Chemical Indicators

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Chemical Indicators
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Titration

• A titration is an important quantitative
  analytical technique.
• It is a process of determining the amount of a
  specific unknown substance by reacting it
  completely with a measured volume of a standard
  reagent.
• The standard reagent must react with the unknown
  in a definite and known relationship.

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Acid Base Titration

• Consider a titration of 25 ml of NaOH with a
  standardized solution of 0.10 M HCl. HCl
  NaOH ? NaCl H2O
• If the titration required 45 ml of the 0.10 M
  HCl, the concentration of the NaOH can be
  calculated.
• 0.10 M HCl x 0.045 L 0.0045 mole HCl
• 0.0045 mole HCl x 1 mole NaOH / 1 mole HCl
• 0.0045 mole NaOH
• 0.0045 mole NaOH / 0.025 L 0.18 M NaOH

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A Chemical Indicator

• HIn H2O H3O In-
• H3O In-
• Ka HIn
• In-
• pKa pH log HIn

Color I
Color II
Constant of the Indicator
pH determined by other components in the solution
not the indicator
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Example Typical Acid-Base Indicator
Phenol Red

• pKa 7.9
• H2O H3O

H3O in this equilibrium is determined by the
composition of the solution
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Effect of pH on In- / HIn Ratio

• HIn H2O H3O In-
• In-
• pKa pH log HIn
• At the midpoint of the indicator range
• In- HIn
• Therefore, equal concentrations of blue yellow
  green
• In- / HIn 1
• log In- / HIn log 1 0
• pH (midpoint range) pKa

Blue
Yellow
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Indicator Color on pH Scale
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Effect of pH on In- / HIn Ratio

• HIn H2O H3O In-
• In- In-
• pKa pH log HIn or pH pKa log
  HIn
• Consider the situation when
• In- gt 10 x HIn
• The yellow of In- masks the blue of HIn so
  solution appears yellow
• In- / HIn 10
• log In- / HIn log 10 1
• So at 1 pH unit higher and above the solution
  appears yellow

Blue
Yellow
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Indicator Color on pH Scale

• In- 10 x HIn
• pH pKa log HIn or pH pKa log
  HIn
• pH pKa 1 and all pHs gt pKa 1

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Effect of pH on In- / HIn Ratio

• HIn H2O H3O In-
• In- In-
• pKa pH log HIn or pH pKa log
  HIn
• Consider the situation when
• HIn gt 10 x In-
• The blue of HIn masks the yellow of In- so
  solution appears blue
• In- / HIn 0.10
• log In- / HIn log 0.10 -1
• So at 1 pH unit lower and below the solution
  appears blue

Blue
Yellow
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Indicator Color on pH Scale

• In- In-
• pH pKa log HIn pH pKa log 10 x
  In-
• pH pKa - 1 and all pHs lt pKa - 1

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Summary of Titration of Strong Acid with a Strong
Base

• There is a large jump in the pH at the
  equivalence point. This jump extends from pH 4
  to about pH 10.
• (The equivalence point is at pH 7)
• Any indicator with a color change within this
  range (4 ? 7) can be used to signal the end point
  of the titration.

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