Question

how many equivalence point(s) will appear on the titration curve during titration of phosphoric acid, H3PO4 with a strong base such as KOH

Answer 1

Titration of the phosphoric acid H3PO4 is an interesting case. Although often listed together with strong mineral acids (hydrochloric, nitric and sulfuric) phosphoric acid is relatively weak, with pKa1=2.15, pKa2=7.20 and pKa3=12.35. That means titration curve contains only two inflection points and phosphoric acid can be titrated either as a monoprotic acid or as a diprotic acid. In the first case acid has to be titrated against indicator changing color around pH 4.7 (for example methyl orange), in the second case - against indicator changing color around pH 9.6 (for example thymolphthalein). Phenolphthalein can't be used, as it starts to change color around pH 8.2, when phosphoric acid is titrated in about 95%.

there are two  equivalence point(s) will appear on the titration curve during titration of phosphoric acid, H3PO4 with a strong base such as KOH. graph is shown below

main points in the graph

• The curve starts at a higher pH than a titration curve of a strong base(KOH)
• There is a steep climb in pH before the first midpoint
• Gradual increase of pH until past the midpoint.
• Right before the equivalence point there is a sharp increase in pH
• pH steadies itself around the midpoint because the solutions at this point in the curve are buffer solutions, which means that adding small increments of a strong base will only barely change the pH
• Increase in pH near the equivalence point
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