acid base titration

ACID BASE TITRATION &COMPLEXOMETRIC

TITRATIONAnalytical Chemistry

Gary D Christian

Strong Acid Versus Strong Base

• Acid base titration involves a neutralization reaction

• Titration curve is constructed for end point detection

• The titrant is always a strong acid or strong base• The analyte may be either strong acid or base or

weak acid or base• The equivalence point is where the reaction is

theoretically complete

Titration Curve for 100 mL of 0.1 M HCl versus 0.1 M NaOH

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Phenolphthalein Transition range

mL NaOH

pH

Equation Governing a Strong Acid (HX) or Strong Base (BOH) Titration

Fraction F Titrated

Strong Acid Strong Base

Present Equation Present Equation

F=0 HX [H+]=[HX] BOH [OH-]=[BOH]

0<F<1 HX/X- [H+]=[remaining HX] BOH/B+ [OH-]=[remaining BOH]

F=1 X- [H+]=Kw B+ [H+]=Kw

F>1 OH-/X- [OH-]=[excess titrant] H+/B+ [H+]=[excess titrant]

Detection of The End Point: Indicator

• The point at which the reaction is observed to be complete is called end point

• The selection of indicator become more critical as the solution become more dilute

• The goal is for the end point to coincide with the equivalence point

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Dependence of the magnitude of the end point break on concentration

Phenolphthalein Transition range

Bromothymol blue Transition range

Methyl orange Transition range

HCl 1.0 M vs NaOH 0.1 M

HCl 0.01 M vs NaOH 0.01 M

HCl 0.001 M vs NaOH 0.001 M

Indicator

• Indicator of acid base titration is a weak acid or weak base that highly colored

• The color of the ionized form is markedly different from that of the unionized form

HIn = H+ + In-

Red Blue• Henderson-Hasselbalch equation

• Your eye can generally discern only one color if it is 10 times as intense as the other

HIn

InpKpH In

log

Indicator

10

1log InpKpH

1

10log InpKpH

pH=pKIn - 1

pH=pKIn + 1

• Most indicator require transition range of about two pH units

• The pKa of the indicator should be close to the pH of the equivalence point

Weak Acid Versus Strong Base

• The curve is flattest and the buffering capacity the greatest at the midpoint

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Phenolphthalein Transition range

Methyl red Transition range

Ka =10-4

Ka =10-8

Ka =10-10

Equation Governing a Weak Acid (HA) or Strong Base (BOH) Titration

Fraction F Titrated

Strong Acid

Present Equation

F=0 HX [H+]=Ka M

0<F<1 HX/X- [H+]=Ka CA-/CHA

F=1 X- [H+]=Kw/Ka CA-

F>1 OH-/X- [OH-]=[excess titrant]

• Strong acid are actually good buffers, except their pH change with dilution

• Weak acid titration require careful selection of the indicator

• Acid weaker than pKa = 8 must be titrated in non aqueous solvent

Titration of Sodium Carbonate

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Methyl orange Transition range

QUESTION• Explain why boiling the solution near the end point in the titration of

sodium carbonate increases the sharpness of the end point• What is the minimum pH change required for a sharp indicator color

change at the end point?• What criterion is used in selecting an indicator for particular acid

base titration?• Calculate the pH at 0, 10, 25, 50 and 60 mL of titrant in the titration

of 25 mL of 0.2 M HA (Ka=2x10-5) with 0.1 M NaOH.• What volume of H2SO4 is required to titrate 0.293 g of 90% pure

LiOH.• A 0.527 g sample of a mixture containing Na2CO3, NaHCO3 and inert

impurity is titrated with 0.109 M HCl, requiring 15.7 mL to reach the phenolphthalein end point and a total of 43.8 mL to reach the modified methyl orange end point. What is the percent each of Na2CO3 and NaHCO3 in the mixture?

COMPLEXOMETRIC TITRATION

• The formation complex can also serve as a basis of acurat and convenient titration for metal ions.

• We are going to describe titration of metal ions with the very useful complexing agent EDTA.

Complexes: Formation constant

• Ammonia is simple complexing agent with one pair of unshared electrons that will complex copper or silver ions:

• Cu2+ + :NH3 = [Cu(NH3)]2+

• Ag+ + NH3 = Ag(NH3)+

• Kf = [Ag(NH3)+]/[Ag+][NH3] = 2.5x107

• Kf is called the formation constant or stability constant

CHELATES: EDTA

• An organic agent that has two or more group capable of complexing with a metal ion is called a chelating agent

• The complex formed is called a chelate• Titrations with a chelating agent is called a chelatometric

titration, a type of complexometric titration• The most widely used chelating agent in titration is

ethylendiamintetraacetic acid (EDTA)

NCH2CH2N

CH2COOH

CH2COOHHOOCCH2

HOOCH2

EDTA Equilibria

• H4Y = H+ + H3Y-

• H3Y = H+ + H2Y-

• H2Y = H+ + HY-

• HY = H+ + Y-

• Ka1 = [H+][H3Y-]/[H4Y]

• Ka2 = [H+][H2Y-]/[H3Y-]

• Ka3 = [H+][H2Y-]/[HY-]

• Ka4 = [H+][Y-]/[HY-]

Effect of pH on EDTA Equilibria

Frc

1.0

0.8

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0.2

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0 2 4 6 8 10 12 14 pH

H4YH3Y- H2Y- HY- Y-

EDTA Titration Curve

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pH 10

pH 7

pH 5