CP CHEMISTRY
Indicators Lab
Name: ________________________________
___ period
Background:
Acid/base indicators are chemicals that have one color in an acidic solution and
another color in a basic solution. They can be used to determine whether a solution
is acidic, basic, or neutral. They can be used to estimate the pH of a solution. They
are also used in titrations to determine when the equivalence point of the
neutralization has occurred.
Purpose:
Compare the colors of three indicators (bromthymol blue,
phenolphthalein, and Universal indicator) in a series of different pH
solutions and use them to estimate the pH of an unknown solution.
Procedure:
1.
Put on safety goggles.
2.
Obtain a 24 count well plate and bottles of various reagents: vinegar, seltzer
water, household ammonia, 0.1 M NaHCO3, pH 7 solution, and unknown pH
solution.
3.
Use the data table below to identify which solutions will go into which wells
of the well plate. Put the well plate on top of a white piece of paper to assist
you in determining the colors seen once the indicators have been added.
NOTE: the solutions with the various pH values are shown across the top of
the well plate and the indicators are listed along the side.
4.
Put 20 drops of each solution in the correct wells. Then add 1 drop of
bromthymol blue to each well in the first row. Add 1 drop of Universal
indicator to each well in the second row. Add 1 drop of phenolphthalein to
each well in the third row. Using a different toothpick for each well, stir the
contents of each well to ensure complete mixing.
5.
Record the colors seen in each well of the well plate in the data table below.
Estimate the pH of the unknown solution.
6.
Rinse out the well plates thoroughly in the sink and throw all other trash into
the garbage. Leave the well plate to dry on a paper towel.
CP CHEMISTRY, Indicators Lab, page 2
Data:
Indicators
pH Solutions
Vinegar
pH = 3
Seltzer
water
pH = 5
Neutral
pH = 7
0.1 M
NaHCO3
pH = 10
Household
Ammonia
pH = 12
Unknown
___ pH
solution
Bromthymol
Blue
Universal
Indicator
Phenolphthalein
1.
2.
At what pH do the indicators change color?
Bromthymol blue
____________________________________
Universal Indicator
____________________________________
Phenolphthalein
____________________________________
Which indicator gives no information in the acid to neutral pH range?
Conclusions:
1.
When bromthymol blue is green, what is the relationship between hydrogen
ions, H+, and hydroxide ions, OH-, in the solution? (>, <, or =) Explain.
CP CHEMISTRY, Indicators Lab, page 3
2.
What is the approximate pH of your unknown solution? Explain how you
were able to determine this based on your data.
3.
Is your unknown solution acidic, basic, or neutral? How do you know?
4.
Which indicator would be most useful in testing a variety of solutions?
Explain.