1. Aim # 31: How can we use indicators to estimate the pH of a solution?
H.W. # 31
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2. Do Now Complete the following table: Solution [H+] [OH-] pH
very acidic
<<1.0 x 10-7
>1.0 x 10-7
neutral
basic
>7

3. Indicators
Explain how you can estimate how hot or cold it is outside by looking out the window.
pH can be estimated similarly.
A substance whose color depends upon pH is called an indicator.

4. Experimenting With Indicators
In front of you are two unknown solutions, “X” and “Y”.
On the piece of paper, make a diagram of the well plate at your station.
Put 5 drops of X into 5 different wells Label these wells X-1, X-2, X-3, X-4, and X-5 on the diagram.
Put 5 drops of Y into 5 new wells.
Label these wells Y-1, Y-2, Y-3, Y-4, and Y-5 on the diagram.

5. Experimenting With Indicators (continued)
5. In the two #1 well plates, put one drop of
phenolphthalein.
6. In the two #2 well plates, put one drop of
methyl orange.
7. In the two #3 well plates, put one drop of
bromthymol blue.
8. In the two #4 well plates, put one drop of
litmus.
9. In the two #5 well plates, place one strip of the
universal indicator paper.

6. Record Your Observations
Make a table to record your observations for each well. 1 2 3 4 5 X Y

7. Drawing Conclusions
Use Reference Table M, which provides the pH range within which each indicator changes color, to determine the approximate pH of solutions X and Y.
How do your results compare with the color changes shown in the reference table?
Are there any inconsistencies?
Which solution is an acid?
Which solution is a base?

8. The Chemistry of Indicators
An Indicator is really a weak acid (HIn) whose color is different from that of its conjugate base (In-).
HIn ↔ H+ + In-
(yellow) (blue)
In which direction will the equilibrium shift if the indicator is placed in a more acidic solution?
In which direction will the equilibrium shift if the indicator is placed in a more basic solution?