Module 5 Intro

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.