Equilibria Involving Complex Ions
Monitoring pH During a Titration Figure 16.10 pg 780 1. The pH meter measures the conductivity of the solution due to the [H 3 O + ]. 2. The endpoint of the titration is reached at the equivalence point in the titration — at the inflection point of the titration curve. 3. If you just need to know the amount of titrant added to reach the endpoint, we often monitor the titration with an indicator .
Indicators • Many dyes change color depending on the pH of the solution. • These dyes are weak acids, establishing an equilibrium with the H 2 O and H 3 O + in the solution. HInd ( aq ) + H 2 O ( l ) Ind ( aq ) + H 3 O + ( aq ) • The color of the solution depends on the relative concentrations of Ind :Hind.
Monitoring a Titration with an Indicator • For most titrations, the titration curve shows a very large change in pH for very small additions of titrant near the equivalence point. • An indicator can therefore be used to determine the endpoint of the titration if it changes color within the same range as the rapid change in pH.
Ranges of Color Changes for Acid – Base Indicators Table 16.1 pg 783
pH Colors and Approximate pH Range of Some Common Acid-base Indicators acidic basic change occurs over ~2 pH units bromthymol blue Which flask is the most acidic and basic? What is the valid pH range for bromthymol blue?
Titration Terminology • a solution of known concentration ( titrant ) • When the moles of acid = moles of base, the titration has reached its equivalence point .