1. Module 5 Intro

1.33. Page 4

Lesson 5

Module 5—Acids and Bases

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Estimating the pH of Solutions Using Indicators

 

Indicators can be used as an inexpensive method to estimate the pH of a solution. You may have noticed as you completed the previous Self-Check questions that a single indicator only provides limited information about pH. When estimating pH, more than one indicator is used. Work through the following example to see how indicators are used to estimate the pH of a solution.

 

Example: A solution is tested with three indicators. The results are summarized in the following table. Estimate the pH of this solution.

 

Indicator

Colour

bromothymol blue

blue

phenolphthalein

colourless

phenol red

red

 

 

 


 

Step 1: Use the Acid-Base Indicators table to determine the pH range.

 

Acid-Base Indicators at 298.15 K

 

Indicator

Suggested Abbreviation(s)

pH Range

Colour Change as pH Increases

Ka

methyl violet

HMv(aq)/Mv–(aq)

0.0–1.6

yellow to blue

~2 × 10–1

bromothymol blue

HBb(aq) / Bb–(aq)

6.0–7.6

yellow to blue

5.0 × 10–8

phenol red

HPr(aq) / Pr–(aq)

6.6–8.0

yellow to red

1.0 × 10–8

phenolphthalein

HPh(aq) / Ph–(aq)

8.2–10.0

colourless to pink

3.2 × 10–10

 

According to the table, the three indicators show the following:

  • bromothymol blue: blue = above pH 7.6
  • phenolphthalein: colourless = below pH 8.2
  • phenol red: red = above pH 8.0

Step 2: Use a number line to narrow the range of pH values.

 

A number line is labeled from pH 6 and extending to pH 10, and the number line has arrows showing it continues in both directions. A line indicating the range over which phenol red appears red starts over pH 8 and extends along the line to higher pH values. A second line shows the range over which phenolphthalein is colourless. This second line starts at pH 8.2 and extends toward lower pH values. A third line shows the pH range over which bromothymol blue is blue starts at pH 7.6 and extends toward the higher pHs. Two vertical dotted lines appear at pH 8 and pH 8.2, the overlapping section of all three lines representing the indicators. These two vertical dotted lines are accompanied by a label stating the pH of the solution is between 8.0 and 8.2.


 

The estimated pH of the solution is between 8.0 and 8.2.

 

Self-Check

 

SC 5. Three test tubes with 5 mL of the same solution are in a test tube rack. A different indicator is added to each test tube. Here are the results.

 

Test Tube

Indicator

Colour

1

phenolphthalein

colourless

2

cresol red

yellow

3

chlorophenol red

red

 

Use the data to estimate the pH of the solution.

 

Check your work.
Self-Check Answers

 

SC 5. According to the data, the three indicators show the following:

  • phenolphthalein: colourless = below pH 8.2
  • cresol red: yellow = below pH 7.0
  • chlorophenol red: red = above pH 6.8
Therefore, the estimated pH of the solution is between 6.8 and 7.0.