Measuring pH and Alkalinity of Sea Water: Lab
Background knowledge:
Living organisms need specific pH ranges in order to carry out the chemical reactions of their metabolism.
Those ranges are different for different organisms, or even different in different systems of organisms
(digestive system vs vascular system), however it is generally important that organisms maintain a relatively
stable pH. Alkalinity is a measure of how well a liquid can resist changes in pH. It is generally caused by the
presence and effectiveness of buffers; chemicals that can accept or donate hydronium (H+) ions when needed.
Buffers are extremely important chemicals for maintaining homeostasis in a variety of systems.
The alkalinity and pH of ocean water is important for the organisms living in the contain ecosystems. The salts
present in sea water act as a buffer helping to maintain the pH. Specifically, Calcium Carbonate will accept
hydronium ions out of the water, keeping their concentration relatively low. This allows the organisms in the
ocean, specifically on coral reefs, to grow and reproduce successfully.
Question:
1. What happens when you put acid into fresh water?
Hypothesis: ______________________________________________________________________
2. Does sea water resist changes in pH better than fresh water (buffering capacity)?
Hypothesis: ______________________________________________________________________
Materials:
Sea Water
De-ionized water
Artificial Buffer
pH meter
200 mL beaker
HCl
Procedure:
1.
2.
3.
4.
Obtain one beaker and fill it with de-ionized fresh water.
Place the pH meter in the beaker.
Read the pH and record in table 1 under “0” drops for the fresh water reading.
Add one drop of HCl (Hydrochloric Acid). Stir gently with the pH probe. Allow for 30 seconds to get an
accurate reading.
5. Record the pH of the solution.
6. Repeat steps 4-5 four more times, adding one drop at a time up to 5 drops.
7. Rinse out the beaker and fill it with salt water.
8. Repeat steps 2-6 above.
9. Rinse out the beaker and fill it with buffer solution.
10. Repeat steps 2-6 above.
11. Create a triple line graph showing the results.
Table 1
Number of drops
(HCl)
0
1
2
3
4
5
pH reading:
Fresh Water
pH Reading:
Salt Water
pH Reading:
Buffer solution
Lab Questions:
1. What happened to the pH of fresh water when you added the acid?
____________________________________________________________________________
a. Was your first hypothesis supported or rejected? ______________________________
2. Compare the fresh water results to the salt water results. What happened to the pH of salt water
when you added the acid?
____________________________________________________________________________
____________________________________________________________________________
b. Was your second hypothesis supported or rejected? ____________________________
3. Based on your results, do you think ecosystems in the sea are affected by acid rain and pollution?
Support your argument with evidence.
___________________________________________________________________________________
____________________________________________________________________________
___________________________________________________________________________________
____________________________________________________________________________
___________________________________________________________________________________
____________________________________________________________________________
4. In our bloodstream, there is a chemical called bicarbonate (similar to calcium carbonate in the ocean).
What do you think this does for the pH of our blood?
____________________________________________________________________________
___________________________________________________________________________________
____________________________________________________________________________