Aim # 31: How can we use indicators to estimate the pH of a solution? H.W. # 31 Study class notes Complete handout sheets (EXTRA CREDIT!)

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

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.

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.

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.

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

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?

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?