[shuryanc]

The titrate contain a mixture of Na2CO3 and NaOH and titrate with HCl.
The first indicator I use is phenolphthalein and the second is methyl orange.
The thing I want to know is: why can't the order of indicators be reversed? Thank.

[AWK]

Na₂CO₃+HCl=NaHCO₃+NaCl At equivalence point (pH about 8-9) phenophtalein becomes colourless
NaHCO₃+HCl=H₂CO₃+NaCl At this equivalence point (pH about 7 - note carbonic acid should be decomposed by warming) the second indicator changes its color.

This is just optimized procedure and there is nothing to discuss

[Borek]

http://www.titrations.info/acid-base-titration-polyprotics-and-mixtures

[shuryanc]

So if the order is changed, the first equivalent will need a long time to reach and the second equivalent point will not exist? Any more reasons? Thank(I have given you snack(both))

[Pradeep]

The active pH range of Methyl orange 3.5-5.5
The active pH range of Phynopthalein 9-11

The initial pH of the Na2CO3 solution is always supposed that above 11.

When U add HCl

Na2CO3 and NaOH will react with HCL. So the pH of the solution will come down. When that pH comes down to below 9 phnopthalein willl be colorless.

when that pH comes down to below 3 methyl orannge will be yello (what ever the changed color)

So color change of the phynopthalein comes first. So u have to use it first and then met Orange.

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