Introduction to Oceanography: Abiotic
pH
Research Question: How and why does the pH of the estuary change with depth?
Part A: Background Information
pH is a measure of the concentration of hydrogen ions in a solution. The more hydrogen ions present, the more acidic a solution. The pH scale runs from zero which is very acidic to fourteen which is very basic. A pH of seven is neutral. Distilled (fresh) water has a pH of seven. An example of an acid you might have at home is a lemon with a pH of about 2. Soap is a base with a pH of about 12. The pH of the ocean is slightly basic, usually between 8.0 and 8.4.
Part B: Sampling Method
Source: Project Oceanology
Van Dorn Bottle set up (left) Field pH kit (right)
The Van Dorn bottle allows water to be sampled from any depth in the water column. It consists of a narrow plastic tube, with rubber plungers that allow it to close. To deploy, the van dorn bottle plungers are held open with wire, and the apparatus is lowered into the water using a cord and winch. When the bottle is at the desired depth, a “messenger” weight is dropped along the cord to a mechanism which releases the plungers, closing the bottle. This allows scientists to sample purely from the desired depth, without allowing the sample to mix with other water while it is being retrieved.
Once the water samples have been collected a pH field kit is used to measure the pH from each sample depth. The field kit includes an indicator solution. This solution is added to the sample water and the color change that occurs determines the pH.
To learn more about the Van Dorn Bottle watch this brief video:
Part C: Prediction and Reasoning
Study the background information provided on pH (above),and look at the field notes (below).
Field Notes
Sample Location: Mouth of the Thames River estuary. (View map here)
Time of Year: Early May
Weather Conditions: Day time air temperatures are about 61 Fahrenheit and evening air temperatures are
Sample Depth: pH samples were collected at the surface (0 meters), the middle of the water column
(4 meters) and the bottom (9 meters). This is called a depth profile.
about 48 Fahrenheit.
Write answers to the following prompts on your sheet of paper.
1. Make a prediction: How do you expect the pH to change from top to bottom?
2. Explain Your Reasoning: WHY do you think the pH will look this way?
Part D: Analyze the Data
Look at the dataset below. On your piece of paper, illustrate the data by making a special kind of graph called a depth profile. The axes look a little different than you might be used to; simply plot column 1 numbers on the vertical axis and column 2 on the horizontal. The origin is still (0,0). Connect your points with a curved line. Remember good graphs have a title, a labeled x-axis (including units), a labeled y-axis (including units) and an appropriate scale.
This table shows the pH in the water column.
Part E: Interpret the Results and Make Arguments from Evidence
Write answers to the following prompts on your sheet of paper.
1. Make a claim that answers the research question (one sentence).
2. What evidence was used to write your claim? Reference specific parts of your graph.
3. Explain your reasoning. Make sure to connect your answer to what you have learned about the relationship between depth and pH.
4. Was your prediction supported by the results? Use evidence to explain why or why not.
5. What do these numbers mean for the health of Long Island Sound? Are these readings within the normal range for this area? Standard Ranges
6. How would you follow up? Describe a new question that should be investigated to build on these results, and what future data should be collected to answer your question.
7. Extension: Organisms within an estuary can be significantly affected by the pH of their environment. Check out this fact sheet and this infographic from NOAA about how low pH or “acidic” water could impact crab life cycles. Write a paragraph that (1) identifies at least one factor that might cause pH to change in the estuary, (2) explains how a change in pH might affect crabs, and (3) explains how this affects (or is affected by) people.
Congratulations! Your final analysis should include the following components:
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A statement of the research question that you chose/were assigned
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Your prediction and your reasoning
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Your labeled graph
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Your answers to the results questions
Share your results with your teacher!