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I have bought a colour changing lipstick with the colour Phloxine B.(Other ingredients, see below...) Normally this colour changes to pink at pH 5. So this means on skin it changes its colour because the human lips have a pH about pH 5-6.

I have made a test: I cut pieces of the lipstick and inserted them into a solution of HCl (no colour change), dest. water (pH7) (no colour change) and a solution of NaOH (colour change!)

My question is: Why doesn't it change its colour in water?

Ideas so far:

  1. due to the solubility of the liphophile lipstick and the surface tension of water, there is no real contact between water and the lipstick possible and therefore also no reaction

  2. the citric acid that is an ingredient of the lipstick solves in the water so the water gets acid and there is no rise of the pH value

  3. the reaction between water and the lipstick is not possible because water doesn't have enough OH- -Ions (but why does skin have those ions?!)

On top of that i made a few more tests: The lipstick changes colours when spreaded on: tables, paper, skin, plastic-lids

it doesn't change colours on: metall, glass, porcelain, plastic with a very plain surface.

Can you please help me out?

Ingredients: polyisobutene, caprylic/capric triglyceride, cera microcristallina (microcrystalline wax), euphorbia cerifera (candelilla) wax, prunus amygdalus dulcis (sweet almond) oil, cannabis sativa (hemp) seed oil, menthol, tocopheryl acetate, glycine soja (soybean) oil, tocopherol, caprylyl glycol, ethylhexyl palmitate, glycerin, caprylhydroxamic acid, beta-sitosterol, squalene, ricinus communis (castor) seed oil, ethoxydiglycol, citric acid, CI 19140 (yellow 5 lake), CI 45410 (red 27 lake), CI 61565 (green 6).

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2 Answers 2

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When you mix two samples with different pH, the resulting pH depends on the pH values of the two samples and their buffer capacity (a measure of "how much it takes" to changes their pH). In this case, pure water has a very low buffer capacity, so your idea #2 explains what you observed.

The skin has a higher buffer capacity, so it "wins" over the lipstick.

You mention the ingredient phloxine B as the one responsible for the color change. This substance has multiple names (red 27, red 28, acid red 92, phloxine B, phloxine O), perhaps based on what pH it was isolated in (as free acid or as salt). It undergoes fairly complicated changes with change in pH involving a lactone ring closure.

enter image description here

Source: https://www.mdpi.com/2673-8015/1/1/1

More common pH indicators like phenolphthalein also show this reversible formation of a lactone ring.

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  • $\begingroup$ The first print of the colour index is from the 1920s, when colours and dyes were much more of interest for BASF, Hoechst, Lucius, Ciba, etc. than today (some of them no longer active as such). At a time when chemical analysis relied on performing reactions to obtain derivatives, and combustion analysis; ahead of the advent of IR, MS, NMR; crystallography to the labs we (too often) take for granted. $\endgroup$
    – Buttonwood
    Feb 7 at 16:00
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The intentionally added colors are the ingredients behind the CI numbers where CI stands for the colour index, an inventory by the Society of Dyers and Colourists. The numbers of five digits each are group dyes and pigments by the functional group deemed to be of principal influence for the color, though this does not correlate with the color perceived, nor exclude the presence of additional structure features which may influence the perceived color, or intensity.

CI 19140 (yellow 5 lake), CI 45410 (red 27 lake), CI 61565 (green 6).

While the printed catalogue is telephone book like heavy, Wikipedia has compiled a selection amended by the CAS registry number (an other index of chemicals, which is not constrained to dyes), and links to separate articles about the individual dyes, colorants, pigments. Hence

Out of this set of three, phloxine is the red dye which eventually was discolored by the action of aqueous lye. As mentioned in @Karsten's answer, $\ce{OH-}$ of the base opened the 5-membered lactone cycle. This alters the electronic interaction of the remaining rings and the interaction of the molecule to visible light, i.e. what frequency of the spectrum is absorbed (and by consequence, the color perceived), and how probable such an absorption is). The underlying principle relates to the one of phenolphthalein.

Image credit: structure formulae are extracted from property boxes of the corresponding Wikipedia pages.

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  • $\begingroup$ @JeanClaudeDelaveine Because I don't know your chemical education (there is at least one journal which includes this in its name), I refrained as much as possible from words too special to chemistry. $\endgroup$
    – Buttonwood
    Feb 7 at 16:05
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    $\begingroup$ You can use special chemistry words, don't worry. I studied chemistry (haha who believes this now?!). Thank you, I will try to check this journal! $\endgroup$ Feb 7 at 22:07

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