Buffer Solutions and Acid-Base Indicators Demonstration Sheet

17.3 Buffer Solutions and Acid-Base Indicators

Chemical Concepts Demonstrated: Buffer solutions, acid-base indicators

Demonstration:

Solution A is both .200 M boric acid and .050 M citric acid.

Solution B is .100 M Na₃PO₄.

Create a buffer solution of a specific pH using the following chart:

pH desired: 2.0	Volume (mL) Solution A: 195	Volume (mL) Solution B: 5
2.5	184	16
3.0	176	24
3.5	166	34
4.0	155	45
4.5	144	56
5.0	134	66
5.5	126	74
6.0	118	82
6.5	109	91
7.0	99	101
7.5	92	108
8.0	85	115
8.5	78	122
9.0	69	131
9.5	60	140
10.0	54	146
10.5	49	151
11.0	44	156
11.5	33	167
12.0	17	183

These buffer solutions can be used to probe the effective ranges of various acid-base indicators, including the following:

Indicator:	pH range:	Color change:
methyl violet	0.0 - 1.6	yellow to blue-violet
thymol blue	1.2 - 2.8	red to yellow
bromophenol blue	3.0 - 4.6	yellow to blue-violet
methyl orange	3.2 - 4.4	red to yellow-orange
methyl red	4.4 - 6.2	red to yellow
litmus	5 - 8	pink to blue
bromocresol purple	5.2 - 6.8	yellow to purple
bromophenol red	5.2 - 6.8	yellow to red
bromothymol blue	6.2 - 7.6	yellow to blue
cresol red	7.2 - 8.8	yellow to red
thymol blue	8.0 - 9.6	yellow to blue
phenolphthalein	8.0 - 10.0	colorless to pink
alizarin yellow	10.0 - 12.0	yellow to red-violet

A sample universal indicator (covering a pH range of 1 through 13) can be prepared by dissolving .04 g of methyl red, .02 g of methyl orange, .02 g of phenolphthalein, .10 g of thymol blue, and .08 g of bromothymol blue in 100 mL of 95% ethanol. As the pH of the solution increases, the indicator changes color from red to orange to yellow to green to blue and finally to purple.