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:

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.

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:

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

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