Select your language

Suggested languages for you:
Log In Start studying!
StudySmarter - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
Free
|
|

All-in-one learning app

  • Flashcards
  • NotesNotes
  • ExplanationsExplanations
  • Study Planner
  • Textbook solutions
Start studying

Polyprotic Acid Titration

Save Save
Print Print
Edit Edit
Sign up to use all features for free. Sign up now
Polyprotic Acid Titration

You have probably heard of phosphoric acid before. Phosphoric acid is an acid that is widely used in fertilizers, detergents, and even as an acidity regulator in cereal bars and jams! Phosphoric acid is considered a polyprotic acid. So, if you are interested to know about titrations of polyprotic acids, keep reading!

  • First, we will take a look at what a polyprotic acid titration curve looks like.
  • Then, we will learn the attributes of a weak polyprotic acid titration curve.
  • Next, we will look at polyprotic acid titration calculation and problems.
  • Lastly, we will look at some examples of polyprotic acid titration graphs.

Polyprotic Acid Titration Curve

First, let's review monoprotic acids. Monoprotic acids are acids that can yield only one H+ (also written as, H3O+) per molecule of acid. They possess a single ionizable hydrogen atom in each molecule.

where, A-, symbolizes the anionic (negatively charged) species. Examples of monoprotic acids include HCl, HClO4, and HNO3.

where all of the above three different reactions produce the same cationic (positively charged) species, H3O+ (also written as, H+) but yield different anionic species. The first reaction produces the, Cl-, anion, the second reaction produces the ClO4-, anion and the last reaction produces the NO3-, anion.

Now, let's define what a polyprotic acid is.

A polyprotic acid is an acid that has more than one ionizable hydrogen (H) atom.

In other words, they have more than one acidic proton, so they can undergo more than one dissociation reaction Examples of polyprotic acids include H2SO4 and H3PO4. Let's look at the dissociation of H3PO4(aq). Phosphoric acid has three acidic protons, and therefore, its ionization happens in three steps.

Notice in this case, that these three dissociation steps are part of a single reaction.

pH, Ka, and pKa

The acid dissociation constant measures the degree of dissociation of hydrogens in an acid. Scientists use the acid dissociation constant (Ka) to determine the strength of acids. We can use the equations below to find Ka and pKa, which is the -log10 of Ka.

Ka is sometimes called acid ionization constant or acidity constant!

Let's look at the Ka equations for a polyprotic acid H3A (also called a triprotic acid). What is happening here is that water gains a proton (H+) to become a hydronium ion (H3O+), whereas phosphoric acid loses a proton and becomes a conjugate base! Since this is a triprotic acid, we have three ionizations that occur:

Now that we know what monoprotic and polyprotic acids are, let's jump into the titrations! Titrations are used by chemists to determine the unknown concentration of an acid or base. Then, we can use the data gathered from the titration to draw a titration curve.

A titration curve (sometimes called a pH curve) is a graph that shows how the pH of a solution changes when an acid or base is added to it.

You can learn more about titrations by checking out "Types of Acid-Base Titrations".

Titration curves are also used to find out the number of acidic protons in an acid. A typical titration curve for a weak polyprotic acid looks like this:

This pH curve has three different acid dissociation constants, meaning that the acid has three acidic protons. For now, just take the time to familiarize yourself with what it looks like. We will learn how to interpret them in a bit.

Attributes of a Weak Polyprotic Acid Titration Curve

Let's go back to our previous example showing the dissociation of phosphoric acid. H3PO4 is a weak polyprotic acid that is capable of losing three protons per molecule. In H3PO4, the values for Ka are:

  • Ka1 =
  • Ka2 =
  • Ka3 =

For each successive dissociation, the acid dissociation constant (Ka) value gets smaller because it becomes more difficult to remove the ionizable hydrogen (Ka1 > Ka2 > Ka3).

If we calculated the pKa values for each Ka, we would get:

  • pKa1 =
  • pKa2 =
  • pKa3 =

The titration curve below shows the titration of H3PO4 with a strong base.

Titrations of Polyprotic Acids Titration curve of weak phosphoric acid with a strong base Attributes of a weak Polyprotic acid titration curve StudySmarterTitration curve of weak phosphoric acid with a strong base, Isadora Santos - StudySmarter Original.

Let's interpret this pH curve. The graph shows the changes in pH that occur when phosphoric acid is titrated with a strong base, adding OH- ions to the solution. So, before adding any strong base (such as NaOH), all we have is a solution of a weak acid (in this case, H3PO4). The arrows point to the half-equivalence point.

The half-equivalence point is the point in a titration where there are equal concentrations of the acid and its conjugate base. For a weak acid, [HA] = [A-] , at the half-equivalence point; where [HA], is the concentration of the acid and [A-], is the concentration of the conjugate base.

The Conjugate Base is the anionic species A-, which results from dissolving an acid HA, in water.

In this titration curve, there are three different half-equivalence points. At the half-equivalence point, the pH is equal to the pKa value. For example, if the value for pKa at the first half-equivalence point is equal to 2.15, then pH at that point will also be 2.15.

The titration curve also shows three equivalence points. At this point, moles of H+ = moles of OH-.

The equivalence point is the point where the number of moles of titrant added is equal to the number of moles of analyte that was originally present.

In our case, the titrant is the base OH-, added and the analyte is acid H+.

Polyprotic Acid Titration Calculations

Have you heard the quote "practice makes perfect"? This also applies here! So, let's take a look at a calculation problem involving a polyprotic acid!

Find the pH of a 0.1 M solution of carbonic acid (H2CO3). The Ka values are: Ka1 = 4.3 x 10-7, and Ka2 = 4.8 x 10-11.

To calculate the pH of a polyprotic acid, we need to consider all the dissociation reactions involved. In this case, carbonic acid is a diprotic acid, so it has two dissociation reactions.

Since we are dealing with a weak acid, first we need to make an ICE chart for the first dissociation. The acronym "ICE" stands for "Initial concentration, "Change in concentration" and "Equation for concentration". This is important because we must figure out the concentration of HCO3- before making an ICE chart for the second dissociation reaction! Once we find the total concentration of protons, then we can calculate pH!

  1. Add the initial concentration of carbonic acid (0.1 M) to the ICE chart.
  2. Next, we add the change in concentration in terms of x.
  3. Then, we write the equation for concentration at equilibrium.

Titrations of Polyprotic acids ICE chart first ionization reaction of carbonic acid Polyprotic acid titration calculations StudySmarterICE chart for the first ionization reaction of carbonic acid, Isadora Santos - StudySmarter Originals.

Now, we can calculate the value of x by adding the terms in lines "I" and "C" and then substituting the terms in line "E" into the equation for Ka1.

Now that we have the value of x, we can use it as the values for the initial concentration of HCO3- and H+ at the second dissociation reaction. So, let's make another ICE chart.

Titrations of Polyprotic acids ICE chart second dissociation reaction of carbonic acid Polyprotic acid titration calculations StudySmarterICE chart for the second dissociation reaction of carbonic acid, Isadora Santos - StudySmarter Original.

Now, we can calculate the value of y using the equation of Ka2.

Finally, we can calculate the total concentration of protons and use it to find pH. In the first ICE chart, we found the concentration of H+ to be 2.1 x 10-4. But, since the second Ka is tiny compared to the first Ka, we can completely neglect it!

So, in this case, pH will be equal to -log10 (2.1 x 10-4), which gives us a pH of 3.68

Polyprotic Acid Titration Problems

In some exam problems, you might be asked to find the pKa values of a polyprotic acid by looking at a titration curve. For example, let's say that you are given the titration curve for the titration of 0.1 M citric acid, H3C6H5O7 with a 0.1 M solution of a strong base (NaOH).

The pH of the three equivalence points is given. But, to find pKa, we need to first figure out the pH at the half-equivalence points. Remember that at the half-equivalence point, pH = pKa.

  • The pH at the first half-equivalence point is 3.13. So, pKa1 = 3.13
  • The pH at the second half-equivalence point is 4.76. Therefore, pKa2 = 4.76
  • The pH at the third half-equivalence point is 6.40. Therefore, pKa3 = 6.40

Now, if a question asks you to calculate the Ka values, use the pKa values that you found and use the following equation to calculate Ka for each pKa!

Polyprotic Acid Titration Graph

Lastly, let's look at an example of a polyprotic acid titration graph. The titration graph for 0.1 M of maleic acid (C4H4O4) with 0.1 M sodium hydroxide shows two equivalence points. In this titration curve, the pKa1 = 1.83, where the pKa2 = 6.07.

Now, I hope that you are feeling more confident in your ability to some problems involving titrations of polyprotic acids!

Titrations of Polyprotic Acids - Key takeaways

  • Monoprotic acids are acids that have can yield one H+ per molecule of acid.
  • A polyprotic acid is an acid that has more than one ionizable hydrogen (H) atom.
  • In polyprotic acids, for each successive dissociation, the acid dissociation constant (Ka) value gets smaller because it becomes more difficult to remove the ionizable hydrogen (Ka1 > Ka2 > Ka3).

References

  1. Moore, John T, and Richard Langley. McGraw Hill : AP Chemistry, 2022. New York, Mcgraw-Hill Education, 2021. ‌
  2. Theodore Lawrence Brown, et al. Chemistry : The Central Science. 14th ed., Harlow, Pearson, 2018. ‌
  3. “Titration of a Polyprotic Acid.” Www.chem.fsu.edu, Bluedoor Labs Chemistry, www.chem.fsu.edu/chemlab/bluedoor_background.html. ‌

Frequently Asked Questions about Polyprotic Acid Titration

A polyprotic acid is an acid that has more than one ionizable hydrogen (H) atom and can undergo more than one dissociation reaction during titration.

To calculate the pH of a weak polyprotic acid, we need to find the total concentration of protons, taking into consideration all the dissociation reactions the acid can undergo. 

To titrate a polyprotic acid, we can use titration, which is a technique used to find the unknown concentration of an acid or base, also known as the analyte, by adding an acid or base with a known concentration, called a titrant.

We can find the equivalence point of a polyprotic acid by performing a titration. 

Polyprotic acids will have more than one equivalence point present in the titration curve.

Final Polyprotic Acid Titration Quiz

Question

______ are acids that can yield only one proton per molecule of acid.

Show answer

Answer

Monoprotic acids

Show question

Question

Which of the following are examples of monoprotic acids?

Show answer

Answer

HCl

Show question

Question

Which of the following are considered polyprotic acids?

Show answer

Answer

H3PO4

Show question

Question

How many dissociation reactions happen in H3PO4

Show answer

Answer

1

Show question

Question

In the first dissociation reaction of phosphoric acid, water ____ a proton (H+) to become a hydronium ion (H3O+), whereas phosphoric acid ____ a proton and becomes a conjugate base!

Show answer

Answer

gains; loses

Show question

Question

A titration curve is a graph that shows how the pH of a solution _______ when an acid or base is added to it.

Show answer

Answer

changes

Show question

Question

In a polyprotic titration, for each successive dissociation, the acid dissociation constant (Ka) value gets _____.

Show answer

Answer

bigger

Show question

Question

The______  is the point in a titration where there are equal concentrations of each species.

Show answer

Answer

half-equivalence point

Show question

Question

 In a weak acid, _____ at the half-equivalence point. 

Show answer

Answer

 [HA] = [A-]

Show question

Question

The equivalence point is the point where the number of moles of titrant added is ____  the number of moles of analyte that was originally present.

Show answer

Answer

equal to

Show question

Question

How many equivalence points does a titration graph of a titration of 0.1 M carbonic acid with a strong base show? 

Show answer

Answer

2

Show question

Question

If the pH at the half-equivalence point is 3.13, what is the pKa value?

Show answer

Answer

3.13

Show question

Question

How many equivalence points would you expect to see in the titration curve of HCl? 

Show answer

Answer

Only one because HCl is a monoprotic acid

Show question

60%

of the users don't pass the Polyprotic Acid Titration quiz! Will you pass the quiz?

Start Quiz

Discover the right content for your subjects

No need to cheat if you have everything you need to succeed! Packed into one app!

Study Plan

Be perfectly prepared on time with an individual plan.

Quizzes

Test your knowledge with gamified quizzes.

Flashcards

Create and find flashcards in record time.

Notes

Create beautiful notes faster than ever before.

Study Sets

Have all your study materials in one place.

Documents

Upload unlimited documents and save them online.

Study Analytics

Identify your study strength and weaknesses.

Weekly Goals

Set individual study goals and earn points reaching them.

Smart Reminders

Stop procrastinating with our study reminders.

Rewards

Earn points, unlock badges and level up while studying.

Magic Marker

Create flashcards in notes completely automatically.

Smart Formatting

Create the most beautiful study materials using our templates.

Sign up to highlight and take notes. It’s 100% free.