given Acid color of methyl orange is
red, phenolphtalein is colorless , Alkaline color is, yellow and
red , respectively
Wouldn't methyl orange stay Red in color
when the end pt of a SA-WB titration, is at a acidic pH?? Why will
methyl orange turn Yellow only at the end pt , that occured at a
acidic pH(for pg 157 SA-WB titration example, end pt pH≈5.5)?
(given pH range methyl orange 3.1-4.4)
If it stays the same color from beginning to end point, how the
hell would u know you've to stop adding the strong acid from the
burette?!? Then the indicator would be utterly useless, wouldn't
it?!?
And u seem to suffer from a ridiculously basic (pun intended)
conceptual error, that "alkaline color" for the indicator means the
solution must actually be alkaline (ie. >7). Nonsense! The
"alkaline color" of the indicator could still be in acidic pH, eg.
4.5 for yellow colored methyl orange. It's "alkaline" only relative
to the "acidic color" for which the pH is lagi acidic, eg. 3.0 for
red colored methyl orange.
The underlying idea, is that the suitable indicator must change
color sharply over the vertical portion of the titration curve (the
sharp change is due to the fact that at equivalence point, it is no
longer a buffer region, so just a single drop of excess strong acid
added will change the pH sharply, resulting in the sharp color
change), ie. the pKa of the indicator must be as close as possible
to the pH of the solution at equivalence point.
So if u started using with a weak base in the conical flask (and
stop ur acronym nonsense, Mr lazy
WA SA BI), the methyl orange color would begin as yellow, until
just enough strong acid is added from the burette to go past the
buffer region into the equivalence point, thereby sharply changing
the pH of the solution upon addition of 1 excess drop of strong
acid, thereby sharply changing the methyl orange color to red,
which indicates the change in pH of the solution has just transited
past the pKa of the indicator (which if correctly chosen should be
close to the equivalence point pH for this titration), which in
turn indicates that the equivalence point has been reached.
Lastly, don't forget that end point merely approximates
equivalence point. But as long as you repeat the titration until
you obtain 2 concordant or consistent titers, you're good to go,
that's already as accurate as is humanly possible.
And you prolly won't fully understand what I've said above, coz
you (like most Singapore JC students) prolly have a terribly weak
understanding of this topic. I've already given u an adequate
response. If u still dun fully understand, go ask ur school teacher
or private tutor. Move on to ur next qn on the next topic.
