Colourful Chemistry: Chemistry of UNIVERSAL INDICATOR

Chemistry of UNIVERSAL INDICATOR jameskennedymonash
jameskennedymonash.wordpress.com

By definition, an indicator is a substance that changes colour in different pH environments. Universal indicator is a brown-coloured solution—containing a mixture of indicators—that can be added to any substance to determine its pH. Like all indicators, universal indicator changes colour in different pH environments. At low pH, it appears red, and at high pH, it appears blue or violet. At neutral pH, it appears green. Universal indicator can form a continuous spectrum of colours that give an approximate reading of the concentration of protons in a sample.

Water and propan-1-ol are used as solvents. They are both polar and dissolve all the other ingredients in the solution. Sodium hydroxide (NaOH) is an alkaline solution that adjusts the pH of the universal indicator to ensure that each colour is shown at the correct pH value. It is necessary to add NaOH to the universal indicator because some of the indicator compounds (e.g. methyl red) are acidic themselves, which would affect the colour of the other indicators present. NaOH is added to neutralise the solution.

Methyl red is red at pH <5 and yellow at pH >5. It provides orange and red hues to the universal indicator solution at low pH. The end point of an indicator compound is defined as the pH at which it changes colour. The end point of methyl red, therefore, is somewhere around pH 5.

Bromothymol blue is blue at pH >6 and yellow at pH <6. It gives blue and indigo hues at high pH. Its end point is therefore around pH 6.

Thymol blue has two end points: it is red below pH <2, blue at pH >8 and yellow in the middle. Thymol blue allows universal indicator to differentiate low and very low pH by providing another red hue below pH 2. Thymol blue is yellow at pH 7, which, when combined with bromothymol blue (which is blue at pH 7), give a green colour.

Finally, phenolphthalein gives universal indicator a deep violet colour at very high pH.

This 2-miunte BBC video is a great introduction to universal indicator:

10 thoughts on “Colourful Chemistry: Chemistry of UNIVERSAL INDICATOR

  1. I am searching the preparation procedure of Universal indicator from these components which I found in many websites on internet. But in this site also I am disapponited after not finding the extact amount of each component amd method of preparation of ultimate universal indicator which can be used finally for testing different solutions. If you have any problem in sharing it in internet can you send it to me via e-mail attachment? My email id is subhdatta@gmail.com.

    Like

  2. thank you for share this posters they are very cool But these images can not be used because it would be difficult translation
    And if the are pdf
    Is easily translated

    Like

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