Question

3-Why might the tube with the plant placed in the dark exhibit an increase in pH?

Answer 1

For example let us take coral species

It has been seen that the pH decrease (the water becomes more alkaline) and enters the photosynthetically active coral tissue. But when in darkness, the pH increases (the water becomes less alkaline or more acidic).

The Photosynthetically active tissue uptake CO2 and releases oxygen (O2), while in the dark the opposite occurs. The tissue, in the dark, is undergoing respiration so it uptakes O2 and releases CO2.

CO2 dissolve in water to produce carbonic acid that, after that dissociates into biocarbonate and hydrogen ions.

pH is a measure of hydrogen ion concentration in a solution. Therefore, when CO2 reacts in the water there seems be a related increase or decrease in pH due to the increase or decrease in hydrogen ion concentration. By measuring pH in water near photosynthesizing organisms, we can indirectly assess whether photosynthesis is taking place.