Ionic Equilibria: Part II Buffers and Titration Curves Unit 9 Ionic Equilibria: Part II Buffers and Titration Curves

Chapter Goals In this chapter we will study; Solutions that have both weak acids and weak bases present Indicators Titrations curves

The Common Ion Effect and Buffer Solutions If a solution is made in which the same ion is produced by two different compounds the common ion effect is exhibited. Example: Soln made with acetic acid and sodium acetate Buffer solutions are solutions that resist changes in pH when acids or bases are added to them. Buffering is due to the common ion effect. This is a special case of le Chatelier’s Priniciple

The Common Ion Effect and Buffer Solutions There are two common kinds of buffer solutions: Solutions made from a weak acid plus a soluble ionic salt of the weak acid. Solutions made from a weak base plus a soluble ionic salt of the weak base

The Common Ion Effect and Buffer Solutions Solutions made of weak acids plus a soluble ionic salt of the weak acid One example of this type of buffer system is: The weak acid - acetic acid CH3COOH The soluble ionic salt - sodium acetate NaCH3COO

The Common Ion Effect and Buffer Solutions Example 19-1: Calculate the concentration of H+and the pH of a solution that is 0.15 M in acetic acid and 0.15 M in sodium acetate. This is another equilibrium problem with a starting concentration for both the acid and anion. Since acetic acid is weak then only a small amt will dissociate (x) But the soluble ionic salt will completely ionize

The Common Ion Effect and Buffer Solutions Substitute the quantities determined in the previous relationship into the ionization expression.

The Common Ion Effect and Buffer Solutions Apply the simplifying assumption to both the numerator and denominator.

The Common Ion Effect and Buffer Solutions [H+] is 89 times greater in pure acetic acid than in buffer solution.

The Common Ion Effect and Buffer Solutions Henderson-Hasselbach equation. The Henderson-Hasselbach equation is one method to calculate the pH of a buffer given the concentrations of the salt and acid.

Weak Bases plus Salts of Weak Bases Buffers that contain a weak base plus the salt of a weak base One example of this buffer system is ammonia plus ammonium nitrate.

Weak Bases plus Salts of Weak Bases We can derive a general relationship for buffer solutions that contain a weak base plus a salt of a weak base similar to the acid buffer relationship. The general ionization equation for weak bases is:

Weak Bases plus Salts of Weak Bases Henderson-Hasselbach equation. So, to find the pH of a basic buffer we will then have to use: pH = 14 - pOH See text for derivation

Buffering Action Example 19-4 in text

Acid-Base Indicators An indicator is an organic dye Its colour depends on the concentration of H3O+ ions or the pH in the solution

Acid-Base Indicators The point in a titration at which chemically equivalent amounts of acid and base have reacted is called the equivalence point. The point in a titration at which a chemical indicator changes color is called the end point. A symbolic representation of the indicator’s color change at the end point is:

Acid-Base Indicators

Titration Curves You can consider a titration curve in 4 parts:- Initial solution (before titrant is added) Region before the equivalence point. This may or may not be buffered. The solution is buffered in this region if the substance being titrated is a weak acid or base. Equivalence point. Its location depends on the concentrations of the acid and base solutions Region beyond the equivalence point. This becomes nearly flat as more excess titrant is added.

Strong Acid/Strong Base Titration Curves Titration curve of HCl with NaOH. The ‘vertical’ section of the curve is quite long (b) Titration curve of NaOH with HCl . The curve is similar to (a), but INVERTED

Weak Acid/Strong Base & Weak Base/ Strong Acid Titration Curves Titration curves for Weak Acid/Strong Base. Vertical section much shorter because the solution is buffered before equivalence point. Titration curves for Strong Acid/Weak Base look similar to Weak Acid/Strong Base Titration Curves but they are inverted.

Weak Acid/Weak Base Titration Curves have very short vertical sections. The solution is buffered both before and after the equivalence point. Visual indicators cannot be used; colour change too gradual

Polyprotic acids Titration Curves Polyprotic acids are acids with more than one ionizable H atoms Example; H2SO4, H3PO4 Titration curves of polyprotic acids have more than one equivalence point.

Buffer zone Titration curve of phosphoric acid (weak acid) with a strong base, NaOH The first point on the curve corresponds to a solution of H3PO4 only. The pH at this point is due solely to the H3PO4 in the solution. As soon as some base is added, some H2PO4- is produced. The solution now contains H3PO4 (a weak acid) and its conjugate base dihydrogen phosphate, and thus it is a buffer.