Chapter 3, Problem 2P

(a)

Summary Introduction

To determine: The key point in the given titration curve where glycine is present as ⁺H₃N-CH₂-COOH.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The titration of amino acids with a base occurs in three stages. Firstly when the solution is acidic, that is, the $\text{pH}$ is low; secondly when isoelectric point of the amino acid is attained where amino acids exists in its zwitterionic form, and third when the concentration of hydroxyl ions is the highest and the solution acts as a buffer solution.

The amino acid exists in the protonated form in the starting of the titration where concentration of hydrogen ions is high and the concentration of hydroxyl ions is very low. The glycine ion in the form ⁺H₃N-CH₂-COOH consists of protonated amino acid group and protonated carboxylic acid group is present at low $\text{pH}$ which corresponds to the point I in the titration curve. Therefore, glycine exists as ⁺H₃N-CH₂-COOH in the point I in the given titration curve.

(b)

Summary Introduction

To determine: The key point in the given titration curve where average net charge of glycine is $+\frac{1}{2}$.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of $\text{pH}$ versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The glycine molecule consist of $+\frac{1}{2}$ charge in the titration curve when half of the hydrogen ions present in the solution get neutralised by the hydroxyl ions under the acidic conditions, when the carboxylic acid is in protonated form. The neutralization of half of the total hydrogen ions present occurs when the concentration of hydroxyl ion is 0.5 M. Therefore, the charge on glycine is $+\frac{1}{2}$ when the titration is at point II in the given titration curve.

(c)

Summary Introduction

To determine: The key point in the titration curve when half of the amino groups are ionized.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The titration of glycine with sodium hydroxide starts with the ionization of hydrogen ions present on the carboxylic acid group. When all the hydrogen ions present on the carboxylic acid group get neutralised, then the ionization of amino group begins that occurs at point III in the given titration curve. After this stage, the ionization of amino group starts and it reaches at half ionization when 0.5 more equivalents of hydroxyl ions are added to the solution. This drives the neutralization of half of the amino groups of glycine which corresponds to the point IV in the titration curve.

(d)

Summary Introduction

To determine: The key point in the given titration curve where pH of solution is equal to the pK_a of carboxyl group.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The stage in the titration curve when pH of the solution is equal to the pK_a of carboxylic acid group is when half of the carboxylic acid groups present in the solution get neutralized. This leads to the equal concentration of carboxylic acid and its conjugate base. The neutralization of half of the carboxylic acid groups occurs at the key point II. Therefore, pH of the solution is equal to pK_a of carboxylic acid group at key point II in the given titration curve.

(e)

Summary Introduction

To determine: The key point in the given titration curve where pH of solution is equal to the pK_a of protonated amino group.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The stage in the titration curve when pH of the solution is equal to the pK_a of protonated amino group is the stage when half of the amino groups get neutralized which leads to the equal concentration of amino groups and its conjugate base. The neutralization of half of the amino groups occurs at the key point IV. Therefore, pH of the solution is equal to pK_a of carboxylic acid group at key point IV in the given titration curve.

(f)

Summary Introduction

To determine: The key point in the given titration curve where glycine has maximum buffering capacity.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

Buffer is the solution which resists any change in pH on the addition of small amounts of alkali or acid. The range in the given titration curve where the change in $\text{pH}$ is minimum corresponds to the key point points II and IV. Therefore, glycine molecule has maximum buffering capacity at the key point points II and IV.

(g)

Summary Introduction

To determine: The key point in the given titration curve where average net charge on glycine is zero.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The charge on the amino acid is zero in the titration curve when the number of protonated amino groups present in the solution is equal to the ionized carboxylic acid groups that occur at the isoelectric point. The isoelectric point is the point in the titration when the amino acid exists in zwitterionic form in the solution having net charge zero. This is defined by key point III in the given titration curve.

(h)

Summary Introduction

To determine: The key point in the given titration curve where carboxylic acid group is completely titrated.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The charge on the amino acid is zero in the titration curve when the number of protonated amino groups present in the solution is equal to the ionized carboxylic acid group which occurs at the isoelectric point. The isoelectric point is the point when the amino acid exists as zwitter ion in the solution having net charge zero which is defined by the key point III in the given titration curve. At this point, all the carboxylic acid groups present in the solution get titrated and the titration is at its first equivalence point. Therefore, carboxylic acid group has been completely titrated at the key point III in the given titration curve.

(i)

Summary Introduction

To determine: The key point in the given titration curve where glycine is completely titrated.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The point in the titration curve where glycine is fully titrated is the end point of the titration where all the hydrogen ions present on the amino group and the carboxylic acid group is completely ionized by the hydroxyl ions of the base. Therefore, glycine is completely titrated at the key point V in the given titration curve.

(j)

Summary Introduction

To determine: The key point in the given titration curve where glycine is predominantly present as⁺H₃N-CH₂-COO^-.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The charge on the amino acid is zero in the titration curve when the number of protonated amino groups present in the solution is equal to the ionized carboxylic acid group which occurs at the isoelectric point. The isoelectric point is the point when the amino acid exists as zwitter ion in the solution having net charge zero which is defined by the key point III in the given titration curve. The species which predominantly exists at the isoelectric point is ⁺H₃N-CH₂-COO^-. Therefore, glycine exists as ⁺H₃N-CH₂-COO^- in the key point III.

(k)

Summary Introduction

To determine: The key point in the given titration curve where the average charge on glycine is -1.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The point in the titration curve where glycine is fully titrated is the end point of the titration where all the hydrogen ions present on the amino group and the carboxylic acid group are completely ionized by the hydroxyl ions of the base. At this point amino group is present as NH₂ and carboxylic acid group is present as COO^- having -1 net charge on the molecule. Therefore, glycine has net charge equal to -1 at the key point V in the given titration curve.

(l)

Summary Introduction

To determine: The key point in the given titration curve where glycine is present as 50:50 mixture of ⁺H₃N-CH₂-COOH and⁺H₃N-CH₂-COO^-.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The stage in the titration curve when half of the carboxylic acid groups get ionized leads to the equal concentration of carboxylic acid and its conjugate base in the solution. The ionization of half of the carboxylic acid groups leads to the formation of equal concentration of the species ⁺H₃N-CH₂-COOH and ⁺H₃N-CH₂-COO^- which occurs at the key point II. Therefore, the given species predominantly exists as 50:50 mixture of ⁺H₃N-CH₂-COOH and ⁺H₃N-CH₂-COO^- at key point II in the given titration curve.

(m)

Summary Introduction

To determine: The end point in the given titration curve which is known as isoelectric point.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The stage in the titration where the number of protonated amino groups present in the solution is equal to the ionized carboxylic acid group is known as the isoelectric point. The isoelectric point is the point when the amino acid exists as zwitter ion in the solution having net charge zero which is defined by the key point III in the given titration curve.

(n)

Summary Introduction

To determine: The key point in the given titration curve which is end of the titration.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

The point in the titration curve where glycine is fully titrated is the end point of the titration where all the hydrogen ions present on the amino group and the carboxylic acid group are completely ionized by the hydroxyl ions of the base. The amino group exist as NH₂and carboxylic acid group as COO^- and there are no more hydrogen ions present for ionization. This is the end of titration. Therefore, the glycine is completely titrated at the key point V in the given titration curve.

(o)

Summary Introduction

To determine: The region in the given titration curve which corresponds to the worst pH region for buffering power.

Introduction:

Titration of amino acids can be done in two ways; one by the addition of alkali and another by the addition of acid because they contain one amino group and one carboxylic acid group. The titration curve of amino acids is a plot of pH versus the concentration of alkali or acid.

Explanation of Solution

Explanation:

Buffer solution is the solution which resists any change in pH on the addition of small amounts of alkali or acid. The range in the given titration curve where the change in pH is minimum corresponds to the best range for the solution to acts as buffer solution. The region from key points I, III, and V involves the maximum change in the pH of the solution due to which this region is the worst range for buffering power.