Ask A Scientist Chemistry Archive

name         Linda B.
status       educator
age          30s

Question -   We are having trouble getting our pH indicators to react as expected on dilute
solutions of HCL, acetic acid, and boric acid. Indicators used are: Methyl Red, Methyl Orange, 
Congo Red, Alizarin Yellow, and Bromocresol Green.


For example, the Congo Red turns purple in pure HCL when we would expect red @ that pH. Can 
indicator dyes lose their potency with age, or could something else be causing the problem?
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I am not quite sure what your definition of "pure" is with regard to the concentration of 
various acids, but here is a table of what color/pH you should see for a 0.1 molar solution of 
the acids and the indicators you list:
                                                                 color/pH
Indicator [pH range]          HCl(1.1)             acetic(2.9)              boric(5.2)


Congo red [3.0-5.0]            blue                     blue				red
Methyl red [4.4-6.2]           red                      red                 orange
Methyl orange [3.2-4.4]        red                      red                 yellow
Alizarin yellow [5.5-7.2]      yellow                   yellow              yellow
also [11.1-12.2]
Bromcresol green [3.8-5.4]     yellow                 yellow                 blue-green

Be aware that the concentration of the indicator solution makes a difference in the intensity of 
the color. You can find a rather extensive list of indicator solutions and their pH range of 
utility in the Appendix of  "The Merck Index" as well as in the individual entries of the 
various indicators in that reference. Of course, the pH of a particular solution of the acids 
(or bases) can have a substantial effect on the pH of the solution.

Vince Calder
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