213 PHC Dr. Mona AlShehri 4 th Lecture

(1) Gary D. Christian, Analytical Chemistry, 6 th edition.

 Define buffer and buffer ratio.  Calculate the pH of acidic and basic buffers.  Describe the indicator theory.  Select a suitable indicator for a particular reaction.

 A solution that resists change in pH when a small amount of an acid or base is added  A mixture of a weak acid and its salt or a weak base and its salt at predetermined concentrations or ratios

 Why do we use buffer? ◦ Maintaining the pH for a reaction at an optimum value ◦ Keeping the pH constant

HA  H + + A - (Weak acid) (Salt) Apply Henderson-Hasselbalch equation pH = pK a + log [A - ] / [HA] pH = pKa + log [salt] / [weak acid]

 Calculate the pH of a buffer solution containing 0.1M acetic acid and 0.1M sodium acetate (K a = 1.75 x )

B + H +  BH + Weak base salt Applying Henderson-Hasselbalch equation pOH = pKb + log [BH + ]/ [B] or pH = pK a + log [B] / [BH + ]

 Calculate the pH of a solution containing 0.07M NH 3 and 0.08M NH 4 Cl (pK b = 4.76)

The ratio of salt / acid [salt] / [acid] = 1 pH = pK a By changing the buffer ratio, buffers of different pH values can be prepared

Baking soda Gastric fluid Vinegar Lemon juice Orange juice Coffee Soft drinks Milk Saliva Urine Blood Pure water Sea water Soapy water Bleach Household ammonia

Indicator

The point at which the reaction is observed to be complete is called the end point The end point can be detected by adding an indicator to the solution and visually detect a color change

 It is a weak acid or weak base that is highly colored  The color of the ionized form is markedly different from that of the unionized form

HIn  H + + In -  unionized ionized form form pH = pK In + log [In - ] / [HIn] (Henderson-Hasselbalch equation)

[In - ] / [HIn] = 1/10 (only color of unionized form is seen) pH = pK a + log (1/10) = pK a – 1 [In - ] / [HIn] = 10/1 (only color of ionized form is seen) pH = pK a + log (10/1) = pK a + 1

The pH in going from one color to the other has changed from pK a - 1 to pK a + 1 (a pH change of 2 units) Most indicators require a transition range of about 2 pH units

At midway of transition range, the concentration of the two forms are equal, and the pH = pK a The pK a of the indicator should be close to the pH of the equivalence point

At midway of transition range, the pOH = pK b A weak base indicator should be selected such that pOH = pK b

A good indicator must have the following properties:  The color change must be easily detected.  The color change must be rapid.  The indicator molecule must not react with the substance being titrated.  To minimize errors, the indicator should have a pK in that is within one pH unit of the expected pH at the equivalence point of the titration.

 You can make a very effective pH indicator in a few steps with hot water and red cabbage leaves. Follow the steps below: Record your reu results

 Buffer Definition.  buffer ratio.  pH Calculation of weak acid and weak base buffers.  The indicator theory.  Weak acid and weak base indicators.