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PH and litmus

  1. Jul 30, 2010 #1
    I'm not a chemistry student. So, I humbly request you to limit yourself to the basics, please do not indulge in infighting. If you ignore the request, you would be investing your efforts at the wrong place and perhaps it would be considered a pretentious show of knowledge.

    Litmus is a water-soluble mixture of different dyes. It works between the pH range 4.5-8.3. To cover the full of spectrum of pH, 1-14, universal indicator is used.

    The dyes used in the litmus are extracted from lichens, especially Roccella tinctoria.

    Lichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner, usually either a green alga or cyanobacterium.

    Here dye is certain chemical which can be single molecule or a group of different molecules and behaves in a particular way under different pH conditions.

    1: As it is stated above that the litmus can only used when the range is between 4.5-8.3, what would happen to the litmus or dyes or their color patterns when pH is outside this bound?

    2: How did someone come up with the idea that some weird composite organisms called lichens can be used to tell the acidity of the solution? It was not like that someone accidently dropped few grains of table salt or sugar into the solution and discovered that it could be used for acidity test.

    Wikipedia says: Chemical reactions other than acid-base reaction can also cause a color-change to litmus paper. For instance, chlorine gas turns blue litmus paper white – the litmus paper is bleached. This reaction is irreversible and therefore the litmus is not acting as an indicator in this situation.

    3: What does it mean by 'irreversible' here? Perhaps, it means that chlorine somehow permanently damages the dyes and even if it is evaporated or removed fully, the dyes wouldn't retain their original property and couldn't be used for the test again.
  2. jcsd
  3. Aug 2, 2010 #2
    1) Litmus can be used outside the 4.5-8.3 range. It will be red below 4.5 and blue above 8.3
    2)Well, I'd say it was just a coincidence: someone dropped in an acid or base and found out that it changes collors. I think it would be hard to derive from first principles which types of substances are indicators.
    (In fact, just a few weeks ago, I found out that some types of wine can be used as indicators. I was washing some dishes and a red wine spot turned blue. Than I did a few other tests and confirmed this is the case. So the whole idea of "someone just accidentaly dropped litmus in acid and found out it changed color" is not as absurd as it may sound)
    3)Litmus is red in acidic medium. You can turn it blue by adding base. You can turn it red again using acids. You can turn it blue again by adding base. You can turn it.... You got the idea: when litmus is used in acidity/basicity tests, it's very simple to reverse its color.
    When you use it for testing chlorine, however, litmus can't be brought to its original state. So litmus reacted with chlorine irreversably.
  4. Aug 3, 2010 #3


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    Remains to be said that lichens quite generally contain interesting dyes and that chromatography of these dyes is even used to distinguish species.
    Some lichens were (and are) also used to dye cloth.
  5. Aug 3, 2010 #4


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    The fascination of medieval society with the color purple is likely the impetus for the experimentation with the extraction and production of litmus. At some point during manufacture, the dye contains both blue and red (purple) components in alkaline solution. This was http://www.ravensgard.org/gerekr/Orchil.html" [Broken]
    Last edited by a moderator: May 4, 2017
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