Agenda
• Collect Pre-lab #3
• Lab quiz #2
• Samples that may be used for Microworlds
today may include cabbage cells
• Doing Lab #3: pH and buffers today
• (JV, set up the pH 11 buffer & safety seal it.)
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
• If using anything higher
than pH 10 goggles
must be warned.
• (Vote to skip pH 11?)
A: pH Indicators
• Change color depending
on the pH.
• Phenol red turns RED
when solution is basic
and turns YELLOW
when solution is acidic
(near neutral it is
orange).
• In Exercise 1 you will
use cabbage extract as
pH indicators.
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
household solutions
• Pay attention to how
the parts of this
experiment fit the
Scientific Method
• But be quick, divide and
conquer!
Buffers Control pH
• In Exercise 3 you will study buffers. Buffers resist
changes in pH (see next slide).
• Do NOT make a solution NEUTRAL.
• Allow 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
• dilute mystery buffers?
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
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 HCl
Rinse electrode WELL
REPEAT experiment with 1mL increments of NaOH
Record in Fig 4, 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!
Extra Credit: Design an Experiment
• Design an original
Scientific Process – turn in one
experiment to investigate per group
an aspect of pH or
– Question (see next slide)
buffering capacity
– Hypothesis & Prediction
– Dependent, independent and
• Use pH meter to
constant variables,
measuring buffering
– Control & experimental groups
capacity and pH
– Brief materials and methods
• Be quick on other
– Results
exercises to have time to
– Discussion/conclusions (brief)
do a good job here!
– (Skip the lab report introduction
• Worth 1 pt f/ each section
and references!)
on right (6 pts)
– If your group does not do this you
may join one that is.
Questions for Designing your
Experiment
Design your experiment to test a hypothesis to one of these two
questions:
1. Is red cabbage extract good at buffering water? (Only use 10
ml of cabbage extract to conserve.) Speculate on why or why
not it is in the discussion section.
2. What is the pH ranking of the following aqueous solutions:
ocean water (if available), cabbage extract, tap water,
distilled water, pond water, spit (if someone has some
drinks, you may add these, but no sipping in the lab)?
Speculate on why their pH differs in the discussion section.
(Only use 10 ml of cabbage extract to conserve.)
Notes to Exercise 3:
• 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!