Name____________________________________________Date_______Per._____
Discovery Lab: The Role of Buffers and pH
Background Information:
On Earth, water is the substance that makes life as we know it possible. So common is water that it
is easily taken for granted; but water is in fact a most exceptional substance with many extraordinary
properties. These include surface tension, capillary action, high heat of vaporization, a solid which is
less dense than the liquid form, its usefulness as a solvent and its tendency to ionize slightly.
Since the dissociation of water produces one H+ for every OH -, the concentrations of these two ions
will be equal in pure water. The concentration of each 10 –7 M. This means that there is only one tenmillionth of a mole of H+ per liter of pure water.
What would cause an aqueous solution to have an imbalance in its H+ and OH- concentrations? When
substances called acids dissolve in water, they add additional hydrogen ions to solution; when bases
dissolve in water, they reduce the H+ concentration (usually indirectly, by releasing hydroxide ions
which then combine with the hydrogen ions to form water. Other bases work by directly
accepting hydrogen ions.)
We use pH to express the concentration of H+ in solution. The logarithmic pH scale ranges from 0 to
14. The pH is defined as the negative log (base 10) of the [H+], in moles per liter. Since [H+] in a
neutral solution is 10 -7 M, the pH would be –(log 10-7), or –(-7), or 7. The larger the hydrogen ion
concentration (the more acidic), and the smaller the value of pH. A pH of 7 is that of a neutral
solution; under 7, acidic and over 7, basic.
RULE to REMEMBER
pH is INVERSELY related to hydrogen ion concentration ( H+)
As hydrogen ion levels increase
As hydrogen ion levels decrease
H+
pH
the pH goes down.
the pH goes up.
(becomes more acidic)
(becomes more basic)
H+
pH
The internal pH of most living cells is close to 7. Even a slight change in pH can be harmful, because
the structures and functions of molecules in the cell are very sensitive to the concentrations of
hydrogen and hydroxide ions. Acids and bases will alter biological molecules, yet the cell’s own
metabolism produces acids and bases; so, a logical question is how can cells tolerate the acidic or basic
substances in their environments?
Biological fluids resist changes to their own pH when acids or bases are introduced because of the
presence of buffers, substances that minimize changes in the concentrations of H+ and OH-. Buffers
in human blood, for example, normally maintain the blood pH at approximately 7.4. If the pH drops
below 7.35, acidosis occurs. If pH goes above 7.45, alkalosis occurs... We would not survive for more
than a few minutes if our blood pH dropped to 7 or climbed to 7.8.
A buffer works by accepting hydrogen ions from the solution when they are in excess, and donating
hydrogen ions to the solution when they have been depleted. Most buffers are weak acids or weak
bases that combine reversibly with hydrogen ions. The acid and the base are in equilibrium with one
another. They are most effective at stabilizing the pH of a solution when their acid and base forms
are present in equal concentration.
Bicarbonate Buffer System
H+
Hydrogen Ion
Free hydrogen ion
`
+
HCO3-
`H2CO3
Bicarbonate ion
Carbonic acid
`
weak base
weak acid
Part A: Measuring pH
The pH indicator paper indicates the pH of a solution by a color change, which can then be
compared to a reference color chart. First place your prediction for each substance then measure
the pH of various solutions provided and record.
Define the following terms:
Acid:____________________________________________________________________
Base:____________________________________________________________________
If a solution has a pH of 4 it is _____________________________.
If a solution has a pH of 9.5 it is _____________________________.
If a solution has a pH of 7 it is _____________________________.
Sample item
pH
Acid or base?
Sample item
pH
1. Vinegar
4.
2
5. Sodium Bicarbonate
(NaHCO3)
3
6.
Experimental Plan
(a) Materials per group:
1 ml pipettes
0.1M NaOH
pH meter
Acid or base??
Detergent
Ammonia
(NH3)
4- 100 mL beakers
stirring rod
beaker of distilled water
0.1M HCl
pH 7 buffer solution
(b) Procedure:
1. Label two beakers “acid” and “base”.
2. Pour 10 ml of 0.1 M HCl solution into the “acid” beaker; pour 15 ml of 0.1M NaOH into the other.
3. Distilled water & acid.
a. Pour 25 ml distilled water into a 100 ml Beaker.
b. Record its pH at 0 substance added.
c. Now begin to get pH data; add acid 0.5 ml at a time. Swirl the flask and record the pH using
the pH meter
d. after each increment has been added.
4. When you have added a total volume of 5 mls acid, stop. Discard the solution in the flask, wash
and dry the glassware, and repeat step #3 but use distilled water and add base.
5. Buffer solution & acid.
a. Pour 25 ml buffer solution into a 100-ml Beaker flask.
b. Check its pH at 0 substance added.
c. Collect data; add 0.5 ml acid at a time, swirl, and take pH.
6. When you have added a total volume of 5 mls. acid, stop, Discard the solution in the flask, wash
and dry the glassware, and repeat step #3 but use the buffer and base instead.
Results
1. Completed data tables B & C.
2. Graphs 1 & 2, buffer capacity. To construct a buffer capacity curve, write the pH scale on the
vertical axis (this is the dependent variable), and write the mls. acid or base added on the
horizontal axis (this is the independent variable). Directly compare the distilled water vs.
buffer solution curves by having them appear on the same graphs (for either acid addition or
base addition).
Table B: Addition of different
concentrations of an acid to dH2O
inorganic buffer.
# of ml
pH
of HCL
Water
0
and an
pH Inorganic
Buffer
Table C: Addition of different
concentrations of an acid to dH2O and an
inorganic buffer.
# of ml
pH
pH of NaOH
Water Inorganic
Buffer
0
.5ml
.5ml
1.0 ml
1.0 ml
1.5ml
1.5ml
2.0 ml.
2.0 ml.
2.5 ml
2.5 ml
3.0 ml
3.0 ml
3.5 ml
3.5 ml
4.0 ml
4.0 ml
4.5 ml
4.5 ml
5.0 ml
5.0 ml
Graph 1: Title_________________________________________________________
mls. added of .1 M HCL
Graph 2: Title________________________________________________________
mls. added of .1 M NaOH
Analysis:
1. Explain why buffers might be important for your cells?___________________________
_________________________________________________________________________
2. Did the buffer completely stop a change in pH?_________________________________
3. Is more buffer in your cells better for you? _______Explain _____________________
4. ______________________________________________________________________
5. Describe the buffering capacity (or lack of it) of water __________________________
______________________________________________________________________
______________________________________________________________________
6. Describe the buffering capacity (or lack of it) of the inorganic buffer solution.________
______________________________________________________________________
7. Which two “themes of biology” are best represented by this buffering system and explain the
connection.__________________________________________________________
______________________________________________________________________
______________________________________________________________________
8. Read the explanation and description of alkalosis and acidosis. Explain in your own words how
the bicarbonate buffer system works to help keep systems at a steady pH.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
9. What Kind of feedback system does this buffering system represent positive or negative?
Explain.
___________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
WORD BANK: *carbonic acid, *hydrogen ion, *carbon dioxide, *bicarbonate ion,* water.
METABOLIC ALKALOSIS
Cause: Either a decrease in hydrogen ion concentration in the blood (variety of causes) or an
excessive intake of bicarbonate.
H
+
-
+
_____
+
HCO3
H2CO3
H2O
______
_____
______
CO2
_____
1. Label the molecules (and ions) above with their correct names.
The body reacts to the increase of pH (more basic) by triggering the release of H+ from the weak
acid. This shifts the equation to the left as hydrogen ions are released.
More free hydrogen ions
…so the pH decreases (becomes more acidic) and homeostasis is restored. If there is too much of a
decrease in H+ or too much bicarbonate for the body to handle it, metabolic alkalosis may result.
Contributing Conditions:
Overdose of alkaline drugs (TUMS, Rolaids, Baking Soda), Diuretics (excess H+ in the urine therefore
out of blood), Negligent administration of Sodium Bicarbonate
METABOLIC ACIDOSIS:
Cause: Build up of H+ ions in the body. Hydrogen ions will combine with bicarbonate ions to form
carbonic acid (weak acid) which has no effect on the body.
+
H
_____
+
-
HCO3
______
H2CO3
H2O
_____
______
+
CO2
_____
2. Label the molecules (and ion) above with the correct names.
Carbonic acid rapidly breaks down into carbon dioxide and water. An increase in carbon dioxide
concentration in the blood will lower the pH ( more acidic) of Cerebral Spinal Fluid (of the brain). This
is detected in the medulla, stimulating an increase in respiratory rate and depth to blow off the
excess carbon dioxide causing the blood to become less acidic and the pH to rise. Hence you have a
metabolic acidosis with a compensatory respiratory alkalosis.
Contributing Conditions
Exercise, Cardiac Arrest, Shock, Renal Failure, Diarrhoea, Vomitus, Meds. e.g. Aspirin