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I want to make an investigation where I measure how the color of light affects the rate of photosynthesis.

The idea is to put an Elodea plant in a test tube and shine with different color of light during time. I want to measure the dissolved oxygen in the beaker every minute for 5 minutes for different colors. As I have no access to a dissolved oxygen sensor I thought I could take out different samples of water every minute and separate them in bottles to conduct the Winkler method on each one. Hence, calculating the mg/L of dissolved oxygen and make a line of best fit for each color.

Is this a functioning idea? Could I get feedback?

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Good thought experiment. Winkler's method is applicable to dissolved oxygen. A concern, if I were doing this experiment is the formation of oxygen bubbles. Wilker's titration would not be able to detect the oxygen which escaped as bubbles.

What you can try is stuffing a large number of aquatic plants in a one or two liter beaker and invert a funnel and a testtube over it as shown below : This is a standard experiment over centuries. Now you can illuminate the beaker with various colors and measure the volume of oxygen. Now oxygen has some solubility in water, when you see bubbling, water is already saturated with oxygen.

In order to make your experiment reliable try Winkler's method and this gas-volumetric method.

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