1. Buffer capacity, titration curves and indicators
Chapter 15, ppt 2
Buffer capacity, titration curves and indicators

2. Buffering Capacity
Definition: The amount of protons or hydroxide ions a buffer can absorb without a significant change in pH.
Large buffering capacity means it contains a large amount of buffering components and it can absorb a lot of protons/hydroxide ions and show little pH change
NOTE: The pH of a buffered solution is determined by [A-]/[HA]. The capacity is determined by how large [HA] and [A-] are

3. Optimal Buffering
We want to avoid large changes in the [A-]/[HA] ratio
Best buffered solution will have [HA]=[A-], or [A-]/[HA] = 1

4. Example
We wish to buffer a solution at pH = Which one of the following bases (and conjugate acid salts) would be most useful?
NH3 (Kb = 1.8 X 10-5)
C6H5NH2 (Kb = 4.2 X 10-10)
N2H4 (Kb = 9.6 X 10-7)

5. Titration Term Review
Titrant in buret = solution of known concentration
Equivalence point = stoichiometric point = moles of acid equal moles of base
Endpoint = color changes due to pH and depends on indicator used
pH curve = titration curve is a plot of the pH of the solution being analyzed as a function of the amount of titrant added

6. Titrations and pH Curves
Strong Acid - Strong Base
Weak Acid with Strong Base
Weak Base with Strong Acid
Mole is too large of a unit when working with mililiters, so generally a millimole (mmol) is used…1000 mmol = 1 mol and mmol/mL = M

7. Strong Acid - Strong Base Titration
pH changes gradually until the titration is close to the equilvalence point where a dramatic change occurs
pH = 7.00 at equivalence point
Curve points right/left based on beginning solution
Polyprotic acids have multiple curves

8. Weak Acid-Strong Base Titration
pH at the equivalence point of a titration of a weak acid with a strong base is always greater than 7.00 (basic)
pH is determined by the amount of excess OH- present
Curve looks different before and the same after the equivalence point
The AMOUNT, not strength of acid determines the equivalence point. The STRENGTH affects the pH at the equivalence point, however. This pH affects the titration curve.

9. Titrations vs. Ka
Titration curves can be used to determine equilibrium constant values
EX: 2.00 mmol of a monoprotic weak acid is dissolved in mL of water and is titrated with M NaOH. After 20.0 mL NaOH has been added, the pH is What is the Ka value for the acid?
Answer: 1.0 X 10-6

10. Weak Base - Strong Acid Titration
The pH at the equivalence point will be less than 7.00 (acid)

11. Determining the Equivalence Point
pH meter can be used and a plot of the titration curve can be made.
Acid-base indicator can be used to see the endpoint (NOT SAME AS EQUIVALENCE POINT, however various indicators can be used so this error won’t be a big deal).

12. Acid-Base Indicators
Indicators are represented by HIn. As the H+ ions react with OH- ions from the basic titrant (are removed from the HIn), In- ions remain. These In- ions cause the color to change based on the indicator present.
Indicators can be chosen where the endpoint and equivalence point are very close.
Determine the pH at the equivalence point
Use pH range chart on page 732