Post on 24-Dec-2015
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Choosing an Appropriate Indicator
• Not every acid-base indicator is appropriate for signaling the neutralization point of a reaction.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• Recall that neutralization occurs when moles of H1+ from the acid = moles of OH1- from the base.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• Also that the pH at neutralization may be below 7, at 7, or above 7.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• The indicator chosen to signal the neutralization point must match the type of acid and base used.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• If the acid is strong and the base is weak, then the indicator chosen must change color below 7.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• If the acid is strong and the base is weak, then the indicator chosen must change color below 7.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• If the acid is strong and the base is strong, then the indicator chosen must change color around 7.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• If the acid is strong and the base is strong, then the indicator chosen must change color around 7.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• If the acid is weak and the base is strong, then the indicator chosen must change color above 7.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
• If the acid is weak and the base is strong, then the indicator chosen must change color above 7.
Table M
Common Acid-Base Indicators
Indicator
Approximate pH Range for Color
ChangeColor Change
methyl orange 3.2 – 4.4 red to yellow
bromthymol blue 6.0 – 7.6 yellow to blue
phenolphthalein 8.2 – 10 colorless to pink
litmus 5.5 – 8.2 red to blue
bromcresol green 3.8 – 5.4 yellow to blue
thymol blue 8.0 – 9.6 yellow to blue
Choosing an Appropriate Indicator
Choosing an Appropriate Indicator
1) Which of the following acid-base indicators would be appropriate to signal the end-
point of a neutralization between nitric acid (HNO3) and ammonium hydroxide (NH4OH)?
(1) phenolphthalein (3) litmus
(2) bromcresol green (4) thymol blue
strong acidweak base
Choosing an Appropriate Indicator
2) In which of the following scenarios would a drop of thymol blue change from yellow to blue?
(1) When it started in a beaker of 50 mL of 0.01M HCl and 50 mL of 0.10 M NaOH was added
(2) When it started in a beaker of 50 mL of 0.01M NaOH and 50 mL of 0.10 M HCl was added(3) When it started in a beaker of 50 mL of 0.01M HF and 50 mL of 0.10 M KOH was added(4) When it started in a beaker of 50 mL of 0.01M KOH and 50 mL of 0.10 M HF was added
Starts with a pH below 8. Ends with a pH above 9.6
Equal moles of H1+ and OH1- are being added.
strong acid &
strong base
weak acid &
strong base
Choosing an Appropriate Indicator
3) Choose an acid-base indicator that would appropriately signal the end-point of the
neutralization of HBr and LiOH?
________________________________
or
________________________________
bromthymol blue
litmus
strong acid strong base
Choosing an Appropriate Indicator
4) Choose an acid-base indicator that would appropriately signal the end-point of the
neutralization of KOH and HC2H3O2?
________________________________
or
________________________________
phenolphthalein
thymol blue
weak acidstrong base
Choosing an Appropriate Indicator
5) Choose an acid-base indicator that would appropriately signal the end-point of the
neutralization of HNO3 and Ca(OH)2?
________________________________
or
________________________________
methyl orange
bromcresol green
strong acid weak base