Chemical Indicators

Titration A titration is an important quantitative analytical technique. 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. 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. The standard reagent must react with the unknown in a definite and known relationship.

Acid – Base Titration Consider a titration of 25 ml of NaOH with a standardized solution of 0.10 M HCl. HCl + NaOH → NaCl + H 2 O Consider a titration of 25 ml of NaOH with a standardized solution of 0.10 M HCl. HCl + NaOH → NaCl + H 2 O If the titration required 45 ml of the 0.10 M HCl, the concentration of the NaOH can be calculated. If the titration required 45 ml of the 0.10 M HCl, the concentration of the NaOH can be calculated M HCl x L = mole HCl 0.10 M HCl x L = mole HCl mole HCl x 1 mole NaOH / 1 mole HCl = mole HCl x 1 mole NaOH / 1 mole HCl = mole NaOH mole NaOH mole NaOH / L = 0.18 M NaOH mole NaOH / L = 0.18 M NaOH

A Chemical Indicator HIn + H 2 O H 3 O + + In - [H 3 O + ] [In - ] K a = [HIn] K a = [HIn] [In - ] pK a = pH – log [HIn] Color IColor II Constant of the IndicatorpH determined by other components in the solution – not the indicator

Example: Typical Acid-Base Indicator pK a = 7.9 pK a = H 2 O + H 3 O + + H 2 O + H 3 O + [H 3 O + ] in this equilibrium is determined by the composition of the solution Phenol Red

Effect of pH on In - / HIn Ratio HIn + H 2 O H 3 O + + In - [In - ] [In - ] pK a = pH – log [HIn] At the midpoint of the indicator range… [In - ] = [HIn] [In - ] = [HIn] Therefore, equal concentrations of blue & yellow = green Therefore, equal concentrations of blue & yellow = green [In - ] / [HIn] = 1 [In - ] / [HIn] = 1 log [In - ] / [HIn] = log 1 = 0 log [In - ] / [HIn] = log 1 = 0 pH (midpoint range) = pK a pH (midpoint range) = pK a BlueYellow

Indicator Color on pH Scale Mid-range pK a xxxxxxx xxxxxxx xxxxxxx xxxxxxx x

Effect of pH on In - / HIn Ratio HIn + H 2 O H 3 O + + In - [In - ] [In - ] [In - ] [In - ] pK a = pH – log [HIn] or pH = pK a + log [HIn] Consider the situation when… [In - ] > 10 x [HIn] [In - ] > 10 x [HIn] The yellow of [In - ] masks the blue of [HIn] so solution appears yellow The yellow of [In - ] masks the blue of [HIn] so solution appears yellow [In - ] / [HIn] = 10 [In - ] / [HIn] = 10 log [In - ] / [HIn] = log 10 = 1 log [In - ] / [HIn] = log 10 = 1 So at 1 pH unit higher and above the solution appears yellow So at 1 pH unit higher and above the solution appears yellow BlueYellow

Indicator Color on pH Scale [In - ] 10 x [HIn] [In - ] 10 x [HIn] pH = pK a + log [HIn] or pH = pK a + log [HIn] pH = pK a + log [HIn] or pH = pK a + log [HIn] pH = pK a + 1 …and all pH’s > pK a pK a xxxxxxx xxxxxxx xxxxxxx xxxxxxx x xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxx xxxxx xxxxx xxxxx x Mid-range

Effect of pH on In - / HIn Ratio HIn + H 2 O H 3 O + + In - [In - ] [In - ] [In - ] [In - ] pK a = pH – log [HIn] or pH = pK a + log [HIn] Consider the situation when… [HIn] > 10 x [In - ] [HIn] > 10 x [In - ] The blue of [HIn] masks the yellow of [In - ] so solution appears blue The blue of [HIn] masks the yellow of [In - ] so solution appears blue [In - ] / [HIn] = 0.10 [In - ] / [HIn] = 0.10 log [In - ] / [HIn] = log 0.10 = -1 log [In - ] / [HIn] = log 0.10 = -1 So at 1 pH unit lower and below the solution appears blue So at 1 pH unit lower and below the solution appears blue BlueYellow

Indicator Color on pH Scale [In - ] [In - ] [In - ] [In - ] pH = pK a + log [HIn] = pH = pK a + log 10 x [In - ] pH = pK a - 1 …and all pH’s < pK a pK a xxxxxxx xxxxxxx xxxxxxx xxxxxxx x xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxx xxxxx xxxxx xxxxx x Mid-range xxxxx xxxxx xxxxx xxxxx x xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx x

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