I work within a science department in a school. We are demonstrating Universal Indicator and something odd has happened. Our bleach solution (~2.5% Sodium hypochlorite) turns blue when UI is added but within about 1 min it goes green and then after 1-2 mins it goes yellow. Overnight it goes clear but when more UI is added it is yellow. Why would this happen? The bleach smells like bleach and has been used in the last week for the luminol reaction which worked.

  • $\begingroup$ Sorry to add more... repeated but with hydrogen peroxide to see if this would have the same effect but it does not... Repeated with bleach, UI and pH probe and it is only the colour which is changing not the pH... what is the mechanism of bleach removing the colour does hydrogen peroxide bleach by a different mechanism??? $\endgroup$
    – Yogi
    Sep 29 '15 at 10:36
  • $\begingroup$ What? Can you explain that in more detail. I don't understand what you are trying to say. $\endgroup$
    – bon
    Sep 29 '15 at 10:39

Looks like your bleach bleaches your indicator, just like it does pretty much everything else. After all, we don't call it "bleach" for nothing.

Sure, pH may be in fact slowly decreasing, and the bleach would eventually decompose and lose its properties, but these processes do not typically occur in a matter of minutes.

As to the mechanism, it is complicated, as are indicators themselves (usually, large organic molecules). One might think of a huge conjugated system of double bonds that produces color; all of a sudden, an oxidant appears and starts randomly snapping bonds in half. Soon the conjugated system is gone, and so is the color.

  • $\begingroup$ Hydrogen peroxide is also a bleaching agent but is having zero effect on the colour of the solution is this just because it use a different mechanism of bleaching, is there a way other than using a pH probe to prove to the students that bleach is alkali? $\endgroup$
    – Yogi
    Sep 29 '15 at 11:00
  • $\begingroup$ The pH has not decreased since I started ~ 1hr $\endgroup$
    – Yogi
    Sep 29 '15 at 11:03
  • $\begingroup$ Well, yes, hydrogen peroxide reacts differently, so it could just happen that it is not as good at snapping these bonds. As for pH, you've just proven it: adding an indicator shows the alkaline reaction right away. One may complain that it vanishes, so maybe the alkali is gone; but then you always may add one more drop of indicator. $\endgroup$ Sep 29 '15 at 12:27
  • $\begingroup$ I thought that too but when I add more drops of indicator it is now yellow and does not go through the same colours... thanks for your time sorry did not say that before $\endgroup$
    – Yogi
    Sep 29 '15 at 12:42
  • $\begingroup$ Well, then probably the alkali is already gone (there was not much of it in the first place) due to the reaction with something. BTW, isn't your indicator in the form of an alcohol solution? $\endgroup$ Sep 29 '15 at 21:26

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