The simplest quantitative method for determining pH is with the use of indicators. An indicator
is a colored substance usually derived from plant material that can exist in either an acid or
base form. The two forms have different colors. If one knows the pH at which the indicator
turns from one form to the other, one can then determine from the observed color whether the
solution has a pH higher or lower than this value. Methyl orange changes color over the pH in-
terval from 3.1 to 4.4. Below pH 3.1 it is in the acid form, which is red. In the interval from 3.1
to 4.4 it is gradually converted to the basic form, which is yellow. By pH 4.4 the conversion is
complete.
See Appendix B for a chart of acid-base indicators.
Scenario:Part I: pH of a Strong Acid
A student obtains 100.0 mL of 0.100 M HCl. The student prepares serial dilutions of the acid
to obtain solutions of the following concentrations: 0.0500 M, 0.0100 M, 0.00500 M and
0.00100 M. The student determines the pH of each of the solutions with a pH meter and then
from an assortment of indicators available and using Appendix B, adds the appropriate indica-
tor to samples of the solutions produced.
Part II: pH of a Weak Acid
This time the student obtains 100.0 mL of 0.100 M acetic acid solution and repeats the steps
from Part I.
Part III: pH of Various Salt Solutions
0.100 M solutions of NH 4 Ac, NaCl, NaAc, NH 4 Cl, NaHCO 3 and Na 2 CO 3 are available. The
student measures each solution with the calibrated pH meter and also adds appropriate indica-
tors to samples of each solution and observes color changes.
Part IV: pH of Buffer Solutions
The student prepares a buffer solution by adding 4.00 g of sodium acetate to 200 mL of 0.250
M acetic acid. The solution is then diluted to 100.0 mL with distilled water. The student then
obtains 4 beakers labeled 1, 2, 3 and 4. To beakers 1 and 2 she adds 50.0 mL of the buffer solu-
tion. To beakers 3 and 4 she adds 50.0 mL of distilled water. She then uses a calibrated pH me-
ter and obtains the pH of the solutions in the beakers. To beakers 1 and 3, she pipets 25.0 mL of
0.250 M HCl, mixes well and determines the pH using the calibrated pH meter. The pH of each
beaker was as follows: (1) 4.32, (2) 1.01, (3) 5.25, (4) 12.88. She also selects an appropriate in-
dicator and records the color produced. To beakers 2 and 4 she adds 25.0 mL of 0.250 M
NaOH and repeats the process as she did with the HCl.
Analysis:Because the data obtained from this lab is quite extensive, sample questions will be
asked to determine knowledge of the processes involved.
Part III: AP Chemistry Laboratory Experiments