[pantone159]

I am curious as to the 'red' color of this substance in basic solutions.  Is it a purer/more saturated red color than phenolphthalein (which is more pink than bright red)?

My eventual goal is to be able to make a colorless (acid) to any-color-in-the-rainbow (basic) indicator solution.
I have phenolphthalein (colorless to pink) and thymolphthalein (colorless to blue).  I haven't been able to find o-cresolphthalein (colorless to red) or m-nitrophenol (colorless to yellow).  With the set, I should be able to make nearly any color I want.  (See Shakhashiri, Vol 3, 8.2 for details.)

If o-cresolphthalein is a better red color than phenolpthalein, then I'll keep my eye out for it.  If it isn't any better of a color, then I should forget that one, and concentrate on m-nitrophenol.

[Mitch]

tried www.aldrich.com and/or looking up the uv-vis spectra?

[pantone159]

Following your suggestion, I just looked at Aldrich.  I didn't find much describing the colors, at least that I could understand.  There was a 'Colour Index Number' of 764 for phenolphthalein, but nothing for o-cresolphthalein.  (I'm not sure what do do with that number, either.)  Both entries describe the color change as 'colorless to red'.  There is a reference to 'Stains and Dyes Ref' that I might try and look up.

It does give lambda-max of 566 nm for o-cresolphthalein and 552 nm for phenolphthalein.  552 is pretty pure green, while 566 is green-yellow.  That, by itself, makes me thing that phenolphthalein would have a more 'pure' color, but just one wavelength isn't enough to tell.

Where would I find the visible spectra data?  It would take a little work but I could convert it fairly easily into a RGB color value.

(In any case, I'm finally learning how to spell phenolphthalein!  )

[Mitch]

552 probably refers to nm. So if it absorbs at 552nm, I think you can figure out what color it appears.

[pantone159]

I need to know more than the transmittance at just that one wavelength.

I made a spectrum of 100% at most wavelengths, dropping to 0% at 552 nm (phenolphthalein), and another at 100% except dropping to 0% at 566 nm (o-cresolphthalein).  Upon further crunching, both of these come out as light pink, with the phenolphthalein being a little bit more saturated.  (I.e. less white.)

I can actually observe the real phenolphthalein color (I just did, actually) and it is much more saturated (brightly colored) magenta/pink than my calculation.  I am sure that the correct transmittance at other wavelengths is something other than 100%, and the precise details will affect the precise color.

In summary:  The max absorption wavelength is enough info to tell me that these are probably both reddish colored, but not enough to make more subtle color judgements.

If I could find the vis spectra data, I'd be happy to crunch the whole thing and get some better answers.

[pantone159]

BTW - I did manage to get samples of both compounds (phenolphthalein and o-cresolphthalein) to compare the colors.

o-cresolphthalein is not any better of a red color (as in more 'pure' red with less purple color in it) than phenolphthalein, in fact it is a little worse (as in more purple-ish, farther from pure red), although the two colors are quite similar.  In addition, o-cresolphthalein was harder to dissolve (in 95% EtOH), and since it is considerably more expensive and harder to find than phenolphthalein, I find nothing to recommend it to somebody who is looking for pretty colors.

[AWK]

http://www.chronolab.com/download/diagnostics/pdf/Calcium.pdf

[pantone159]

Is there a difference between 'o-cresolphthalein complexone' and just 'o-cresolphthalein'?

I added similar amounts of o-cresolphthalein (labeled as CAS 596-27-0) to
  a) limewater (approx saturated Ca(OH)2), and
  b) a solution made basic with NH3 (containing approx equal amount of NH4Cl)
and observed very similar purple-magenta colors in each.  No difference in color based on Ca++ concentration.

I don't have ethanolamine for the buffer (or the 8-hydroxyquinoleine) so I couldn't do the test as specified, but I would have thought I would see a color difference.

[pantone159]

Is there a difference between 'o-cresolphthalein complexone' and just 'o-cresolphthalein'?

Yes, these are not the same.  'o-cresolphthalein complexone' goes by CAS 2411-89-4 and formula C32H32N2O12, while plain-ole o-cresolphthalein is CAS 596-27-0 and formula C22H18O4.

[UnintentionalChaos]

Curcumin is bright red in basic solution, with the downside that it is bright yellow everywhere else. Its not even slightly purple in tone.