Costa Rica Rainforest Experiment
"Keeping It Natural"

- Equipment
- Data Collection Procedure
- Conclusions and Extensions

PASPORT pH Sensor
(PS-2102)

Purpose

To measure the pH of several water samples and determine how well each can sustain aquatic life.

Overview

In this activity, students will collect several water samples from different parts of the Selva Verde preserve and use the PASPORT pH Sensor to measure the pH of each sample. Students will then predict whether aquatic life can live at the pH of the sample and suggest factors that influence the pH of each sample.

Background Information

The Danish chemist Soren Sorenson is credited with introducing the pH scale in 1909, giving scientists a means of characterizing a substance’s acidity based on a scale of 0 to 14.  This scale is logarithmic; meaning every one-unit change in pH represents a tenfold change in acidity.  Therefore, a river with a pH of 5 is one hundred times more acidic than a river with a pH of 7.

Most aquatic life depends on water that has a pH range between 5 and 9.  Water with a pH outside that range is unable to sustain most life.  In any ecosystem, the pH of a water system depends on both natural conditions and the interaction of humans in the environment.  Weather and climate play important roles as well.  For example, since rainwater tends to be slightly acidic, acidification can occur when rainfall accumulates in natural water systems.  Rainfall that percolates through dense forest litter and bogs also has a tendency to be more acidic due to the interaction of rainwater with the organic materials.

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Equipment

Data Collection Procedure

GLX Setup:

1. Connect the probe to the pH Sensor and plug the pH Sensor into the port on top of the GLX. 

2. Calibrate the pH Sensor.

3. From the Home screen, press I to access the Sensors screen and again to access the Sensors menu.  Press 3 to calibrate.

4. You will need to perform a 2-point calibration for this sensor with two standard solutions.  The default for the pH sensor is pH 4 and pH 7.  These can be changed by simply pressing c. 

Natural GLX Screen

5. Place the pH electrode into the first standard solution.  Wait for the reading to stabilize and press H to Read Pt 1.  Remove the pH electrode and rinse with distilled water.  Do the same for point 2 with the second standard solution.  Press OK.

6. Press F to switch to Manual sampling mode.

Record Data:

1. Collect a sample from the Selva Verde preserve. Press s to begin recording data.

2. Insert the pH electrode into the first sample. Wait for the reading to stabilize and press f to record the pH. When the keyboard data prompt appears, use the keyboard to type in the location.

3. Rinse the pH electrode. Collect another 9 samples and repeat the above steps. For best results, take the pH immediately after collection. Remember to rinse the pH electrode between samples. Press s to stop recording data.

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Conclusions and Extensions

1. Which samples were acidic, which were basic, and which were neutral?  Discuss what environmental factors at each location might influence the pH of these samples.

2. Was there any correlation between a sample’s acidity and its physical appearance or characteristics?  If so, what conclusions can be drawn?

3. Why is pH important to all living things?  In physiological systems, what role do buffers play in maintaining homeostasis?

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