Agenda
•
•
•
•
Collect Pre-lab #3
Lab quiz #2
Doing Lab #3: pH and buffers today
1st four Microworlds entries due at END of lab
today
Lab #3: pH and Buffers
pH
ACIDS
BASES
• Releases or causes the
release of H+ (hydrogen
ions) into solution
• pH lower than 7
• Hydrochloric acid (HCl)
• Sulfuric acid (H2SO4)
• Acetic Acid (vinegar)
• Carbonic acid
• Removes H+ (hydrogen ions)
from solution
• pH higher than 7
• Sodium Hydroxide (NaOH)
• Magnesium Hydroxide
(Mg(OH)2)
• Potassium Hydroxide (KOH)
pH
• Each step of the pH scale = 10-fold decrease in H+
concentration
What fold difference in H+
concentration is there
between pH:
•7 and 9?
•11 and 4?
Safety Precautions
• Wear GOGGLES at all
times when there are
people at your table
working with hazardous
chemicals
A: pH Indicators
• Change color depending
on the pH.
• Phenol red turns RED
when solution is basic
and turns YELLOW
when solution is acidic
A: pH Indicators - anthocyanins
• Red cabbage has
anthocyanins
• Boiling will extract the
anthocyanins which can
then be used as a pH
indicator
• Ex.1, create a set of
standards at pH 3, 5, 7,
9 (Skip 11)
A: pH Indicators - anthocyanins
B: Comparing pH of Beverages and
Medicines
• Ex.2: Determine the pH
value of various
solutions
• Pay attention to how
the parts of this
experiment fit the
Scientific Method
• But be quick, divide and
conquer!
Buffers Control pH
• Resist changes in pH
• Does NOT make a solution NEUTRAL
• Allows biological molecules to maintain their
structure, shape, characteristics
• Buffers are important for life
10
Buffered vs non-buffered solution
Add HCl to buffer
Add HCl to water (low
buffering capacity)
11
Bicarbonate Buffer System
• Carbon dioxide in blood
forms carbonic acid,
which separates into H+
and bicarbonate
Excess OH- ions combine with H+ to form H2O
Excess H+ ions combine with bicarbonate to form carbonic acid
12
Bicarbonate Buffer System
• Organs assist in maintaining
blood pH homeostasis
• Lungs remove excess CO2
• Kidneys remove excess
HCO3• Other buffers in the body
13
Buffers in the environment
• Some SOILS are an excellent buffer
– LIMESTONE
14
Ex 3: Using pH meter to determine
buffering capacity
• Why would we use a pH
meter instead of red
cabbage extract?
• BUFFER: resists pH
changes when a small
amount of acid or base
is added
• Must have an acid
component to add in H+
and a base component
to take out H+
Ex 3: Using pH meter to determine
buffering capacity
• How can you tell the
pink line is a good
buffer while the blue
line is not?
Ex 3: Using pH meter to determine
buffering capacity
• At what pH(s) is this
buffer useful?
Ex 3: Using pH meter to determine
buffering capacity
• At what pH(s) is this buffer useful?
Ex 3: Using the pH meter
• Ask me how to dilute your mystery buffers.
• Only one can be used as is
• Dilute two ½ and one ¼
IMPORTANT NOTES:
• pH electrodes are fragile, handle gently
• After being in ANY solution, rinse the electrode
with distilled water, & keep it wet!
• Immerse the electrode fully and swirl to get
accurate reading
Ex 3: Using the pH meter
•
•
•
•
•
•
•
•
Add 20mL of your mystery buffer (some dilute)
Add 1mL HCl, swirl, & record pH (1ml ≈ 2 dropper fulls)
Add 1mL HCl (2mL total), swirl, record pH on Fig. 3
Continue adding 1mL and recording pH until you’ve added
10mL total
Rinse electrode WELL
REPEAT experiment with 1mL increments of NaOH
Record in table, zero is in middle of x-axis
How to divide & conquer: two groups choose the same
mystery buffer, one adds HCl, the other NaOH, now share
your data!
Ex 4: Designing an Experiment
• Design an original
• Scientific Process – turn in
experiment to investigate
one per group
an aspect of pH or
– Question
buffering capacity
– Hypothesis
– Dependent variable
• Use red cabbage to
– Independent variable
measure pH
– Control group
• Use pH meter to
– Brief Explanation
measuring buffering
– Predictions
capacity
– Results/conclusions
• Be quick on other
– ADD IN: control variables
exercises to have time to
(constants)
do a good job here!
Note Changes to Exercise 3
• Make dilutions of mystery
buffers as indicated earlier
or ask me.
• Clean all glassware & put it
away!
• Return the trays and all
materials used to the place
you obtained them from in
a better condition than you
got them in!
These dilutions should help
you get a graph similar to the
one below, demonstrating the
buffering capacity.