Analytical ChemistryNeutral Titration

Introduction

Neutral titrations are considered the most volumetric analysis titrations practiced since they feature high accuracy and precision along side being quick and easy to practice. Hence, it has been used to estimate a number of organic and inorganic substances that feature acidic or basic qualities.

These titrations produce water and salt. The salt might be of a neutral effect. Therefore, the eq. point is situated in a neutral medium. It also might be of an acidic effect as a result of a weak base titration such as titrating ammonia with NH4Cl which forms ammonium, that contains an acid which leaves the eq. point in the acidic portion. The eq. point can also be situated in an alkaline medium like, the titration of a weak acid such as acetic acid with sodium hydroxide, then sodium acetates are formed which provide acetates of a base effect.

1. Titrating a strong acid with a strong base and vice versa

A. Titrating a strong acid with a strong base:

For e.g. titrating HCl with sodium hydroxide. Such titration characterizes by the enormous changes in the pH units around the eq. point.

HCl + NaOH NaCl + H2O

H+ + OH- H2O

this reaction is almost complete:

Keq = [H2O] = 1 = 1 x 10-14

Kw 1 x 10-14

The ZonePrinciple Elements in the

Titration FunnelpH Calculation

Before titration HCl pH = - log [HCl]

Before eq. point HCl + NaCl pH = - log mlHCl MHCl – mlNaOH MNaOH

mlHCl + mlNaOH

(strong acid solution R=1)

At eq. point NaCl pH = 7

After eq. point NaCl + NaOH pOH = - log mlNaOH MNaOH – mlHCl MHCl

mlHCl + mlNaOH

pH = 14 – pOH(strong base solution)

A. Titrating a strong acid with a strong base:

12

10

8

6

4

2

0

0.1 M

0.01 M

0.001 M

0.1 M

0.01 M

0.001 M

Ph.Ph

BcP

MO

NaOH (ml)

pH

A. Titrating a strong acid with a strong base:

B. Titrating a strong base with a strong acid:

The ZonePrinciple Elements in the

Titration FunnelpH Calculation

Before titration NaOH pOH = - log [NaOH]

Before eq. point NaOH + NaCl pOH = - log mlNaOH MNaOH – mlHCl MHCl

mlHCl + mlNaOH

At eq. point NaCl pH = 7

After eq. point NaCl + HCl pOH = - log mlHCl MNaOH – mlNaOH MNaOH

mlHCl + mlNaOH

12

10

8

6

4

2

0

0.1 M

0.001 M

0.1 M

0.001 M

HCl (ml)

pH

B. Titrating a strong base with a strong acid:

2. Titrating a weak acid with a strong base and vice versa:

A. Titrating a weak acid with a strong base:

Here, we obtain eq. point when the weak acid HA converts to NaA where the anion A- works as a base:

A- + H2O OH- + HA

Therefore, the eq. point will take place in the basic portion. For e.g. titrating 100 ml of the acid HA with the concentration of 0.1 molar and the contuent dissociation Ka = 10-5, with sodium hydroxide concentration 0.1 molar.

1. pH before addition

2. Before eq. point

3. At eq. point

4. After eq. point

1. pH before addition:We can determine the hydrogen ion concentration of the weak acid solution as following:[H+] = Ka Ca = 10-5 x 0.1 = 10-3 MpH = 3.00

2. Before eq. point:This means that we are coming across a solution that contains a weak acid and its conjugate, so this solution is a buffer solution. If the amount of the added base was 10 ml that means that the produced salt:[NaA] = 10 x 0.1 = 9 x 10-3 M

100 + 10and the remaining acid concentration:[HA] = 100 x 0.1 – 10 x 0.1 = 0.08M

100 + 10[H+] = Ka x Ca

Cs

= 10 -5 x 0.8 = 8.9 x 10-5 M

9 x 10-3

pH = 4.05

A. Titrating a weak acid with a strong base

After adding 50 ml of the base, half the acid would be equivalent, and therefore the acid concentration would be equal to the produced salt concentration:[NaA] = 50 x 0.1 = 5 = 0.03 M

100 +50 150[HA] = 100 x 0.1 – 50 x 0.1 = 5 = 0.03 M

100 + 50 150[H+] = Ka = 10-5

pH = 5.00which means:pH = pKa

This means that the hydrogen ion concentration equals the acid contuent dissociation and that is when it is equal to the acid by 50% or what is called midpoint.

A. Titrating a weak acid with a strong base

3. At eq. point:

The acid has transformed 100% to it’s salt NaA. Hence, the medium will be basic (because the salt is produced from the reaction between a weak acid and a strong base).

[NaA] = 100 x 0.1 = 0.05 M

100 + 100

[OH-] = kw Cs = 10-14 x 0.05 = 7.1 x 10-6 M

ka 10-5

pOH = 5.2

pH = 14 – 5.2 = 8.8

A. Titrating a weak acid with a strong base

A. Titrating a weak acid with a strong base

4. After eq. point:

The medium will be a strong base and will calculate the pH within it by the knowledge of the added sodium hydroxide concentration, and will neglect the effect of the basic salt because of its weak contribution ( its constant dissociation to OH- equals 10-9)

The following picture shows the change in pH with the titration progress. Observe how much pH changes at eq. point and its relation to the strength of the titrated acid.

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10

8

6

4

2

0

Ka = 10-3

Ka = 10-5

Ka = 10-7

Ka = 10-9

20 40 60 80 100 120 140 160 180 200

pH

The titration percentage (%)

The curves of titrating weak acids with sodium hydroxide solution

Example 1

Titrating NaOH with CH3COOH

NaOH + CH3COOH CH3COONa + H2O Before titration:

weak acid solution pH = -log Ka Ca

Before eq. point:A solution containing (CH3COONa + CH3COOH) and it is a buffer solution

pH = pKa + log Cs

Ca

Cs = mlNaOH MNaOH (R= 1)

mlNaOH mlCH3COOH

Ca = mlCH3COOH MCH3COOH - mlNaOH MNaOH

mlNaOH + mlCH3COOH

At eq. point:A solution that contains (NaOH + CH3COONa) neglects the effect of the salt which is a weak base and is calculated as a NaOH solution:

pOH = -log mlNaOH MNaOH - mlCH3COOH MCH3COOH

mlNaOH mlCH3COOH

B. Titrating a weak base with a strong acid

This is the inverse curve of titrating a weak acid with a strong base. In this curve, the eq. point is in the acidic portion because the salt BHX is formed. It gives the kation BH+ which is of an acidic quality.

BH+ + H2O B + H3O+

hence, the solution will obtain an acidic feature at eq. point.

The following picture shows such curves. It also shows the effect of the weak base constant dissociation value at the clarity of the end point (considering the number of pH units that change meanwhile) where the equilibrium constant of the reaction equals:

keq = kb

kw

To achieve the value 10-6 required in the equilibrium constant, kb must not be less than 10-

8.

Accordingly, weak bases with dissociated constants less than 10-8 can not be titrated.

B. Titrating a weak base with a strong acid

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8

6

4

2

0

Kb = 10-3

Kb = 10-5

Kb = 10-7

Kb = 10-9

20 40 60 80 100 120 140 160 180 200

pH

The titration percentage (%)

The curves of titrating weak bases with hydrochloric acid solutions

MO

BcP

Ph.Ph.

Example 2

Titrating NH3 with HCl:

HCl + NH3 NH4Cl (R = 1)

the titration curve derivation is similar to the previous titration curve. Before titration:

weak base solution: pOH = -log kb Cb

Before eq. point:The solution (NH3 + NH4Cl) which is a buffer solution:

pOH = pkb + log Cs

Cb

Cs = mlHCl MHCl

mlNH3 + mlHCl

Cb = mlNH3 MNH3 - mlHCl MHCl

mlNH3 + mlHCl

At eq. point:Salt solution NH4Cl (weak acid)

pH = - log kw Cs

kb

Cs = mlHCl MHCl = mlNH3 MNH3

mlHCl + mlNH3 mlNH3 + mlHCl

After eq. point:Solution (HCl + NH4Cl) is calculated based on the fact that it is a strong acid solution and neglects the effect of the salt NH4Cl.

pH = -log mlHCl MHCl - mlNH3 MNH3

mlHCl + mlNH3

Example 2

As we referred to before, when titrating a weak base with a strong acid, the pOH at mid-titration (at the titration of 50% of the base) is equal to pkb of the titrated base:

kb = [OH-] [NH+4]

[NH3]

pkb = pOH In conclusion, we realize the great importance of the titration curves which give us an idea

about the reaction. It also helps in choosing the best evidence to recognize the end point.

Example 2

The importance of evidence used in equilibrium titrations:

The evidence Evidence range Acidic color Basic color

Methyl orange 3.1 – 4.4 red yellow

Bromocresol purple 5.2 – 6.8 yellow purple

Phenol red 6.8 – 8.4 yellow red

phenolphthalein 8.3 – 10.0 colorless red pink

Methyl orange evidenceColor: basic yellow and acidic red

PhenolphthaleinColor: basic pink and the acidic medium is colorless

The importance of evidence used in equilibrium titrations: