Name: _____________________________________
Acids, Bases, Buffers, and pH
Objectives
1.
Measure the pH of various liquids.
2.
Show how buffers stabilize the pH of a liquid.
3.
Measure the ability of commercial antacids to buffer the pH of liquid.
Introduction
An acid is a substance that releases hydrogen ions when dissolved in the water. The greater the degree of
dissociation, the greater the strength of the acid. Hydrochloric acid is a strong acid. HCl is secreted in our stomachs.
The hydrogen ions aid in digestion and make a toxic environment for bacteria we may ingest. A base is a substance
that releases hydroxide ions or removes hydrogen ions when added to water. The greater the degree of
disassociation, the stronger the base is. Sodium hydroxide is a strong base. If you are suffering from acid
indigestion, your stomach acid level is high, and taking antacids will add OH ions to your stomach. The OH will
combine with an H+ ion, producing a water molecule, which is neutral.
Chemists use a pH scale to indicate the strength of acids and bases. The scale ranges from 0 to14 and indicates the
proportionate concentration of hydrogen and hydroxide ions at the various pH values. When the concentration of
hydrogen and hydroxide ions increases, the concentration of hydroxide ions decreases, and vice versa. A change of
one (1.0) in the pH number indicates a 10-fold change in the hydrogen concentration because the pH scale is a
logarithmic scale.
Pure water has a pH of 7, the point where the concentration of hydrogen and hydroxide ions are equal. Acids have a
pH less than 7 and basses have a pH greater than 7. The greater the concentration of hydrogen ions, the stronger the
acid and the lower the pH number. The greater the concentration of hydroxide ions, the stronger the base and the
higher the pH number.
Living organisms are sensitive to the concentrations of hydrogen and hydroxide ions and must maintain the pH of
their cells within narrow limits. For example, your blood is kept very close to pH 7.4. Organisms control the pH of
cellular and body fluids by buffers. A buffer is a compound or a combination of compounds that can combine with
or release hydrogen ions to keep the pH of a solution relatively constant.
Part A: Determining pH
A convenient way to measure the pH of a substance is with pH paper. The color chart on the side of the dispenser
relates the color of the pH paper to the pH of the solution.
Dip the pH paper into the liquid you are
testing, making sure to wet the entire test
area. Compare the color change on the
paper to the color chart provided with
the test strips to determine the pH.
Depending on the brand of pH test strips,
there may be a single color to compare
or 3 color blocks as show on the left.
Watch a YouTube video to see how this procedure is done.
Procedure
1.
Place a small amount of each of the following solutions in the chart below in its own small paper cup. I
recommend labeling your cups with the solution it carries. Wet a small piece of pH paper in each of the following
solutions. Compare the wet pH paper color with the color scale to determine the pH.
Pro Tip - Read the pH paper right away. The color will change over time.
2.
Record the results of each solution in Table 1.
Solution
pH
Acid / Base / Neutral
Saliva
use your own by spitting in a cup (eww!)
Tap Water
Baking Soda
one tablespoon in ¼ cup water
Juice
note the type you used:
Black Coffee
Vinegar
Liquid Soap mixed with water
50% : 50%
Milk
Substance of your choice: ______________
Table 1 - pH of Common Solutions
Note: If you are not able to acquire any of the solutions above, you may replace it with a household solution of your choice
(e.g. Replace with soda). Be sure to note on your document which solution you replaced and with what. If you are unsure of
your replacement you may contact me prior to continuing with the lab.
Part B: Buffers
In most organisms the pH is kept relatively constant by buffers, which are chemicals that absorb excess H+ as the
pH decreases (becomes acidic) or releases H+ as the pH increases (becomes basic). The addition of a small amount
of acid to a buffered solution produces a small change in the pH, whereas adding the same amount of acid to an
unbuffered solution changes the pH drastically. Most biological fluids (i.e. milk, blood) contain buffers. For
example, human blood contains buffers that maintain a pH of 7.3-7.5; slight changes in pH of blood can cause
illness and even death.
Commercial antacids such as Maalox, Alka-Seltzer, Rolaids, and Tums claim to neutralize stomach acid by
absorbing excess H+ (produced as hydrochloric acid) by the stomach.
Procedure
1.
Select one of the substances from the chart above that is acidic.
2.
Add 15 mL (roughly one tablespoon) of the acidic substance you chose to an empty, clean small paper cup.
3.
create an antacid solution.
4.
Add 15 mL (roughly one tablespoon) of the antacid solution to the acid solution in the cup. Retest the acid
solution and record the pH after adding the buffer.
Type of buffer used ______________________
pH of acid before the buffer ___________
Acid selected _______________________
pH of acid after the buffer ________________
Summary- you are expected to use your online textbook to answer these questions
This Khan Academy video can also help you to answer these questions and understand pH.
Acids, Bases and pH
1. Describe the chemical difference between 2 solutions, one with a pH of 5 and the other with a pH of 8, in terms
of ions in solution.
2. After adding an antacid solution to the known acid, what effect did it have on the pH?
3. How do buffers work and why are they important to biological systems?
4. Explain how solutions are classified as acids or bases on the pH scale. Give 2 examples of acids and 2 examples
of bases.