Name: ______Date: ______Period: ______Page: ______
pH and Indicators Lab
Objectives: SWBAT measure and record the pH values at which indicators change colors, in order to estimate each indicator’s pKa.
Introduction: An acid-base indicator is usually a weak acid with a characteristic vivid color. Upon addition of base, this acid is converted into its conjugate base, which is a different color. When the weak acid and its conjugate base exist in solution in approximately equal concentrations, an intermediate color is evident. For example, the common acid-base indicator bromthymol blue (BTB) is a weak acid that is yellow; its conjugate base is blue. This is illustrated by the following chemical equation:
- + HBTB + H2O ⇔ BTB + H3O
Yellow blue protonated acid unprotonated base HA A-
The acid ionization constant, Ka, for the above reaction is expressed as a ratio of the ionized form of the acid to the form that is not ionized: note [ ] = concentration
- + When [BTB ] = [HBTB], then Ka = [H3O ], or pKa = pH. In other words, the pH at which the indicator changes - color is approximately equal to the pKa of the acid (the pH when [HA]= [A ] = pKa.
- [BTB ] = [HBTB] pKa = pH Blue Yellow green
Purpose: In this experiment you will investigate the color changes of various acid-base indicators as a function of pH. To do this you will set up a colorimetric pH meter using universal indicator, and then set up another pH meter using a single indicator. You can then see the color of the single indicator at each of the 12 pH units and determine at what pH it changes color. From this data you can approximate the indicator’s pKa.
Safety: Wear safety glasses at all times.
Materials: Small-scale pipets of the following solutions: Universal indicator (UI) buffer solutions (pH 1-12) Thymol blue (TB) Bromothymol blue (BTB) phenolphthalein (phen) methyl red (MR) Bromophenol blue (BPB) Small scale reaction surface
Procedure: Caution: ALL the indicators stain you and your clothing!!! 1. Place the copy of the data table on the lab bench and then cover with the reaction surface. 2. Add one drop of the indicator and then 3 drops of the pH buffer solution to the appropriate square. 3. Observe and record the color changes (on your laboratory data table) of all possible indicators. Cleaning up: Carefully clean the small-scale reaction surface by absorbing the contents onto a paper towel. Rinse the reaction surface with a damp paper towel and dry it. Dispose of the paper towels in the waste bin. Wash your hands thoroughly with soap and water. Universal BTB Phen BPB MR TB Indicator p H 1 p H 2 p H 3 p H 4 p H 5 p H 6 p H 7 p H 8 p H 9 p H 10 p H 11 p H 12
U n kn o w n Data T Name(s): ______Signature(s): ______pH and Indicators Analysis
1. Fill in the following chart.
a. What is the color of the conjugate acid form of each indicator?
b. What is the color of the conjugate base form of each indicator?
c. For each indicator, what is the pH at which acids and base forms are in approximately equal concentrations?
d. Estimate the pKa of each indicator.
Color of the Color of conjugate pH when Indicator Color of pKa conjugate acid form base form - [HA] = [A ] = pKa
BTB Yellow Blue 7 green
BPB
TB
*2 pKas
phen
MR
2. List the indicator(s) that change color in acidic solutions?
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3. Which indicator would you choose to show that a solution has changed from pH 3 to 5?
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4. Which indictor would you choose to show that a solution has a change from pH 9 to 10?
______**Return Your Universal Indicator**
Unknown: Get an unknown form your teacher. Use what you have learned in this lab to determine its pH.
What is the pH of your unknown? In complete sentences, explain how you came to this conclusion.
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