Lesson Summary:  This lesson is designed to give students a better understanding of how to measure gasses in water. This lesson sets the stage for the upcoming Experiments, which rely on a thorough understanding of the presence of carbon dioxide and oxygen in the solutions that the students will be manipulating.

Time Estimates:  30 minutes.  

Learning Outcomes: 

1. I can describe the different forms of respiration in terrestrial and marine organisms.
2. I can define the mechanisms required for a gas to be dissolved into a solution.
3. I can compare the effect of temperature and pressure on the solubility of a gas solute in water.
4. I can observe different methods used to measure the concentration of dissolved gases in a solution.
5. I can calculate dissolved carbon dioxide and carbonate species in water.

Classroom Prep:  None

Guiding Questions

1. What are the different challenges facing terrestrial vs aquatic vertebrates when trying to respire? What about  microbial respiration?
2. How do marine and aquatic organisms source the gases necessary for respiration?
3. What is gas solubility, and which factors impact it?
4.  How can we measure dissolved  oxygen and carbon dioxide concentrations in bodies of water?

Key Terms:  solubility, solubility constant

Time

Activities

Learner outcomes

I can…

Let me prove it. (write responses)

5  - 7 mins

Review the paragraph below, then watch this video:

Youtube: Fish breathing

Describe the different forms of respiration in terrestrial and marine organisms.

Explain how water-breathing and air-breathing organisms differ in gas exchange.

Think of an advantage and disadvantage of each?

All organisms require some gas to conduct respiration, photosynthesis in the case of carbon fixation. In different environments,  organisms have found various methods to obtain this gas, each containing its own tradeoffs. Vertebrates typically use lungs, one of the few empty spaces in a body cavity, and require a bone and muscle network to operate correctly. Vertebrates in water (except marine mammals; this case relates mostly to fish) often cannot have a cavity of air within their body because it will expand and contract with changing depth. Fish don’t have lungs like other vertebrates, they have fragile filaments that perform gas exchange with the surrounding water. They are delicate and exposed to the external environment. Invertebrates and microbes can often diffuse gasses through their outer membranes in a similar way. In microalgae, this gas is trapped and brought into the cell through enzymatic activity, as in RuBisCO carbon fixation.

These premises require us to accept that gases can somehow insert themselves into a medium such as water. How does this happen?

5 - 7 mins

Read the paragraph below, then watch this video:

Youtube: The Effect of Temperature on the Solubility of Gases - A Science Experiment with Mr Pauller

Define the mechanisms required for a gas to be dissolved into a solution.

Describe two parameters that affect the solubility of a gas in water.

Gases can indeed dissolve in water, which is good news for all that life in oceans and lakes that need to respire! The rules of solubility govern the action of gaseous chemicals transitioning to an aqueous solution. In turn, solubility is controlled by a solubility constant, which tells you how much of a chemical can be dissolved in a given amount of fluid. These constants change based on the chemical, which other chemicals are in the solution, as well as the parameters explored below.

Read the paragraph below, and try to think about each parameter from an atomic model of a gas or dissolved gas.

Compare the effect of temperature and pressure on the solubility of a gas solute in water.

The amount of carbon dioxide in the atmosphere is highest in the poles. Why might the atmospheric carbon dioxide levels be higher in the poles compared to the rest of the world?

Compare the amount of gas that would enter a cup of water if you exhale into the cup at the top of Mt. Everest and exhale into a different cup at sea level.

Which factors affect the solubility of a dissolved gas?

Temperature: The amount of gas that can dissolve in water depends on the temperature of that water. As the water cools, the atoms and molecules within it begin moving slower, giving more space between those molecules that gases can “slip” their way into the solution. This means that as a fluid cools, the solubility of gases (or a gas's ability to insert itself into solution) increases!

Pressure: The atomic model can also clarify how decreasing pressure leads to an increase in gas solubility. As pressure decreases, the molecules move further apart, allowing more space for gaseous molecules to fit into temperature.

Salinity and chemical equilibrium: Salinity is the concentration of dissolved salts in a given solution. The solubility constant of each salt tells you how much chemical can dissolve in that solution. Hence, as you approach the upper limit, it gets harder and harder to add more without it immediately precipitating or going from aqueous to solid phase within the solution. You can observe this effect by continuously adding sugar to a beaker of water, and noting that you eventually add so much sugar that it does not dissolve and remains in crystalline form.

5 mins

Read the paragraph below regarding different methods to measure dissolved oxygen in water.

Colorimetric method video:

Measuring Dissolved Oxygen

DO meter method video:

Pond Water Quality-Dissolved Oxygen

Titration method video:  

8. Measuring Dissolved Oxygen

Observe different methods used to measure the concentration of dissolved gases in a solution.

Which of the tools described would be appropriate for algae farmers to measure the amount of oxygen in the water? Why would these be good choices?

How would the amount of dissolved gasses in a liquid affect the growth of algae? Would these results differ with different gases? Why or why not?

 Which of the measurement tools described would be appropriate for algae farmers to measure the amount of oxygen in water? Why would these be good choices? Are there tools that would not work well for the algae farmer?  Why not?

There are many methods used by scientists to measure the concentration of dissolved oxygen in water. They range from cheap and "good enough" to very expensive and precise. The plethora of solutions makes describing each technique difficult, but each takes advantage of one of three general concepts:

Chemical methods: chemists have designed strips of paper coated in solutions that change color in the presence of different oxygen concentrations. These methods are cheap and easy to use but do not have the ability to track change over time and are limited the human capacity to discern which concentration is most accurate in cases where the color is between two values on a provided reference scale. Other chemical methods include titration with a known concentration solution and concluding the starting concentration based on the combined solution's timewise pH change.

Destructive methods: Clark-type electrodes allow scientists to use electricity to measure dissolved oxygen concentration in a solution. Electrodes provide electrons to a solution, and their rate of uptake can be measured against known standards of dissolved oxygen to determine the concentration of your solution. They are expensive and require calibration, but can be used to track changes throughout time and are very precise.

Non-destructive methods: optical methods, much like the pulse oximeters clipped to your finger in a hospital, can measure the concentration of dissolved oxygen in water by measuring its absorption of light. They typically consist of a light meter, which shines a specific wavelength of light through water, and an oxygen indicator that glows upon light impingement at varying brightnesses depending on oxygen presence. These allow for tracking change over time, but they are expensive and require calibration using known standards.

7-10 min

Read the below paragraph describing how scientists can measure dissolved carbonate species in water.

Download the co2sys excel spreadsheet.

Calculate dissolved carbon dioxide and carbonate species in water.

CO2 Sys

Read the instructions, and research the temperature and salinity of different bodies of water (high vs. mid-latitude oceans, lakes, rivers, etc.). Research the possible range of pH, DIC, or TA values for the same bodies. Input those values into the co2sys excel spreadsheet, and explore the outputs. This exercise aims to familiarize yourself with standard methods and programs used by marine and aquatic scientists around the world. 

Hypothesize about conditions that would be beneficial for algae growth and explain your rationale. Think about temperature, pressure, and the concentration of different dissolved gases.

In water, the concentration of carbon dioxide is often difficult to measure directly. As we will explore in later lessons, carbon dioxide quickly converts into other carbonate species when it contacts water, turning into either bicarbonate or carbonic acid. As such, oceanographers must indirectly measure and then compute the concentration by taking the values of:

1. Temperature
2. Salinity
3. Atmospheric pressure
4. Partial pressure of CO2 (pCO2, the amount of pressure that molecules of carbon dioxide contribute to the total)
5. pH
6. Dissolved inorganic carbon (DIC, a measure of total dissolved carbonate species mentioned above)
7. Total Alkalinity (TA, another metric computing the concentration of carbonate species, that includes some species not included in DIC)

We do not need all of these values to compute the concentration of CO2. We always need temperature and salinity, and we can choose from one of either TA, DIC, pH, or pCO2. Once we have temperature, salinity, and two of the above, we can plug them into free programs that use established mathematics to compute the concentration of all the carbonate species (including CO2) in water.  

Lesson Summary:  This lesson begins tying previous concepts together in terms of respiration, photosynthesis, and how to measure both of those processes with a pH indicator solution. You will discuss what a pH indicator solution is and discuss how to apply it to future experiments.

Time Estimates:  30 minutes

Learning Outcomes: 

1. I can define and calculate the pH of a solution and describe the pH scale.
2. I can model the equation of the dissociation of carbon dioxide in water.
3. I can predict inputs and outputs of carbonate species from an aqueous system and the resulting change in pH.
4. I can predict and justify the reasoning behind  the change in pH of a solution as a result of microalgal inhabitants.
5. I can recognize and use scientific methods of measuring pH.
6. I can leverage the carbonate cycle to indirectly measure photosynthesis and respiration using a bicarbonate indicator.

Classroom Prep:  None

Guiding questions:

1. What is pH, and how is it measured?
2. Briefly describe the aqueous carbonate cycle.
3. What is the relationship between the carbonate cycle and pH?
4. What effects do photosynthesis and respiration have on pH?
5. What are the various methods used to measure pH?
6. How can we leverage the carbonate cycle to draw conclusions about photosynthesis and respiration?

Key Terms: proton, pH, log transformation, Litmus paper, pH probe, bicarbonate, Le Chatelier’s principle, carbon fixation, pH indicator

Time

Activities

Learner outcomes

I can...

Let me prove it. (write responses)

5-7 min

Read the below paragraph describing acid-base theory and how scientists describe acidity.

Explore:

Acid-Base Solutions 1.2.21

Define and calculate the pH of a solution and describe the pH scale.

What is a proton? Explain how protons relate to pH and acidity.

What is the relationship between the pH value and the concentration of H+ ions and hydroxide (OH-) ions? Write the equation used to calculate the pH of a solution.

Say the concentration of H+ ions (protons) in a solution is 10-7. Calculate the pH of this solution. Side note: this is roughly the pH of your blood!

When an acid comes into contact with water, it releases a hydrogen ion, also known as a “proton.” We use the number of free protons in a solution to calculate the pH of the solution, which tells us how acidic that solution is.

We can measure the concentration of free H+ ions, and then apply a log transformation to that number, which allows us to visualize massive changes in [H+] as relatively small numbers.

1. pH = -log[H+]. This equation tells us that as the H+ concentration increases, the pH decreases in number, and vice versa for a decrease in [H+]. It also tells us that we need a massive change in [H+] to shift the pH a whole number up or down. This means when we compare a pH of 7.0 and a pH of 7.5, the [H+] concentration that each number implies is much larger than a difference of simply 0.5 would suggest.
2. pH is unitless, with a range of 0-14. A pH of 0 is highly acidic, and a pH of 14 is highly basic. An increasing pH means that H+ ions are removed from solution, and a decreasing pH means that H+ ions are added to the solution.

2-3 min

Read the below paragraph describing carbonate speciation in water, then watch the below video:

Sea Sketches: The ocean carbon cycle

Model the equation of the dissociation of carbon dioxide in water.

Draw a representation of what a strong acidic solution would look like under a microscope of hypothetically incredible power, and a representation of what a weak acidic solution would look like.

Using the below diagram, draw a stoichiometrically correct equation for the dissociation of CO2 into the various carbonate species that occur in seawater.

As we learned in previous lessons, gasses can dissolve in water and chemically react with other dissolved or aqueous molecules. Carbon dioxide is no different! When carbon dioxide reacts with water, it forms bicarbonate and hydrogen ions, and then, those react again to form carbonate and two hydrogen ions. The intermediate step to carbonic acid does happen, but the amounts created are negligible.

3-4 min

Read the below paragraph describing how Le Chatelier’s Principle governs the concentration change of different carbonate species in water, and how those changes relate to pH.

Predict inputs and outputs of carbonate species from an aqueous system and the resulting change in pH.

Research Le Chatelier’s Principle and make the following predictions. Thinking specifically about photosynthetic reactions, what variables cause a shift in equilibrium towards the reactants? What variable cause a shift in equilibrium towards reactants?

An algal community is placed in an aquatic solution, what would happen to the concentration of CO2 within that solution over time? How would the pH of that solution then change?

As with any chemical equilibrium, Le Chatelier's principle tells us that addition to one side of the carbonate system results in a shift to the other side because chemical equilibrium must be maintained. This principle tells us that chemical systems are always trying to balance out. If you add reactants to that system, the equilibrium will shift to the products’ side in order to adjust that equilibrium.

As you can see from the diagram of carbonate speciation in the previous section, the carbonate system’s significant steps generate at least one free hydrogen ion. The majority of these ions are from carbon dioxide and bicarbonate. Suppose a large amount of carbon dioxide dissolved in a solution. In that case, many H+ ions are generated as the carbon dioxide reacts with water per Le Chatelier's principle. If carbon dioxide is removed from the water, then the equilibrium drives a decrease in H+ ions as they re-form into water.  

2-3 min

Read the below paragraph describing the effects of microalgal respiration on pH.

Predict and justify the reasoning behind  the change in pH of a solution as a result of microalgal inhabitants.

We now know that microalgal photosynthesis will increase the solution's pH by decreasing free hydrogen ions concentration. What do you think will happen when microalgae respire?

What would an increase in dissolved carbon dioxide concentration do to a carbonate system equilibrium, and what would that do to the pH? In which direction would the chemical equation be “pushed?”

The dark reactions of photosynthesis require carbon dioxide to be used as a backbone for the creation of sugars, so microalgae pull carbon dioxide from the water in a process known as carbon fixation. This process decreases the concentration of dissolved carbon dioxide, which reduces the concentration of free H+ ions, which increases the pH of the solution.

5-7 min

Read the below paragraph describing the various methods used to measure pH.

Recognize and use scientific methods of measuring pH.

Before you read further in this document, think about how the methods described below could be used to measure the pH change of a solution derived from microalgal photosynthesis or respiration.

How would you combine these methods to track changes with time using color?

There are many different instruments used to measure pH, each of which with their own advantages and disadvantages. Litmus paper, a strip of paper stained with a dye that changes chemical structure based on pH, will change color when dipped into a solution. This allows you to compare the stain to a known pH standard. While this is a simple solution, it does not allow you to track changes in pH with time as well as a pH probe. These probes take advantage of the fact that changes in H+ concentration alter the electrical conductivity of the solution. We can calibrate the sensor to a standard of known pH (and therefore known electrical conductivity), and then measure the pH of our test solution by comparing those values. The third common method is a synthesis of these two concepts (ease of use and ability to track time-wise changes), which we will discuss below

7-9 min

Read the below paragraph describing colorimetric pH indicator solutions.

Watch this video:

Bromothymol Blue Respiratory Physiology Experiment

Leverage the carbonate cycle to indirectly measure photosynthesis and respiration using a bicarbonate indicator.

What happened in this video? Why was the color of the solution changing when the person exhaled into it through a straw? What chemical caused the indicator to change color?

As mentioned above, litmus paper is easy to use but bad at tracking change over time, and a pH probe is good at tracking changes but expensive and occasionally hard to use. The solution is a compromise that changes color with time to indicate pH without an electrical sensor's need.

Because there is an intermediate step to bicarbonate in this equilibrium, we can use the molecule as a pH indicator. Combining bicarbonate with sensitive dyes results in a solution that can be added to water and changes color with a pH change. Because we know about the relationship between CO2 and pH, we can conclude the relative rates of photosynthesis in algae by looking at how the color changes over time. The direction of pH change can tell you whether or not the algae are in net photosynthesis (an increase in pH) or net respiration (a decrease in pH).

Algae Research and Supply provide a pH indicator solution composed of Bromothymol Blue and Creosol Red, mixed with bicarbonate. The solution allows us to select the molecule that will serve as a proxy for pH and introduce it into the solution without killing our algae. This mixture is included in our bead solution and allows us to track photosynthesis and respiration-based changes in pH according to the scale shown below

.

Lesson Summary:  This lesson explains how humans can OBSERVE pH to learn about photosynthesis.

Time Estimates:  10- 12 minutes

Learning Outcomes: 

1. I can use chemical indicators as a tool to describe available carbon dioxide for photosynthesis.

Classroom Prep:  None

Guiding questions:

1. What is pH, and how is it measured?
2. Briefly describe the aqueous carbonate cycle.
3. What is the relationship between the carbonate cycle and pH?
4. What effects do photosynthesis and respiration have on pH?
5. How can we leverage the carbonate cycle to draw conclusions about photosynthesis and respiration?

Key Terms: Proton, pH, log transformation, Litmus paper, pH probe, bicarbonate, Le Chatelier’s principle, carbon fixation, pH indicator

Time

Activities

Learner outcomes

I can...

Let me prove it. (write responses)

5  mins

Watch this video:

Bromothymol Blue Respiratory Physiology Experiment

Use a chemical indicator as a tool to describe photosynthesis.

The teacher blew through a straw into a universal indicator solution in the video until it changed color. Did the indicator indicate the solution became acidic or basic?

What chemical caused the indicator to change color?

How does the pH of a solution relate to the amount of CO2 in the solution?

How could you use bromothymol blue to test the pH in a solution where algae beads are located?

How does knowing the pH of the algal bead solution help you to understand respiration and photosynthesis rates?

5 mins

Study this picture:

Use a chemical indicator as a tool to describe photosynthesis.

Explain the differences in the color of the different solutions with the algal beads. Be sure to use the vocabulary terms photosynthesis, respiration, and pH in your explanation.

Review

1. Recall from lesson-1 that one of the things that life does is change the environment

1. Respiration adds CO2
2. Photosynthesis removes CO2

2. CO2 is a gas,  gasses can dissolve in water

1. See lesson on dissolving gasses in water

3. CO2 gas in water forms 3-different molecules depending on the pH

1. CO2 
2. Bicarbonate
3. Carbonate

4. Aquatic plants can use only two of those molecules for photosynthesis

1. CO2 
2. Bicarbonate
3. They can NOT use carbonate

5. Why does pH increase?  

1. As photosynthesis occurs the CO2 and Bicarbonate are fixed into new algae biomass
2. The H+ (acid) in the water are part of the CO2 and Bicarbonate.  When those get used by the plants there are fewer H+, so the water becomes less acidic and more alkali, thus pH goes up!

6. As humans, how can we measure pH

1. Commonly a pH probe is used

1. Uses electricity to compare the difference in voltage between a reference solution and the testing solution

2. Easier is to use chemicals that change color based on pH

1. Bromothymol blue

2. Creosol red

7. The Algae Beads kit from ARS have a Bromothymol Blue and Creosol Red indicator solution

1. By combining both Bromothyol blue and Crysol red, we can get a range of pH that is color changing from 6-9

8. pH Indicator and algae beads

1. The indicator solution is not toxic to the algae beads
2. When the algae consume or release CO2 and form carbonic acid,- the pH changes
3. Over time, we can track the rate of change of the pH and thus the rate of photosynthesis

1. Rates of photosynthesis
2. Rates of respiration