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Pharmaceutical Analytical Chemistry
213 PHCDr. Mona AlShehri
4th Lecture
Acid-Base Titrations (1)
(1 )Gary D. Christian, Analytical Chemistry, 6th 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.
By the end of the lecture you should be able to:
Buffers
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
Definition
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 = pKa + log [A-] / [HA]
pH = pKa + log [salt] / [weak acid]
Buffer of weak acid
Calculate the pH of a buffer solution containing 0.1M acetic acid and 0.1M sodium acetate (Ka = 1.75 x 10-5)
Problem 1:
B + H+ BH+
Weak base salt
Applying Henderson-Hasselbalch equation
pOH = pKb + log [BH+]/ [B]or
pH = pKa + log [B] / [BH+]
Buffer of weak base
Calculate the pH of a solution containing 0.07M NH3 and 0.08M NH4Cl (pKb = 4.76)
Problem 2:
The ratio of salt / acid
[salt] / [acid] = 1pH = pKa
By changing the buffer ratio, buffers
of different pH values can be prepared
Buffer Ratio
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
HomeworkWhat is the pH of the following substances?
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
How to detect the end point of a reaction?
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
Indicators
HIn H+ + In-
unionized ionized form form
pH = pKIn + log [In-] / [HIn]
(Henderson-Hasselbalch equation)
Weak acid indicator
[In-] / [HIn] = 1/10 (only color of unionized form is seen)
pH = pKa + log (1/10) = pKa – 1
[In-] / [HIn] = 10/1 (only color of ionized form is seen)
pH = pKa + log (10/1) = pKa + 1
The pH in going from one color to the other has changed from pKa - 1 to pKa + 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 = pKa
The pKa of the indicator should be close to the pH of the equivalence point
At midway of transition range, the pOH = pKb
A weak base indicator should be selected such that pOH = pKb
Weak base indicator
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 pKin that is within one pH unit of the expected pH at the equivalence point of the titration.
pH ranges and colors of some common indicators
Natural IndicatorRed cabbage juice contains a mixture of substances
whose color depends on the pH value. Each test tube contains a solution of red cabbage juice in
water, but the pH of the solutions varies from pH = 2 - 11
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
Homework:Make your own indicator
Buffer Definition. buffer ratio. pH Calculation of weak acid and weak base buffers.
The indicator theory. Weak acid and weak base indicators.
Summary
Thank you
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