U2 S3 L1 Indicators page : Neutralization Reactions Textbook Items p 616: item 8

Upon completion of this lesson, you should be able to: define acid/base indicators operationally define acid/base indicators theoretically as weak acids and write net ionic equations to represent the reversible nature of an acid-base indicator equilibrium system use Le Châtelier's principle to predict indicator colour changes when acid or base is added to a solution containing a specific indicator species determine the pH of a solution using indicator

An indicator is a weak acid or weak base which changes color in response to specific change in pH. –The pH range at which an indicator molecule changes from the weak acid form to the conjugate base form is a function of the ease with which the proton can be removed from the weak acid form. Example: Litmus is an indicator. It has two forms: – a weak acid form (HLt ) which reflects pink light in low pH conditions –and a conjugate base form (Lt - ) which reflects purplish- blue light in higher pH conditions.

phenolphthalein is colourless below pH 8.2 and deep pink above pH –If you add a drop of phenolphthalein solution to a solution and you see a deep red colour (Ph - ), then you know that the pH of the solution is something above pH –However, if you see no colour (HPh), then the pH of the solution is below pH 8.2. A slight pink colour tells you that the pH is somewhere between pH 8.2 and 10.0 because similar amounts of both forms of the indicator will be present in this pH range.

Phenol red indicator (HPr) has a yellow colour below pH 6.6 and a red colour above pH 8.0. a.Write the net ionic equation for the reaction between phenol red (HPr) indicator and water. b.Predict the position of the indicator equilibrium when a drop of the indicator is added to a flask containing 50 mL of M nitric acid. c.Suggest a way to shift the position of the indicator equilibrium.

Using the Indicator Table: What is the approximate pH range of a solution if it caused methyl red indicator to turn yellow and phenol red indicator to turn yellow?

1.Write a net ionic equation for the reaction of each weak acid form of these indicators with water: a.bromothymol blue, HBb (aq) b.congo red, HCr (aq) c.methyl orange, HMo (aq) 2.A flask contains 100 mL of M ethanoic acid and two drops of bromothymol blue. What form of the indicator should be favoured in the flask? Predict the changes that should occur when excess NaOH is added to the flask. Justify your predictions. 3.A flask contains 100 mL of M ammonia and two drops of congo red indicator. What form of the indicator should be favoured in the flask? Predict the changes that should occur when excess HCl is added to the flask. Justify your predictions. 4.Combinations of indicators can be used to determine the approximate pH's of solutions. Predict the pH range for each solution given these indicator colour results. a.congo red indicator is red and chlorophenol red indicator is yellow. b.metacresol purple indicator is yellow and chlorophenol red indicator is red. c.both thymolphthalein and indigo carmine indicators are blue. 5.Determine the colour and form of each indicator at the stated pH condition. The formula for each indicator in the weak acid form is given in parentheses. a.methyl orange (HMo) at pH 7.0 b.alizarin yellow R (HAy) at pH 9.7 c.litmus (HLi) at pH 8.7 d.metacresol purple (HMp) at pH 3.4 e.thymol blue (H 2 Tb) at pH 3.1 f.thymol blue (HTb - ) at pH 14.0 g.phenolphthalein (HPh) at pH 9.0 h.Methyl violet indicator is a yellow colour in a 1.0 mol/L solution. Is the solution a strong acid, strong base, weak acid, or weak base. Explain.