Temperature and vitamin C in orange juice by DCPIP titration
A student investigates the effect of temperature on the vitamin C content of orange juice. She measures 5.0 cm3 samples of fresh juice into test tubes, heats them in a water bath at 20, 40, 60 and 80 C for exactly 10 minutes, cools them rapidly, and titrates each with 0.0010 mol dm-3 DCPIP solution until the blue colour just disappears. The volumes of DCPIP used are 8.4, 7.9, 6.1 and 4.0 cm3 respectively. Each titration is done once. She concludes that vitamin C is destroyed by heating and that the relationship is exponential because the volume at 80 C is about half that at 20 C. Her teacher asks her to justify the conclusion and to identify the greatest weakness in her investigation. The student must decide whether her data support an exponential model or simply a decrease, and what evidence she would need to make the claim stronger.
1. Identify the independent, dependent and two controlled variables, and state one variable that was not controlled and could confound the result.
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The independent variable is the temperature of heating (20, 40, 60, 80 C). The dependent variable is the volume of DCPIP solution needed to reach the endpoint, which is proportional to the vitamin C content of the 5.0 cm3 sample. Controlled variables include the volume of juice (5.0 cm3), the concentration of DCPIP (0.0010 mol dm-3), and the heating time (10 minutes). The same batch of juice and the same indicator should also be used. One variable not controlled is exposure to light or oxygen during heating and cooling. Vitamin C is easily oxidised by air, so different amounts of dissolved oxygen or different cooling times could change the result independently of temperature. Another uncontrolled variable is the exact endpoint judgement, which is subjective.
2. Calculate the percentage decrease in DCPIP volume between 20 C and 80 C and comment on whether this alone proves an exponential relationship.
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Percentage decrease = (8.4 - 4.0) / 8.4 x 100 = 4.4 / 8.4 x 100 = 52.4%.
So the volume at 80 C is 47.6% of the volume at 20 C, which is roughly halving. However, two points alone cannot establish an exponential relationship. Any curve can be drawn through two points, and a straight-line decrease could also pass through them. To test exponential decay, the student needs more temperatures, ideally closely spaced, and should plot ln(volume) against temperature or time. If the plot is linear, that supports an exponential model. The intermediate data are also informative: 8.4 to 7.9 is only a 6% decrease from 20 to 40 C, whereas 7.9 to 6.1 is a 23% decrease from 40 to 60 C. That pattern does not immediately look like simple exponential decay and may suggest a threshold or a different mechanism.
3. Evaluate the reliability of the investigation and suggest improvements.
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Reliability is weak because each temperature was titrated only once. A single endpoint is subject to random error and subjective colour judgement. There is no measure of repeatability and no uncertainty calculated for the volumes. The 5.0 cm3 sample volume and the DCPIP concentration are controlled, but light, oxygen, cooling rate, and juice batch are not. Improvements include carrying out at least three titrations at each temperature and calculating a mean and standard deviation; using a colorimeter to detect the endpoint objectively; standardising the DCPIP against a known ascorbic acid solution; controlling light and oxygen exposure; using the same juice batch; and measuring the temperature inside the sample rather than only the water bath. These changes would improve precision and make the conclusion about temperature more defensible.