Acid-alkali Titrations

Acid alkali titration is a quantitative analysis method in which we obtain information about a solution, that contains acid or base. This type of titration gives information about the concentration as well as the strength of an unknown acid or base. The endpoint is detected by using acid-base chemical indicators.

When strong acid reacts with a strong base to neutralize each other, the only product is water. The enthalpy of neutralization is always the same for all strong acid and strong bases which is 57 Kj/mol.

What is acid-Base titration?

“The process of obtaining information about a solution that contains acid and base is known as acid-base titration.”

Acid-base titration can give information about:

  • The concentration of an acid and base
  • The strength of an unknown acid and base
  • The pKa value of unknown acid and pKb of unknown base

Terms used in acid-base titration

Titration

It is a method of volumetric analysis in which the volume of one reagent is added to the known volume of another reagent until an endpoint is reached.

Titrant

The titrant is a solution of known concentration which is added to another solution to determine the concentration of the analyte. It is recommended that the titrant should be an acid or a strong base known concentration. It is taken in the burette.

Titrand or Analyte

The analyte is a substance whose concentration is to be identified and measured in the procedure. It is maybe either weak acid or base or strong acid or base. Its known volume must be taken in a conical flask and titrated against the titrant.

Endpoint and Equivalence point 

Endpoint and Equivalence point are the frequently used terms in acid-base titration. The equivalence point is the point in the titration at which the added amount of the titrant is enough to neutralize the analyte solution whereas, The endpoint is the condition at which the reaction is observable with a naked eye. this happens due to the change in the color of chemical indicators.

Titration curve

It is a graphical representation of the change in pH of the solution upon adding the titrant during the titration process.

 

Types of acid-alkali titration

Strong acid versus strong base titration

In this type, the titrant and analyte are present in completely ionized form. On titrating, they react with each other to form a neutral solution. This is because of the high value of Ka and Kb of the strong acid and base respectively.

Strong acid

It is a substance that completely dissociates into ions in the aqueous solution.

The dissociation of strong acid:

acid alkali titration

  • HA = Strong acid
  • H+ = hydrogen ions
  • A = conjugate base of the acid

Strong base

A strong base is a substance that completely dissociates in water and produces a large number of hydroxide ions.

The dissociation of strong base:

acid alkali titration

  • XOH = a strong base
  • X+ = conjugate acid of a base
  • OH = hydroxide ion

For Example

  • Titration of hydrochloric acid with sodium hydroxide

When HCl reacts with NaOH, the net ionic equation remains the same because of the complete dissociation of HCl and NaOH. the resulting NaCl also remains ionized and known as spectator ions. Thus, water is the only product of the neutralization reaction.

acid alkali titration

Dissociation

acid alkali titration

Left behind

acid alkali titration

  • Titration curve of a strong acid and strong base

The titration curve for the 100mL of 0.1M HCl titrated with 0.1M NaOH is given below:

 

    1. At the start of the reaction, there is 0.1M HCl so the pH is approx 1.0. As the reaction proceeds, the concentration of H+ ions decreases due to the formation of H2O.
    2. At 90% neutralization, only 10% H+ remains so the pH gradually increases. At the equivalence point neutralization is complete, therefore, pH will be 7.0.
    3. After passing the equivalence point if we add more base initially the pH increases and at the end slopes off.

Weak acid versus strong base titration

This process involves the direct transfer of a proton from the weak acid to the strong base.

In this type of titration, a weak acid is an analyte whereas the strong base is the titrant.

weak acid vs strong base titration

For Example

  • Titration of acetic acid with sodium hydroxide

The acetic acid is only a few percent ionized. The reaction between acetic acid and sodium hydroxide is given below:

acid alkali titration

  • Titration curve of a weak acid and strong base

The titration curve for the 100mL of 0.1M HOAc with 0.1M NaOH is the following:

  1. Before the titration starts the initial pH of the analyte is higher than that of strong acid.
  2. At the beginning of titration, the pH will increase slowly than a sudden change in pH.
  3. At the equivalence point, moles of NaOH added are equal to the moles of acetic acid in the analyte.

At the equivalence point, the pH of the solution is not neutral and is slightly greater than 7. This is due to the presence of dissociated salt. Acetate ions are present in the solution which are the strong conjugate bases of a weak acid. They react with water to produce hydroxide ions. Therefore,  it increases the pH of equivalence point.

Strong acid versus Weak base titration

The titration of a weak base with a strong acid is similar to the above example. In this type, the analyte is a weak base and the titrant is a strong acid.

strong acid vs weak base

For Example

  • Titration of hydrochloric acid with ammonia.

The reaction between hydrochloric acid and ammonia is given below

  • Titration curve of a weak base and strong acid

The titration curve of a weak base and strong acid just reverses the above example. The titration curve of 100mL of 0.1M NH3 titrated with 0.1M HCl is the following:

Titration curve of a strong acid and weak base

 

 

  • At the start of the titration, we have 0.1M NH3 and the pH will be almost 11.
  • When titrant is added to the analyte NH3 is converted into NH4+. At the midpoint of titration [NH+4] = [NH3] so that pH will be equal to the pKb.
  • At the equivalence point, we have a solution of NH4Cl. At this point, the moles of strong acid added are equal to the initial moles of the base.

Weak acid versus Weak base titration

This type of titration is rare because the endpoint is not so clearly identified. Moreover, suitable indicators are not available, hence mixed indicators system is required.

Acid-base indicators

The indicators for the acid-alkali titration are the highly colored weak organic acids and bases. The color of the nonionized form is different from the ionized form.

For example, if the indicator is a weak acid and designated by Hln. Assume that the ionized form is pink while the nonionized form is colorless

acid alkali titration

The color of the indicator also changed by changing the pH.

Acid base titration: types of indicators

Concepts Berg

What is titration used for?

Titration is a method that is used to determine the unknown concentration of the analyte. It is a method of quantitative chemical analysis.

How is titration done?

During the titration, the titrant(solution with known concentration) is added to the analyte. At the equivalence point, further addition is stopped. The endpoint can be detected by the use of an indicator.

What indicator is used in acid-base titration?

The indicators used for acid-base titration are weak acids or weak bases. For example, phenolphthalein and methyl orange.

Why is titration repeated three times?

Titration is repeated three times to get accurate readings. It is done to obtain a statistically valid answer.

What is the meaning of acid-base titration?

It is a method of quantitative analysis by which we can determine the concentration of acid and base. It is done by neutralizing it with the standard solution of acid and base.

What are the types of acid-base titration?

There are the following types of acid-base titration:

  • Strong acid vs strong base titration
  • Strong acid vs weak base titration
  • Strong base vs weak acid titration
  • weak acid vs weak acid titration

What is the midpoint of acid-base titration?

The point at which enough acid or base has been added to neutralize half of the acid or base is known as the midpoint of the acid-alkali titration.

  • Reference books

Vogel’s quantitative analysis 5th edition

  • Reference links

Strong acid vs strong base titration ( chem.libretext.org)

 

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