the change of pH as titration progresses

Indicator: For the purposes of this tutorial, it’s good enough to know that an indicator is a weak acid or base that is added to the analyte solution, and it changes color when the equivalence point is reached i.e. the point at which the amount of titrant added is just enough to completely neutralize the analyte solution. The point at which the indicator changes color is called the endpoint. So the addition of an indicator to the analyte solution helps us to visually spot the equivalence point in an acid-base titration.

Endpoint: refers to the point at which the indicator changes color in an acid-base titration.

What is a titration curve?

A titration curve is the plot of the pH of the analyte solution versus the volume of the titrant added as the titration progresses.

Let’s attempt to draw some titration curves now.

1) Titration of a strong acid with a strong base

Suppose our analyte is hydrochloric acid HCl (strong acid) and the titrant is sodium hydroxide NaOH (strong base). If we start plotting the pH of the analyte against the volume of NaOH that we are adding from the burette, we will get a titration curve as shown below.

Point 1: No NaOH added yet, so the pH of the analyte is low (it predominantly contains H

_{3}

^{+}

from dissociation of HCl).

As NaOH is added dropwise, H

_{3}

^{+}

slowly starts getting consumed by OH

^{-}

produced by dissociation of NaOH. Analyte is still acidic due to predominance of H

_{3}

^{+}

ions.

Point 2: This is the pH recorded at a time point just before complete neutralization takes place.

Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NaOH added = moles of HCl in the analyte. At this point, H

_{3}

^{+}

ions are completely neutralized by OH

^{-}

ions. The solution only has salt (NaCl) and water and therefore the pH is neutral i.e. pH = 7.

Point 4: Addition of NaOH continues, pH starts becoming basic because HCl has been completely neutralized and now excess of OH

^{-}

ions are present in the solution (from dissociation of NaOH).

2) Titration of a weak acid with a strong base

Let’s assume our analyte is acetic acid CH

_{3}

COOH (weak acid) and the titrant is sodium hydroxide NaOH (strong base). If we start plotting the pH of the analyte against the volume of NaOH that we are adding from the burette, we will get a titration curve as shown below.

Point 1: No NaOH added yet, so the pH of the analyte is low (it predominantly contains H

_{3}

^{+}

from dissociation of CH

_{3}

COOH). But acetic acid is a weak acid, so the starting pH is higher than what we noticed in case 1 where we had a strong acid (HCl).

As NaOH is added dropwise, H

_{3}

^{+}

slowly starts getting consumed by OH

^{-}

(produced by dissociation of NaOH). But analyte is still acidic due to predominance of H

_{3}

^{+}

ions.

Point 2: This is the pH recorded at a time point just before complete neutralization takes place.

Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NaOH added = moles of CH

_{3}

COOH in the analyte. The H

_{3}

^{+}

ions are completely neutralized by OH

^{-}

ions. The solution contains only CH

_{3}

COONa salt and H

_{2}

Let me pause here for a second - can you spot a difference here as compared to case 1 (strong acid versus strong base titration)??? In the case of a weak acid versus a strong base, the pH is not neutral at the equivalence point. The solution is basic (pH ~ 9) at the equivalence point. Let’s reason this out.

As you can see from the above equation, at the equivalence point the solution contains CH

_{3}

COONa salt. This dissociates into acetate ions CH

_{3}

COO

^{-}

and sodium ions Na

^{+}

. As you will recall from the discussion of strong/ weak acids in the beginning of this tutorial, CH

_{3}

COO

^{-}

is the conjugate base of the weak acid CH

_{3}

COOH. So, CH

_{3}

COO

^{-}

is relatively a strong base (weak acid CH

_{3}

COOH has a strong conjugate base), and will thus react with H

_{2}

O to produce hydroxide ions (OH

^{-}

) thus increasing the pH to ~ 9 at the equivalence point.

Point 4: Beyond the equivalence point (when sodium hydroxide is in excess) the curve is identical to HCl-NaOH titration curve (1) as shown below.

3) Titration of a strong acid with a weak base

Suppose our analyte is hydrochloric acid HCl (strong acid) and the titrant is ammonia NH

_{3}

(weak base). If we start plotting the pH of the analyte against the volume of NH

_{3}

that we are adding from the burette, we will get a titration curve as shown below.

Point 1: No NH

_{3}

added yet, so the pH of the analyte is low (it predominantly contains H

_{3}

^{+}

from dissociation of HCl).

As NH

_{3}

is added dropwise, H

_{3}

^{+}

slowly starts getting consumed by NH

_{3}

. Analyte is still acidic due to predominance of H

_{3}

^{+}

ions.

Point 2: This is the pH recorded at a time point just before complete neutralization takes place.

Point 3: This is the equivalence point (halfway up the steep curve). At this point, moles of NH

_{3}

added = moles of HCl in the analyte. The H

_{3}

^{+}

ions are completely neutralized by NH

_{3}

. But again do you spot a difference here??? In the case of a weak base versus a strong acid, the pH is not neutral at the equivalence point. The solution is in fact acidic (pH ~ 5.5) at the equivalence point. Let’s rationalize this.

At the equivalence point, the solution only has ammonium ions NH

_{4}

^{+}

and chloride ions Cl

^{-}

. But again if you recall, the ammonium ion NH

_{4}

^{+}

is the conjugate acid of the weak base NH

_{3}

. So NH

_{4}

^{+}

is a relatively strong acid (weak base NH

_{3}

has a strong conjugate acid), and thus NH

_{4}

^{+}

will react with H

_{2}

O to produce hydronium ions making the solution acidic.

Point 4: After the equivalence point, NH

_{3}

addition continues and is in excess, so the pH increases. NH

_{3}

is a weak base so the pH is above 7, but is lower than what we saw with a strong base NaOH (case 1).

4) Titration of a weak base with a weak acid

Suppose our analyte is NH

_{3}

(weak base) and the titrant is acetic acid CH

_{3}

COOH (weak acid). If we start plotting the pH of the analyte against the volume of acetic acid that we are adding from the burette, we will get a titration curve as shown below.

If you notice there isn’t any steep bit in this plot. There is just what we call a ‘point of inflexion’ at the equivalence point. Lack of any steep change in pH throughout the titration renders titration of a weak base versus a weak acid difficult, and not much information can be extracted from such a curve.

To summarize

In an acid-base titration, a known volume of either the acid or the base (of unknown concentration) is placed in a conical flask.
The second reagent (of known concentration) is placed in a burette.
The reagent from the burette is slowly added to the reagent in the conical flask.
A titration curve is a plot showing the change in pH of the solution in the conical flask as the reagent is added from the burette.
A titration curve can be used to determine:
1) The equivalence point of an acid-base reaction (the point at which the amounts of acid and of base are just sufficient to cause complete neutralization).
2) The pH of the solution at equivalence point is dependent on the strength of the acid and strength of the base used in the titration.
-- For strong acid-strong base titration, pH = 7 at equivalence point
-- For weak acid-strong base titration, pH > 7 at equivalence point
-- For strong acid-weak base titration, pH < 7 at equivalence point

[Attribution and references]

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Aram Durgerian
Posted 7 years ago. Direct link to Aram Durgerian's post “Why do amino acids, which...”
Why do amino acids, which I'm assuming are weak acids, have equivalence points below 7, as opposed to above 7 seen above, when titrated with a strong base like NaOH?
Button navigates to signup pageButton navigates to signup page
(12 votes)
Answer
- Hillary B.
  Posted 7 years ago. Direct link to Hillary B.'s post “Amino acids behave as pol...”
  Amino acids behave as polyprotic acids that have more than one dissociable proton because of the amide and carboxylate functionality. They essentially act as diprotic acids with two different equivalence points. One equivalence point below 7 and one above seven for the amino acid alanine for example.
  
  Typically, the conjugate base that is formed from the FIRST dissociation of a polyprotic acid is still considered a weak acid itself and will undergo acid hydrolysis with water. Therefore producing a pH at the first equivalence point that is less than 7.
  Comment on Hillary B.'s post “Amino acids behave as pol...”
  (29 votes)
Phoebe Houle
Posted 7 years ago. Direct link to Phoebe Houle's post “What is the half-way to t...”
What is the half-way to the equivalence point?
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(6 votes)
Answer
- jesse strayer
  Posted 6 years ago. Direct link to jesse strayer's post “The half equivalence poin...”
  The half equivalence point is the point where half of the titrant necessary to neutralize the analyte has been added. So the amount of protonated [HA] and deprotonated [A-] species of the analyte is equal.
  
  When [A-] and [HA] are equal, the henderson hasselbalch equation, pH = pKa + log([A-]/[HA]) simplifies to pH = pKa because [A-]/[HA] = 1 and log(1) = 0.
  Button navigates to signup page
  (34 votes)
alixvanpoperinghe
Posted 6 years ago. Direct link to alixvanpoperinghe's post “Why does it take the same...”
Why does it take the same volume of base to reach the equivalence point regardless of whether it is a strong or weak acid?
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(5 votes)
Answer
- elena
  Posted 5 years ago. Direct link to elena's post “Assuming the weak base an...”
  Assuming the weak base and the strong base both have the same concentration, then saying that they have the same volume also means the have the same number of moles. Ultimately it is the number of moles that matters because an equivalence point is reached when the number of moles of base reacting is the same as the number of moles of acid reacting (so the two are neutralized).
  
  The reaction between a weak acid and a strong base goes essentially to completion, so for problems we say that it will. So a weak acid reacting with a strong base reacts just as much as a strong acid reacting with a strong base. Since the weak and strong acids have the same molarities, it will require the same volume (and thus same number of moles) to reach the equivalence point when they react with a strong base.
  Button navigates to signup page
  (7 votes)
فاطمة المحتسب
Posted 7 years ago. Direct link to فاطمة المحتسب's post “why the normality of NaOH...”
why the normality of NaOH solutions may change if the solution is exposed ti air for an extend period of time?
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(3 votes)
Answer
- tyersome
  Posted 7 years ago. Direct link to tyersome's post “CO₂(g) from the air will ...”
  CO₂(g) from the air will dissolve in any aqueous (water based) solution. It reacts with water molecules to form carbonic acid (a weak acid) H₂CO₃(aq).
  
  This acid releases protons that will partially neutralize the basic NaOH.
  Button navigates to signup page
  (5 votes)
surabikiki
Posted 6 years ago. Direct link to surabikiki's post “Can u show where show whe...”
Can u show where show where the end point is sin a graph
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(4 votes)
Answer
- Meryl
  Posted 4 years ago. Direct link to Meryl's post “The endpoint is where the...”
  The endpoint is where the indicator changes color, which is around the top (end) of the steep curve.
  Button navigates to signup page
  (3 votes)
Apllo Trn
Posted 8 years ago. Direct link to Apllo Trn's post “Why does a WB + WA titrat...”
Why does a WB + WA titration curve look like that?

I thought a buffer can't be made from a SA or SB. It has to be a Wa and Cb. But what is going on here? The titration curve looks like it is behaving as a buffer
Button navigates to signup pageComment on Apllo Trn's post “Why does a WB + WA titrat...”
(4 votes)
Answer
- Dylan Ofri
  Posted 8 years ago. Direct link to Dylan Ofri's post “it depends on the ka/kb o...”
  it depends on the ka/kb of the wA and weak base, if you are using a weak acid and its conjugate base you can use the pKa and it will give you the equivalence point.
  Button navigates to signup page
  (3 votes)
natureforever.care
Posted 7 years ago. Direct link to natureforever.care's post “Can you give intuitive si...”
Can you give intuitive sight of what is meant by steepness of a slope?
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(2 votes)
Answer
- tyersome
  Posted 7 years ago. Direct link to tyersome's post “The size of the angle bet...”
  The size of the angle between the slope and horizontal.
  
  A larger angle (closer to vertical) is steeper.
  Button navigates to signup page
  (5 votes)
olivialeelatham
Posted 8 years ago. Direct link to olivialeelatham's post “In the "Titration of a st...”
In the "Titration of a strong acid with a weak base" section, HCl is used as the titrant, and NH3 serves as the titrand/ analyte solution. However, in the previous 2 videos, NH3 is used as the titrant, and HCl serves as the titrand/ analyte solution. Therefore, the resultant titration curves for the videos and this article are different even though they represent the same neutralization reaction. Does it matter whether the acid versus the base is used as the titrant versus the titrand/ analyte? Or are both titration curves accurate representations of this neutralization reaction? Thank you!
Button navigates to signup pageButton navigates to signup page
(3 votes)
Answer
- Thaq
  Posted 8 years ago. Direct link to Thaq's post “You are correct that both...”
  You are correct that both curves are representations of the same neutralization reaction, but the difference lies in what happens before and after the equivalence point. Thus, you end up with different graphs
  
  Before the equivalence point, you have an excess of the titrand/analyte solution in comparison to the titrant. In this lecture, the strong acid is the analyte so the pH up until the equivalence point will be quite acidic. As you can see in the videos, with the weak base as the analyte, the pH is initially much higher (until the equivalence point).
  
  You will see a similar relationship in terms of the pH after the equivalence point
  
  However, there should be no difference in the pH at the actual equivalence point as the neutralization reaction is the same: NH3 + H3O+ = NH4+ + H20
  
  Also note that the pH of the half-equivalence point will differ in these two examples for the same reason
  Button navigates to signup page
  (2 votes)
Chi Ecnn
Posted 6 years ago. Direct link to Chi Ecnn's post “How do you know the indic...”
How do you know the indicator that is suitable for a weak acid and a strong base or a strong acid and a weak base
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(3 votes)
Answer
suyashatmg17
Posted 5 years ago. Direct link to suyashatmg17's post “Why the part of the curve...”
Why the part of the curve beyond the equivalence point is similar in the graphs from strong and weak acid even though you titrated a strong acid and a weak acid??
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(2 votes)
Answer
- Addison Downey
  Posted 5 years ago. Direct link to Addison Downey's post “After titration, there is...”
  After titration, there is no acid left over. There is only base. Therefore, the right side of the graph will approach the pH of the base.
  Button navigates to signup page
  (2 votes)

MCAT

Course: MCAT > Unit 9

Acid-base titration curves

How do we define ‘titration’?

What is a titration curve?

To summarize

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