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Acid-Base Titration

Acid-Base Titration. Acid-Base Titration. H n A + n OH - n A - + n H 2 O. Equivalence Point : The point at which the reaction is just complete End Point : The place where experimentally the reaction is observed to be complete.

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Acid-Base Titration

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  1. Acid-Base Titration

  2. Acid-Base Titration HnA + n OH-n A- + n H2O Equivalence Point : The point at which the reaction is just complete End Point : The place where experimentally the reaction is observed to be complete

  3. • At Equivalence Point : A solution initially containing [A-] moles of HA, now contains an equivalent amount of A-. • The amount of base needed to reach the equivalence point is independent of the strength of the acid. THE WHOLE BASIS OF TITRATION RELYS ON THIS FACT!

  4. • Although, the location of the equivalence point does not change, the shape of the curve does change.

  5. Observing the equivalence point • • There are as many equivalence points as there are replaceable hydrogens in an acid. • • Some equivalence points MAY • occur without a noticeable rise in pH! • • Two requirements are necessary • for a jump in pH to occur. • The successive Ka values must • differ by several orders of • magntude • 2. The pH of the equivalence point • must not be very high or very low.

  6. Observing the equivalence point – I knew there was a use to studying calculus!

  7. Detecting the equivalence point • pH Measurements are the most direct way of measuring the equivalence point • Less exacting methods include using an indicator dye, which is it self an acid or a base. The protonated and deprotonated forms have different colors, and the pKa should be close to the pH expected at the equivalence point. • The observed color changes of an indicator changes slowly over 1.5 to 2 pH units. • Indicators are therefore only useful in titrations of acid and bases that show definite breaks in the titration curve.

  8. Common acid-base indicators

  9. Phenolphthalein H2In In2− In(OH)3− < 0 0−8.2 8.2−12.0 >12.0 colorless pink to fuchsia colorless

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