How did Blackman measure photosynthesis?

When Elodea photosynthesises it consumes dissolved carbon dioxide from the water around it. Frost Blackman used chemical titration to measure the change in concentration of dissolved carbon dioxide in the water round the plant. He measured this change every hour for about eight hours, so that each of his experiments took a whole day.

Blackman estimated the carbon dioxide concentration of the water around Elodea by mixing it with a known volume of barium hydroxide solution. Carbon dioxide dissolved in the water reacts with barium hydroxide to form barium carbonate. The solution became alkaline because of the unreacted barium hydroxide. The solution was neutralised with hydrochloric acid and phenolpthalein was used to indicate when the alkali had been neutralised.

Balckman was able to calculate the concentration of carbon dioxide in the water from the amount of hydrochloric acid needed to neutralise the barioum hydroxide solution.