Eleanor Roosevelt High School
Dimitri Saliani, Principal
LAB # 18: Estimation of pH
Introduction: If you are interested in gardening, you may know
that plants such as azaleas and rhododendrons refer acidic soils. A
part of soil testing consists of the measurement of the pH of the soil.
The pH is important in many other applications in the home and in
industry. For example, maintenance of the proper pH of water is
important for caring for swimming pools. There are several ways to
test pH. One of the most important ways is by means of acid-base
indicators such as phenolphthalein. In neutral and acidic solutions,
phenolphthalein is colorless. In alkaline solutions, it is pink. By
adding a few drops of a weak solution of the indicator to a solution to
be tested, you can tell immediately whether the test solution is
alkaline. Other acid-base indicators change colors at different pH
values. By systematically testing a solution with a series of indicators,
you can often arrive at a good estimate of the pH of a solution.
In this experiment, you will estimate the pH of several solutions
by using acid-base indicators. You will then use the same indicators to
test the pH of several common household chemicals.
1- well plate 0.1 M hydrochloric acid
0.1 M ethanoic acid 0.1 M ammonia
0.1 M sodium hydroxide distilled water
red litmus paper blue litmus paper
methyl red indicator bromothymol blue indicator
1) Place a few drops of each acid or base in a well. You may use
a piece of white paper underneath the well for labeling.
2) Add a drop of methyl red to each well and record the color in
the data table.
3) Repeat for each indicator being sure to rinse the well plate
before each reusing a well.
4) Estimate the pH of each chemical by using Table M in the
Data & Observations:
Solution Methyl Litmus Bromothymol Phenolphthalein Estimated
Red Blue pH
0.1 M HCl
0.1 M NH3
1) Compare the pH of 0.1 M ethanoic acid with that of 0.1 M
hydrochloric acid. Explain any differences.
2) Which of the indicators used in this experiment could most
accurately identify a neutral solution? Explain.
3) How does an acid-base indicator work? You may use a
specific example to explain your answer.