The pH range for phenolphthalein is 8-10 so it would not be appropriate for H3PO4 since the equivalence point is much more acid than that. If you know the pH range for K1 for H3PO4, here is a link from which you can pick the right indicator to use.
http://www.jiskha.com/display.cgi?id=1329337234
Phenolphthalein would not be an appropriate indicator to use to determine the Ka1 of phosphoric acid Why not? what indicator would be appropriate?
I Think its 8-11 as it is a small range but i'don't know if its right.
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Thanx!!
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