Demonstration of Color Changes with pH of Indicators

Buffers between pH 2 and 12 were prepared (see below). The various pH solutions were used to half-fill test tubes, which were lined up in test tube racks with pH 2 on the left and pH 11 on the right (pH 12 didn't fit, but looked just like the pH 11 solutions). A few drops of either methyl orange, bromphenol blue, or phenolphthalein were added to all tubes in a rack. Below are some pictures of these racks (the best I could do with an Apple Quick Take digital camera).

Methyl Orange

Bromphenol Blue

Phenolphthalein

Preparation of Buffers used in the Pictures Shown Above

General notes on preparation and naming of buffer are on a separate web page.

To reproduce this experiment one can prepare a test tube rack full of different buffers and then add a few drops of indicator solutions to each of the solutions in order to visualize the changes in indicator color with pH.

The buffer solutions used were as follows:

pH 2.0	0.010 M HCl
pH 3.0	0.0010 M HCl
pH 4.0	0.10 M pH 4.0 sodium acetate buffer (with chloride ion present)	NOTE: Normally "sodium acetate buffer" should be prepared by adding sodium hydroxide to acetic acid. We prepared the buffer instead by approach #2b. By adding HCl to a sodium acetate solution, the final buffer solution contained chloride ions, which increase the ionic strength.
pH 5.0	0.10 M pH 5.0 sodium acetate buffer (with chloride ion present)
pH 6.0	0.10 M pH 6.0 sodium phosphate buffer	NOTE: The weak acid/weak base pair at work here is H2PO4-/ HPO42-. Nevertheless, it is referred to as a pH 6 sodium phosphate buffer. It is assumed that the reader knows that at pH 6, almost all PO4 units have either one or two protons bound. We prepared the buffer by approach #2a, adding sodium hydroxide to sodium dihydrogen phosphate, but the exact same buffer could be prepared by adding sodium hydroxide to phosphoric acid.
pH 7.0	0.10 M pH 7.0 sodium phosphate buffer (NOTE 2)
pH 8.0	0.10 M pH 8.0 TRIS chloride buffer.	NOTE: Prepared by approach #2b. TRIS = tris(hydroxymethyl)(amino)methane.
pH 9.0	0.10 M pH 9.0 TRIS chloride buffer (NOTE 3)
pH 10.0	0.10 M pH 10.0 sodium carbonate buffer.	NOTE: Prepared by approach #2a from sodium bicarbonate and sodium hydroxide.
pH 11.0	0.10 M pH 11.0 sodium carbonate buffer.
pH 12.0	0.010 M NaOH solution

The various pH solutions were used to half-fill test tubes, which were lined up in test tube racks with pH 2 on the left and pH 11 on the right (pH 12 didn't fit, but looked just like the pH 11 solutions). A few drops of either methyl orange, bromphenol blue, or phenolphthalein were added to all tubes in a rack.