pH indicator

David W Walker Dave at diwalk.demon.co.uk
Wed Apr 17 07:31:14 EST 1996


In article <3172BAF2.55A4 at primenet.com>, Esther Ceren
<estherc at primenet.com> writes
>I am looking for the name of the chemical/pigment that changes it's 
>color from blue to red in the presence of an acid and to green in the 
>presence of an alkali. 
>
>Your help would be greatly appreciated.
>
>Esther

Hi Esther

Most single component indicators are used over quite a narrow pH range,
and relatively few actually encompass the ph7 acid-base transition.

The list of over sixty of these single component indicators in my
admittedly ageing Edition (1972) of the Chemical Rubber Handbook does
not mention any going from red-blue-green through acid-neutral-base.

Some of the commercially used indicators are mixtures. For example, many
wide range 'universal' indicators are red only in strong acid with
various shades of yellows, orange, greens and blues as the pH increases,
so it could be one of these. The large US chemical supplies catalogues
may help.

To really narrow it down you may need to monitor the pH range at which
the colour change occurs and any intermediate colours.

So no specific answer but hope this helps!

Dave Walker 




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