Sodium hydrogencarbonate indicator solution contains sodium hydrogencarbonate. Some students found out on the internet that this indicator solution changes colour with pH. The students also discovered that this indicator is used to determine the concentration of carbon dioxide in a solution. The students decided to use this indicator to measure the rate of photosynthesis in algal balls. Algal balls are unicellular algae that are immobilised in alginate. The students assumed that the rate of carbon dioxide uptake was proportional to the rate of photosynthesis. Fig. 1.1 shows the apparatus that the students used. [Figure 1.1] A laboratory technician prepared a series of test-tubes containing hydrogencarbonate indicator solution from pH 7.6 to pH 9.2. The colour of the indicator solution in each test-tube is described in Table 1.1. [Table 1.1]