Download D.4 pH Regulation of the Stomach

pH Regulation of the Stomach
• The body keeps a tight control over the pH in cells and extra-cellular fluiids, as
changes in the H+ concentration have significant effects on the activity of many
molecules, especially enzymes.
• The gastrointestinal tract generates and maintains different pH environments along
its length, which play an important role in controlling the activity of digestive
enzymes.
The Helicobacter Pylori burrows into the mucus lining of the stomach ,
causing inflammation.
This leads to a loss of mechanisms that protect the stomach wall from the
acidic contents, and so tissue break down occurs.
The treatment for chronic stomach inflammation and ulcers includes
antiobiotic treatment and medines that reduce acid secretion
BUFFERS
BUFFERS
A buffer is a solution composed of a weak acid and its conjugate
base or a weak base and its conjugate acid.
Buffers can resist drastic changes in pH when a small amount of
strong acid or strong base is added.
A buffer resists changes in pH because the acid species neutralizes
OH- ions from a base and the basic species neutralizes the H+ ions
from an acid.
CALCULATING THE pH OF THE BUFFER
• Consider an acid, HA, and one its salts, MA (M could be any cation from
group 1).
• The acid dissociation equilibrium is:
BUFFERS
BUFFERS
Notice that if the acid and the salt have the same concentration, then
pH = pKa. Also, if the base and the salt have the same concentration,
then the pOH = pKb.
BUFFERS
• Example 1
What is the pH of a buffer that is 0.12 M in lactic acid, HC3H3O3, and 0.10 M in
sodium lactate, NaC3H3O3. Ka for lactic acid is 1.4 x 10-4
pKa = -log 1.4 x 10-4 = 3.85
pH = 3.85 + log .10/.12
pH = 3.85 – 0.08
pH = 3.77
BUFFERS
• Example 2,
BUFFERS
• Example 3,
• Calculate the pH of a buffer composed of 0.12 M benzoic acid and 0.20 M
sodium benzoate. Ka for benzoic acid = 6.3 x 10---5
pKa = - log Ka = -log 6.3 x 10---5 = 4.20
pH = 4.20 + log 0.20/0.12
pH = 4.20 + 0.22
pH = 4.42
PREPARATION OF A BUFFER
• The simplest way to prepare a buffer is by choosing an acid with a pKa
close to the desired pH. In this manner, equal amounts of acid and salt
can be used.
• Equal amounts of the acid and salt can be obtained by reacting the
acid with enough strong alkali, such as NaOH, to change ½ of the acid
into salt.
• Example 1
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PREPARATION OF A BUFFER
• Another way a buffer can be prepared is by mixing a weak acid or
weak base with a salt of that acid or base.
• For example:
1. Adding sodium acetate to a solution of acetic acid
or
2. Adding ammonium chloride to a solution of ammonia.
PREPARATION OF A BUFFER
• Example 2,
How would you prepare 1 L of pH 5.00 buffer solution from 0.10 M butanoic
acid solution and solid potassium butanoate?
In this type of problem, assume the volume won’t change when the salt is
added.
1. Obtain 1 L of butanoic acid.
2. Calculate the grams of potassium butanoate needed.
3. Mix the salt with the 1 L of acid.
CHANGES IN pH OF A BUFFER
To calculate the change in pH that occurs in a buffer
solution when a strong acid or strong base is added:
1. Calculate the pH of the buffer before the
addition of the strong acid or strong base.
2. Write the an equation for the reaction and
determine how much of the salt and weak
acid/base are present after the reaction
(use ICE table).
3. Calculate the pH of the resulting solution.
4. Find the difference between the pH before
and after the reaction.
CHANGES IN pH OF A BUFFER
CHANGES IN pH OF A BUFFER
CHANGES IN pH OF A BUFFER
If both the volume and the concentration of the solutions
are given, then first find the number of moles.
CHANGES IN pH OF A BUFFER
CHANGES IN pH OF A BUFFER