Week 9


Finding the extremes of pH to be considered:

When a leaf sample was made to react with bleach (pH 13), bleach decolorized the leaf surface and there was not much of a change in color as the pH was made to reduce gradually. Hence, in order to enable reversible color changes, the extreme ph should be 12(max) and 2(min). The results are tabulated below.



From the table, we conclude that pH 12 will be the limit for our experiments. Bleach (pH 13) decolorizes the chromophores, which are the molecules responsible for the color change present on the surface of the leaf. Once bleach is introduced on the surface, the color changes to green in less than 1 minute and it does not change back to its normal form even when solutions of lower pH are introduced. Bleach, an example of pH 13, has sodium hypochlorite (NaOCl) as the active ingredient. Chlorine, present in this compound, is a strong bleaching agent and it requires nascent oxygen, which is also readily available in the same compound, to bleach substances. Moreover, bleaching action of chlorine is a permanent one. When we say pH 13, it means it has more number of OH- ions and less H+ ions than solutions of pH 12. OH- ions and H+ ions, by themselves do not bring about the bleaching action. Hence, it is clear that chlorine in bleach was the substance which caused the bleaching action and not the ions themselves. So, solutions of pH 13 without chlorine, nascent oxygen and other bleaching agents like SO2, nascent hydrogen etc. can be used in these experiments. Since these were commercially unavailable, we limit the pH to 12 in our experiments.

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