Lab 18 Estimation of pH

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					                   Eleanor Roosevelt High School
                      Dimitri Saliani, Principal
                       Chemistry Laboratory

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
           Reference Table.

 Data & Observations:

Solution      Methyl Litmus Bromothymol Phenolphthalein Estimated
               Red              Blue                        pH

0.1 M HCl

  0.1 M
acetic acid
0.1 M NH3

  0.1 M

     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.

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