Acid-base equilibria
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What indicator should I use to detect the change in pH when titrating 0.0005 mol/dm3 phosphoric acid against 0.0015 mol/dm3 sodium hydroxide solution?
Igloo replies ...
Methyl orange is the common indicator of choice when detecting a solution with a pH in the region of 3.5 - 4.5.
Methyl red is another indicator which can be used if you are concerned with detecting pH changes in the region of 4.5 - 6.0.
Phenolphthalein is the usual indicator of choice when we wish to detect a change in pH at about 9.5 - 10.0.
The problem with this particular titration is that since phosphoric acid is tribasic (we tend to use the term “triprotic” these days), it neutralizes in three stages:
H3PO4 + NaOH -> NaH2PO4 + H2O
NaH2PO4 + NaOH -> Na2HPO4 + H2O
Na2HPO4 + NaOH -> Na3PO4 + H2O
The three salts produced all react with water to someextent to produce solutions of different pH.
A solution of pure NaH2PO4 has a pH of about 4.5, so methyl orange or methyl red should be used to detect the end-point for the first neutralization.
A solution of pure Na2HPO4 has a pH in the region of 9.5, so phenolphthalein should be used for this neutralization.
A solution of pure Na3PO4 has a pH of about 12.0, and there is no suitable indicator to use for the third neutralization process.
If you are using 25.0 cm3 of 0.0005 mol dm-3 phosphoric acid, and titrating with 0.0015 mol dm-3 sodium hydroxide the first end point (using methyl orange or methyl red) will occur at 8.33 cm3.
If you carry out another titration, using once again 25.0 cm3 of 0.0005 mol dm-3 phosphoric acid to detect the second end-point (this time with phenolphthalein as indicator), you will need 16.7 cm3 of 0.0015 mol dm-3 sodium hydroxide.
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updated: 11 February 2004
