pH and Buffering
Capacity of Seawater and Fresh Water
Background Information:
pH is a measure of acidity. Acids
are characterized by their ability to give off H+ ions in aqueous
solutions. pH is a mathematical function that indicates the amount of
H+ present in the water, and is calculated using the equation
pH = — log[H+]
The smaller the pH value, the
more acidic the sample. Acids have pH from 1 to 7, 7 is neutral, and
bases have pH from 7 to 14.
Buffering capacity is the
ability of a solution to resist changes in pH. Why might it be important
for a body of water to be able to resist changes in pH? Would you
predict sea water or fresh water to have a higher buffering capacity?
Why? Quantitatively, buffering capacityis defined as the number
of moles of a strong acid or strong base that are required to change the
pH of 1 liter of the solution by 1 pH unit. You will measure and
calculate this later.
In this investigation, you will
determine the pH of bodies of water you visit, and determine which ones
have the largest buffering capacity. Try to compare a fresh water
sample, a brackish water sample, and a sea water sample.
To become familiar with the
tests, try testing samples of distilled water, tap water, and pond,
lake, or stream water before leaving home.
Materials:
| Wide range pH paper |
3 stirring rods |
| Narrow range pH paper
(6-8) |
3 sample jars |
| 0.1 M HCL in dropping
bottle |
10 mL graduated cylinder |
Test samples of water you find along
the way. Use 3 trials for each sample whenever possible.
Calibrate the dropping bottle you are
using. Count the number of drops needed to fill your graduated
cylinder to 1 mL and if this number is different from 15 drops per mL,
substitute the correct number of drops into Equation 1 in the Data
Analysis Section.
Procedure: for each
sample
1. Take samples of seawater,
brackish water, or freshwater from a site. Record the location and a
description of the site in the
data table provided in the kit.
2. Place 50.0 mL of the seawater,
fresh water, or brackish water sample in each of 3 sample jars. It is
important to use as close to
the same amount of water for each sample as possible under the
conditions.
How might you do this without
any contamination of samples?
3. Check and record the pH of each
using first the wide range and then the appropriate narrow range papers.
Record.
4. Add 1 drop of 0.1 mL HCL to each
sample. Stir, and measure the pH using the narrow range paper. Record
the #drops used and the pH
after each addition of acid.
Repeat until the pH of each
sample is 7.
5. Graph your data on a piece of
graph paper. For best comparison, graph two or three samples on the
same
piece of paper. Use a computer
graphing program such as Graphical Analysis, if available, when you
return
home. (The program must do
best-fit curves.)
Error sources:
1. Why is volume of water sample
used each time important? Drop size?
2. Why is it important to use the
same acid solution for each test?
3. Why is it important to use the
same procedure for each sample? (It would be nice to take your pH
meter
or computer into the field, but
this is not always possible.)
4. Explain the effect of sample size
on the buffering capacity.
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Buffering Capacity Data
Table Sample ID # _____ Site and
Description _________________
| drops HCl |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
| pH |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Data Analysis:
To calculate buffering capacity:
Find the number of moles of
strong acid or base that are required to change the pH of 1 liter of the
sample by 1 pH unit.
The HCl is 0.1 M. There are 15
drops** in 1 mL acid, so 1 drop is equal to 6.7 x 10-5 L.
Sample volume was 100 mL (if not, adjust ratio of sample to 1 L
accordingly.)
If 25 drops of HCl were used per
100 mL sample:
Equation 1: 25 drops
HCl x 0.067 mL x 1 L x
0.1 mole H+ = 1.6 x 10-6 moles acid
100 mL
sample 1 drop HCl 1000 mL 1 L HCl
neutralized per 100 mL of sea water
SO :
(Number of drops) x 6.7 x 10-8
= buffering capacity of seawater, in moles per 100 mL of seawater
**Be sure to calibrate the drops
as directed in the procedure.
Copyright 1997 by Luann
Christensen Lee
Feel free to reproduce this page
for classroom teaching purposes - please include this copyright, and
please do not change the material.
|
