Fig. 9.1 shows the apparatus used to find the vitamin C content of a blackcurrant drink. The drink was added from a syringe, drop by drop, into 1.0 cm3 of D...

Labelled answer diagram:

This is a titration-style vitamin C investigation with DCPIP. Fewer molecules of vitamin C mean more drink is needed to decolourise a fixed amount of DCPIP, so a larger volume needed means less vitamin C.

(a) The drink is added drop by drop from a syringe (a burette is also acceptable) [1]; the 1.0 cm³ of DCPIP would be measured with a measuring cylinder or a graduated pipette [1].

(b) The volume is read from the syringe so that the exact amount of drink needed to decolourise the DCPIP can be measured, which quantifies how much vitamin C is present [1].

(c) Extra volume needed at 30 °C compared with 4 °C \( = 2.40 - 0.80 = 1.60 \text{ cm}^3 \) [1].

(d) Number of times more \( = \dfrac{2.40}{0.80} = 3 \) times [1].

(e) Using concentration \( = \dfrac{0.50}{\text{volume needed}}\times1.0 \): for 4 °C \( \dfrac{0.50}{0.80}\times1.0 = 0.625 = 0.63 \text{ mg cm}^{-3} \). Working [1], answer [1].

(f) Same method: 20 °C \( \dfrac{0.50}{1.20}\times1.0 = 0.42 \text{ mg cm}^{-3} \) [1]; 30 °C \( \dfrac{0.50}{2.40}\times1.0 = 0.21 \text{ mg cm}^{-3} \) [1].

(g) As storage temperature increases, the vitamin C content decreases [1]; more drink is needed to decolourise the DCPIP at higher storage temperatures [1].

(h) The 30 °C sample needed the most drink because warmth speeds up the oxidation / breakdown of vitamin C [1], so fewer vitamin C molecules remain in the drink [1]; a larger volume of the weaker drink is then required to decolourise the same DCPIP [1].

(i) Yes, the results support storing it in a fridge [1]: the sample kept at 4 °C (fridge temperature) had the most vitamin C and needed the least drink, showing the lowest temperature preserves vitamin C best [1].

(j) Any two controlled variables: storage time (5 days), volume / concentration of DCPIP, the size of each drop, or the same drink [2].

(k) Any two ways to judge the end point more reliably: view the tube against a white background; use the same person to judge the colour; add drop by drop with swirling near the end point; repeat and take a mean [2].