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It is the only indicator I have found that does not test below its pka, and can't think of any reason for this.

Neutral Red has a pKa value of 6.8. ( Reference 1, Reference 2 ), but looking at its pH testing range, it only shows from pH 6.8-8.0 (Reference 3)

What would be the reason for this?

Thank you

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  • $\begingroup$ When comparing different tables of data you likely will find inconsistencies. Such inconsistencies are impossible to track down since many references don't give sources of the data. $\endgroup$ – MaxW Aug 4 '19 at 4:29
  • $\begingroup$ @MaxW Yes but every pKa value for neutral red I have found is 6.8 (within a few decimals accuracy), and every pH range for it I have found is 6.8-8 $\endgroup$ – Harry Tong Aug 4 '19 at 4:41
  • $\begingroup$ I didn't try to solve your mystery, but rather read your statement that you had two sources that said pKa was 6.8 but a third that said pH range was 6.8-8.0. $\endgroup$ – MaxW Aug 4 '19 at 5:14
  • $\begingroup$ What do you mean by "cannot test below"? The pH range you cite is the transition range. Neutral red is yellow above, red below, and orange-y inbetween. $\endgroup$ – Karl Aug 4 '19 at 12:55
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The colour of neutral red in aqueous solution changes from violently red (protonated cation) to yellow/mostly colourless (the uncharged amine).

If the $pKa$ is 6.8, then the dye is still very much red at pH 6.8, and you won't be able to see the colour difference to, say, pH 5.

You have to keep adding base up to a pH of eight until the concentration of the colourful cation is so low you cannot see it any more.

Other indicators change from one strong colour to another, there the $pKa$ will be more in the middle of the transition range.

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