Hydrogen Carbonate Indicator: The Colorful Detective of Carbon Dioxide
The Chemistry Behind the Colors
To understand how the hydrogen carbonate indicator works, we first need to understand a little chemistry. The key player is carbon dioxide (CO₂). When CO₂ dissolves in water, it reacts to form a weak acid called carbonic acid (H₂CO₃).
The chemical reaction is: $CO₂ + H₂O \leftrightarrow H₂CO₃$
Carbonic acid then breaks down (dissociates) into hydrogen ions (H⁺) and hydrogen carbonate ions (HCO₃⁻), which is where our indicator gets its name.
This second reaction is: $H₂CO₃ \leftrightarrow H⁺ + HCO₃⁻$
The most important part of this process is the release of hydrogen ions (H⁺). The concentration of these ions determines the pH, or acidity, of the solution. More H⁺ ions mean a lower pH (more acidic). The hydrogen carbonate indicator contains a special pH-sensitive dye that changes color as the pH changes.
The following table shows the precise color changes corresponding to different CO₂ levels.
| Carbon Dioxide Level | Approximate pH | Indicator Color | Interpretation |
|---|---|---|---|
| Very High | < 6.0 | Yellow | A large net production of CO₂ (e.g., high respiration). |
| High | 6.0 - 6.8 | Orange / Orange-Red | A net production of CO₂. |
| Atmospheric (Normal) | ~7.6 | Red | Equilibrium with the CO₂ level in normal air (~0.04%). This is the starting color. |
| Low | 8.2 - 8.4 | Magenta / Purple | A net removal of CO₂ (e.g., high photosynthesis). |
| Very Low | > 8.4 | Deep Purple / Violet | A very high rate of CO₂ removal. |
Investigating Photosynthesis and Respiration
The hydrogen carbonate indicator is most famously used to study the two most important energy processes in living organisms: photosynthesis and respiration. These processes have opposite effects on CO₂ levels, making the indicator a perfect tool for comparison.
Photosynthesis is the process used by plants, algae, and some bacteria to make their own food. They use light energy, carbon dioxide, and water to produce glucose and oxygen. The word equation is:
Carbon Dioxide + Water ⟶ Glucose + Oxygen (in the presence of light and chlorophyll)
Because photosynthesis uses up carbon dioxide, if a plant is photosynthesizing in a sealed container with the indicator, the CO₂ level will decrease. According to our table, this will cause the indicator to change from red to magenta or purple.
Respiration is the process that all living cells use to release energy from food. It uses oxygen to break down glucose, releasing carbon dioxide and water as waste products. The word equation is:
Glucose + Oxygen ⟶ Carbon Dioxide + Water (and energy)
Because respiration produces carbon dioxide, if an organism (like a small animal or germinating seeds) is respiring in a sealed container, the CO₂ level will increase. This will cause the indicator to change from red to orange or yellow.
A Classic Classroom Experiment
Let's look at a typical experiment that demonstrates the power of the hydrogen carbonate indicator. The goal is to show the net gas exchange of pondweed (an aquatic plant) under different light conditions.
Materials Needed: Four test tubes, hydrogen carbonate indicator solution, fresh pondweed (Elodea), aluminum foil, a light source.
Method:
- Prepare four test tubes, each about two-thirds full with the red hydrogen carbonate indicator.
- Place a similar-sized piece of healthy pondweed into three of the test tubes. The fourth tube will have no plant and act as a control1.
- Leave one tube with pondweed in bright light.
- Wrap one tube with pondweed completely in aluminum foil to create darkness.
- Place the third tube with pondweed in dim light.
- Leave the control tube (no plant) in bright light.
- Observe the color changes in all tubes after 30-60 minutes.
Expected Results and Explanation:
- Tube A (Pondweed in Bright Light): The indicator will turn purple. In bright light, the rate of photosynthesis is high and uses CO₂ faster than respiration produces it. There is a net removal of CO₂.
- Tube B (Pondweed in Darkness): The indicator will turn yellow. In darkness, photosynthesis cannot occur, but respiration continues. Respiration produces CO₂, leading to a net increase.
- Tube C (Pondweed in Dim Light): The indicator may stay red or change only slightly. In dim light, the rate of photosynthesis is roughly equal to the rate of respiration. The CO₂ produced by respiration is used up by photosynthesis, so there is no net change in CO₂. This point is called the compensation point.
- Tube D (Control, No Plant): The indicator should remain red. This shows that the color changes in the other tubes are due to the presence of the living plant and not some other factor.
This experiment beautifully illustrates the dynamic balance between photosynthesis and respiration and how light intensity is a key factor.
Common Mistakes and Important Questions
A: It measures carbon dioxide indirectly by detecting changes in pH. It does not measure oxygen at all. A common mistake is to think that a color change to purple means more oxygen has been produced. While it is true that photosynthesis produces oxygen, the indicator is blind to it. The purple color tells us that CO₂ has been removed.
A: The indicator is carefully formulated so that its "neutral" or starting color is red when it is in equilibrium with the normal level of carbon dioxide in the atmosphere (about 0.04%). This provides a clear baseline. A shift to yellow/orange (more acidic) shows an increase above normal, and a shift to purple (more alkaline) shows a decrease below normal.
A: Yes, you can! This is a simple and effective demonstration. Gently blow bubbles through a straw into a test tube containing the red indicator. Your exhaled breath contains about 4% carbon dioxide, which is much higher than atmospheric air. The increased CO₂ will dissolve in the indicator, forming carbonic acid and turning the solution yellow. Safety Note: Always be careful not to suck liquid back into the straw.
The hydrogen carbonate indicator is a brilliantly simple yet powerful scientific tool. By transforming an invisible gas exchange into a vivid color change, it provides a window into the fundamental processes that sustain life on Earth. From understanding how plants make food to measuring the energy production in every cell, this colorful solution makes complex biological concepts accessible and engaging for students. Its application in classroom experiments fosters critical thinking and a hands-on understanding of the delicate balance between photosynthesis and respiration.
Footnote
1 Control: In an experiment, a control is a test that is kept under standard conditions, without the factor being tested. It is used for comparison to ensure that any observed effects are actually due to the variable being studied and not to other factors.