Chemical IndicatorsChemical Indicators

TitrationTitration

A titration is an important quantitative A titration is an important quantitative analytical technique.analytical technique.

It is a process of determining the amount It is a process of determining the amount of a specific unknown substance by of a specific unknown substance by reacting it completely with a measured reacting it completely with a measured volume of a standard reagent.volume of a standard reagent.

The standard reagent must react with the The standard reagent must react with the unknown in a definite and known unknown in a definite and known relationship.relationship.

Acid – Base TitrationAcid – Base Titration

Consider a titration of 25 ml of NaOH Consider a titration of 25 ml of NaOH with a standardized solution of 0.10 M with a standardized solution of 0.10 M HCl. HCl + NaOH HCl. HCl + NaOH → NaCl + H→ NaCl + H22OO

If the titration required 45 ml of the 0.10 If the titration required 45 ml of the 0.10 M HCl, the concentration of the NaOH M HCl, the concentration of the NaOH can be calculated.can be calculated.

0.10 M HCl x 0.045 L = 0.0045 mole HCl0.10 M HCl x 0.045 L = 0.0045 mole HCl 0.0045 mole HCl x 1 mole NaOH / 1 mole HCl =0.0045 mole HCl x 1 mole NaOH / 1 mole HCl = 0.0045 mole NaOH0.0045 mole NaOH 0.0045 mole NaOH / 0.025 L = 0.18 M NaOH0.0045 mole NaOH / 0.025 L = 0.18 M NaOH

A Chemical IndicatorA Chemical Indicator

HIn + HHIn + H22O O HH33OO++ + In + In--

[H[H33OO++] [In] [In--]]

KKaa = = [HIn] [HIn]

[In[In--]]

pKpKaa = pH – log [HIn] = pH – log [HIn]

Color I Color II

Constant of the Indicator pH determined by other components in the solution – not the indicator

Example: Example: Typical Acid-Base IndicatorTypical Acid-Base Indicator

pKpKaa= 7.9= 7.9

+ H+ H22OO + H + H33OO++O

S

OO

HO

OH

phenolsulfonphthalein

O

S

OO

OH

phenolsulfonphthalein

O-

[H3O+] in this equilibrium is determined by the composition of the solution

Phenol Red

Effect of pH on InEffect of pH on In-- / HIn Ratio / HIn Ratio

HIn + HHIn + H22O O HH33OO++ + In + In--

[In[In--]]

pKpKaa = pH – log [HIn] = pH – log [HIn]At the midpoint of the indicator range…At the midpoint of the indicator range…

• [In[In--] = [HIn]] = [HIn]• Therefore, equal concentrations of blue & yellow = greenTherefore, equal concentrations of blue & yellow = green• [In[In--] / [HIn] = 1] / [HIn] = 1• log [Inlog [In--] / [HIn] = log 1 = 0] / [HIn] = log 1 = 0• pH (midpoint range) = pKpH (midpoint range) = pKaa

Blue Yellow

Indicator Color on pH ScaleIndicator Color on pH Scale

Mid-range

+2+1-2 -1 pKa

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Effect of pH on InEffect of pH on In-- / HIn Ratio / HIn Ratio

HIn + HHIn + H22O O HH33OO++ + In + In--

[In[In--] ] [In [In--]]

pKpKaa = pH – log [HIn] or pH = pK = pH – log [HIn] or pH = pKaa + log [HIn] + log [HIn]

Consider the situation when…Consider the situation when… [In[In--] > 10 x [HIn]] > 10 x [HIn] The yellow of [InThe yellow of [In--] masks the blue of [HIn] so solution appears yellow] masks the blue of [HIn] so solution appears yellow [In[In--] / [HIn] = 10] / [HIn] = 10 log [Inlog [In--] / [HIn] = log 10 = 1] / [HIn] = log 10 = 1 So at 1 pH unit higher and above the solution appears yellowSo at 1 pH unit higher and above the solution appears yellow

Blue Yellow

Indicator Color on pH ScaleIndicator Color on pH Scale

[In[In--] ] 10 x 10 x [HIn][HIn]

pH = pKpH = pKaa + log [HIn] or pH = pK + log [HIn] or pH = pKaa + log [HIn] + log [HIn]

pH = pKpH = pKaa + 1 …and all pH’s > pK + 1 …and all pH’s > pKaa + 1 + 1

+2+1-2 -1 pKa

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Mid-range

Effect of pH on InEffect of pH on In-- / HIn Ratio / HIn Ratio

HIn + HHIn + H22O O HH33OO++ + In + In--

[In[In--] ] [In [In--]]

pKpKaa = pH – log [HIn] or pH = pK = pH – log [HIn] or pH = pKaa + log [HIn] + log [HIn]

Consider the situation when…Consider the situation when… [HIn] > 10 x [In[HIn] > 10 x [In--]] The blue of [HIn] masks the yellow of [InThe blue of [HIn] masks the yellow of [In--] so solution appears blue] so solution appears blue [In[In--] / [HIn] = 0.10] / [HIn] = 0.10 log [Inlog [In--] / [HIn] = log 0.10 = -1] / [HIn] = log 0.10 = -1 So at 1 pH unit lower and below the solution appears blueSo at 1 pH unit lower and below the solution appears blue

Blue Yellow

Indicator Color on pH ScaleIndicator Color on pH Scale

[In[In--] ] [In [In--]]

pH = pKpH = pKaa + log [HIn] = pH = pK + log [HIn] = pH = pKaa + log 10 x [In + log 10 x [In--]]

pH = pKpH = pKaa - 1 …and all pH’s < pK - 1 …and all pH’s < pKaa - 1 - 1

+2+1-2 -1 pKa

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Mid-range

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Summary of Titration of Strong Acid Summary of Titration of Strong Acid with a Strong Basewith a Strong Base

There is a large jump in the pH at the There is a large jump in the pH at the equivalence point. This jump extends equivalence point. This jump extends from pH = 4 to about pH = 10. from pH = 4 to about pH = 10.

(The equivalence point is at pH = 7)(The equivalence point is at pH = 7)

Any indicator with a color change within Any indicator with a color change within this range (4 this range (4 7) can be used to signal 7) can be used to signal the end point of the titration.the end point of the titration.