Neutralization Titration (Acid/base titration) Dr. Mohammad Khanfar.

Neutralization Titration (Acid/base titration)

Dr. Mohammad Khanfar

Acid/Base Titration

• Standard Solutions • The standard solutions used in neutralization titrations are

strong acids or strong bases because these substances react more completely with an analyte than do their weaker counterparts, and they therefore provide sharper end points.

• HCI, HCIO4, H2S04, NaOH, and KOH.

Acid/Base Titration

• Acid/Base Indicators • Many substances, both naturally occurring and synthetic,

display colors that depend on the pH of the solutions in which they are dissolved.

• An acid/base indicator is a weak organic acid or a weak organic base whose undissociated form differs in color from its conjugate base or its conjugate acid form.

• Indicator pH range = pKa ± 1• Indicator should fall within the range of inflection of the

strong acid/strong base titration curve.

Acid/Base Titration

• Internal structural changes accompany dissociation and cause the color change

Acid/Base Titration

Methyl orange, pKa 3.7

Titration Of Strong Acids And Strong Bases

• Consider the titration of 50 ml of 0.05 M HCl (analyte) with 0.1M NaOH (titrant).

• First we determine equivalent point• M1V1 = M2V2• V = 25ml• Initial Point • Before any base is added, the solution is 0.0500 M in H30+, and

pH = -log[H30+] = -log 0.0500 = 1.30

• After Addition of 10.00 mL of Reagent • The H+ ion concentration is decreased as a result of both reaction with the

base and dilution. • pH = 1.6

HCl + NaOH NaCl + H2O

Titration Of Strong Acids And Strong Bases

• After Addition of 25.00 mL of Reagent (The Equivalence Point) • At the equivalence point, neither HCl nor NaOH is in excess, and so the

concentrations of hydronium and hydroxide ions must be equal• pH = 7 • After Addition of 25.10 mL of Reagent • The solution now contains an excess of NaOH, pH = 10.12

Titration Of Strong Acids And Strong Bases

• The Effect of Concentration

Titration Of Strong Acids And Strong Bases

Titration curves for HCI with NaOH. Curve A: 50.00 mL of 0.0500 M HCI with 0.1000 M NaOH. Curve B: 50.00 mL of 0.000500 M HCI with 0.001000 M NaOH.

The Effect of Concentration

Titrating a Strong Base with a Strong Acid

• Titration curves for strong bases are derived in an analogous way to those for strong acids.

• Short of the equivalence point, the solution is highly basic, the hydroxide ion concentration being numerically related to the analytical molarity of the base.

• The solution is neutral at the equivalence point and becomes acidic in the region beyond the equivalence point then the hydronium ion concentration is equal to the analytical concentration of the excess strong acid.

Titrating a Strong Base with a Strong Acid

• Titration curves for NaOH with HCl. Curve A: 50.00 mL of O.0500 M NaOH with 0.1000 M HCl. Curve B: 50.00 mL of 0.00500 M NaOH with 0.0100 M HCl.

Strong Acid/Strong Base Titration

• Calcium hydroxide (Ca(OH)2)solution

• Although this is a fairly strong alkaly and is titrated with HCl solution, methyl orange is not used as indicator. The solution absorbs carbon dioxide from the atmosphere to form carbonate (pKa 7.7) but since the free Ca(OH)2 and not the total alkali content is required, phenolphthaline is used as indicator.

Weak acid/strong base and weak base/strong acid

• On addition of a small volume of the strong acid or strong base to a solution of the weak base or weak acid, the pH rises or falls rapidly to about 1 pH unit below or above the pKa value of the acid or base. Often a water-miscible organic solvent such as ethanol is used to dissolve the analyte prior to addition of the aqueous titrant.

• Example. Titration of 50.0 ml of 0.1 M acetic acid (pKa 4.76) with 0.1 M NaOH.

• Initial pH: calculated from [H+] = (ka. c)1/2

• pH before equivalence point: A state of equilibrium between the acid and its conjugate base formed, and the pH is calculated from Henderson–Hasselbalch equation

Weak acid/strong base and weak base/strong acid

• At equivalence point: all acetic acid is consumed and converted to sodium acetate, and pH is calculated starting from [OH-] = (Kb. c)1/2

• pH after equivalence point: After the addition of excess amount of NaOH, both the excess base and the acetate ion are sources of hydroxide ion. The contribution from the acetate ion is small, however, because the excess of strong base represses the reaction of acetate with water.

• pH = 14 – pOH

Weak acid/strong base and weak base/strong acid

Curve for the titration of acetic acid with sodium hydroxide. Curve A: 0.10 M acid with 0.1000 M base. Curve B: 0.0010 M acid with 0.0010 M base.

Weak acid/strong base and weak base/strong acid

Titration curve of 25 ml of 0.1M solution of aspirin (pKa 3.5) titrated with 0.1M NaOH

Weak acid/strong base and weak base/strong acid

Titration curve of 25 ml of 0.1M solution of quinine(pKa 8.05) titrated with 0.1M HCl

Weak acid/strong base and weak base/strong acid

• For an acid, the measured pH when the acid is exactly half neutralized is numerically equal to pKa. For a weak base, the pH at half titration must be converted to pOH, which is then equal to pKb.

• Weak acid with strong base, end point will be basic then phenolphthalein can be used in general

• Weak base with strong acid, end point is acidic so methyl orange can be used in general.

The effect of acid strength (dissociation constant) on titration curve . Each curve represents the titration of 50.00 mL of 0.1000 M acid with 0.1000 M base.The pH change in the equivalence-point region becomes smaller as the acid becomes weaker, that is, as the reaction between the acid and the base becomes less complete.

5-fluorouracilA: Ureide nitrogen, very weak acidic, pKa 13 B: Ureide nitrogen, weak acid, pKa 7.0

GuanithedineA: guanidine moiety, one of the strongest nitrogenous base pKa 11.4B: Tertiary amine, pKa 8.3

SulphadiazineA: pyrimidine ring, very weak basic nitrogen , pKa < 2B: Sulfonamide nitrogen, weak acid, pKa 6.5C: Weakly basic aromatic amine (aniline) pKa < 2

Weak acid/strong base and weak base/strong acid

• Blank titration• In general, blank determinations are used if the volumetric

solution is unstable or if it alters in strength during the assay (heating, interaction with glass, of absorption of atmospheric CO2).

• Titration without analyte.

Weak acid/strong base and weak base/strong acid

• Polyfunctional acids and bases • Some acids and bases can donate or accept more than one

proton, i.e. 1 mole of analyte is equivalent to mort than one mole of titrant. If the pKa values of any acidic or basic groups differ by more than 4, then the compound will have more than one inflection in its titration curve.

pKa 10.32pKa 6.38

Weak acid/strong base and weak base/strong acid

Titration of 1 M sodium carbonate with 1M HCl

Weak acid/strong base and weak base/strong acid