CPR Inorganic Indicators








Chemical Processes and Reactions - Inorganic Chemistry - Indicators

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Simple Indicators

This type of indicator is one which loses H+ or OH- under specific pH conditions.



The colour of the neutral indicator is different to its ionised form. Reversible reactions so the changing pH will change the colour.

If H+ is lost the indicator molecule is a weak organic acid. If the OH- is lost the indicator is a weak organic base.

When H+ is lost from a weak organic acid a conjugate base is formed (In-).

The structures of the molecules are often complicated and H+ loss may be from somewhere unexpected.

The colour change between the neutral and ionised forms may be used to indicate the end point of a titration/ reaction.

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Middle Tint

The recognised colour when the mixture contains equal parts of the unionised and ionised indicator.

e.g. Litmus



e.g. Methyl Orange



The titration should be stopped as the middle tint is reached.

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Indicators Without a Middle Tint

Phenolphthalein has no middle tint. The acidic solution is colourless and the alkaline solution is pink. Therefore we aim for the first appearance of the end point colour.

Permanganate titrations do not use an indicator and therefore the end point is identified as the first colour of a slight excess of the permanganate solution.

Phenolphthalein at pH 0 - 8.2                        Phenolphthalein at pH 8.2 - 12




.                                      Phenolphthalein at pH >12

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Water soluble dye extracted from lichen.

Red below pH = 4.5

Blue above pH = 8.3

A weak organic acid

                 Red Litmus                                                Blue ionised Litmus

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Methyl Orange

Red -> Orange -> Yellow






Weakly Acidic/ Conjugate Base

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Crystal Violet (Gentian Violet) (Methyl Violet)

Antiseptic properties.






                Alkaline                                                                       Acid

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Universal Indicator

Can detect pH from 1-14

Paper soaked in many indicators (phenolphthalein, methyl red and bromothymol blue) which cover most of the pH range and is then dried.

For an indicator to work on a titration: The pH range of the indicator’s colour change must be within the range of the pH change at the end-point.

Using pH graphs if the colour change is in the vertical region of the graph then the indicator can be used.

Phenolphthalein and methyl orange can both be used to titrate a strong acid and a strong base shown in this graph:

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Strong Acid/ Weak Base Titration


Methyl Orange is the correct indicator to use when titrating a strong acid with a weak base because phenolphthalein's colour change is at the very top of the vertical line on the graph.

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Weak Acid/ Strong Base Titration


Phenolphthalein is the correct indicator to use on a weak acid/ strong base titration as show in the graph:

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K and pK

pK  is the difference between strong and weak acids.

A strong acid is assumed to be completely dissociated into ions in solution:



A weak acid is only partially ionised and the dissociation reaction is described as a equilibrium with an equilibrium constant K  .

e.g. for ethanoic acid:

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K and pH

For any acid:


H O is taken as a constant and ignored therefore:



If we know the starting concentration of the acid we can calculate pH.


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K , K and K

pK  + pK  = pK  = 14

So it is possible to work out the pK  for the conjugate base of a weak acid.

e.g. for ethanoic acid:



A weak base has a pK  value the same as a weak acid has a pK  :



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Weak Acid/ Weak Base Titration

Unless a suitable indicator is already known it is not usually possible to titrate a weak acid with a weak base in the normal way. It is usually possible to determine the end point with a conductimetric titration which uses the change in ionic strength of the solution to determine an end point.

Here is a graph of a weak acid/ weak base titration.

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