Determining vitamin C concentration by redox
titration using iodine solution
Determining vitamin C concentration by redox titration using iodine solution
Hypothesis
As the temperature of the sample increases, the content of vitamin C will decrease.
Objective
Whether temperature effects vitamin C content in a solution.
Introduction
Vitamin C or ascorbic acid is an essential for human life and is required for a range of
physiological functions in human body. Besides, it is an essential water-soluble antioxidant and free
radical, it has been suggested to be an effective antiviral agent. (“vitamin C is an essential, 2013).
Though daily requirements of vitamin are changeable according to age, sex, and conditions. It is
around 75 to 90 mg per day for healthy adults and no more than 200 mg per day is recommended.
(vitamin C- health professional fact sheet, 2018). It can be found either in dietary sources like fresh
fruits and vegetables naturally like: oranges, limes, lemons, blackcurrants, parsley and capsicums or
in medical forms such as normal tablets, effervescent tablets and liquid vials. It is the most widely
taken supplement such as: Solgar vitamin C. (vitamins and minerals, 2017)
Additionally, ascorbic acid has been widely used in many fields; one of which of which is the food
industry as an antioxidant. Hence, there is a need to determine the temperature at which vitamin C
denatures at as well as any other factors that affect it.
Pasteurisation is a process named by a French scientist called Louis Pasteur and it is defined as
the “partial sterilisation of a product, such as milk or wine, to make it safe for consumption and
improve its keeping quality”. This process is widely used within the food and drink industry whereby
Packaged foods are treated with mild heat, usually less than 100 oC, in order to eliminate pathogens
and extend shelf life. (Pasteurisation, 2003)
The aim of this experiment is to determine vitamin C concentration in a solution by a redox
titration using iodine. As the iodine is added during the titration, the ascorbic acid is oxidised to
dehydroascorbic acid, while the iodine is reduced to iodide ions.
Ascorbic acid + I2
2 I- + dehydroascorbic acid
Once all the ascorbic acid has been oxidised, the excess iodine is free to react with the starch
indicator, forming the blue-black starch-iodine complex. This is the “end-point” of the titration.
Method
The methods were carried out as described in the laboratory schedule. (Coventry University, 2019).
In industry, one of the most commonly used temperatures in order to pasteurise commercial
produce of fortified fruit and vegetable juice drinks is at 72oC for 6 seconds. Therefore, choosing to
remain with the same temperature as the one used in industry but for longer than 6 seconds, for 10
and 60 seconds in order to be able to directly compare the effect of time on the vitamin C content,
and whether it results in significant changes. Furthermore, increasing the temperature to 75oC which
is higher than that used in industry alongside an increase in heating time of 30 seconds, to be able to
see the if both combined do have a different affect than individually.
Temperature (oC)
Time (seconds)
72
10
72
60
Control
Run 1
Run 2
Run 3
Final volume
iodine
Initial
volume
iodine
Titre
Final volume
iodine
Initial
volume
iodine
Titre
Final volume
iodine
Initial
volume
iodine
Titre
Mean titre
cm3
Vit C per
100ml
Vit C per ml
Results
C1 x V1 = C2 x V2
Temperature
(oC)- 72 (10secs)
75
30
Temperature
(oC)- 72 (60secs)
Temperature
(oC)- 75 (30secs)
C1= iodine (0.005mol L-1)
V1=titre (16.4)
C2= ascorbic acid in the drink
V2= 25ml drink sample
Step 1:
0.005mol L-1 x (16.4x10-3L) = C2x 25ml
0.005mol L-1 x 0.0164L = C2 x 25ml
0.000082mol = C2 x 25ml
Step 2:
Molecular weight of ascorbic acid (C6H8O6) = 176.1g mol-1
176.1g mol-1 x 0.000082mol = 0.014g
0.014g = C2 x 25ml
0.014g / 25ml = C2
C2= 0.00056g of ascorbic acid per ml of drink sample
Step 3:
0.00056g x 100ml = 0.056g per 100ml
0.056g x 1000ml = 56mg per 100ml
Vitamin C concentration (g/100ml)
Vitamin C concentration (g/100ml)
C2 = 56mg per 100ml
Temperature to which vitamin C solution was
heated (oC)
Figure 2- Temperature effects on vitamin C
Time to which vitamin C solution was heated
(seconds)
Figure 1- Time effects on vitamin C
Figures 1 and 2 show a negative correlation. In other words, the variables are inversely proportional
to one another.
Discussion
There is a health and safety issue that is needed to be considered in this experiment, which only
relates to handling iodine as the other solutions are harmless. (Iodine LabChem, 2012) Iodine is
harmful if it is in contact with skin and inhaled; even when it’s in a diluted solution it stains the skin.
Therefore, control measures are required to reduce risks. Wearing eye protection for all but the
most dilute solution, second is to use forceps or protective gloves especially for larger amounts
when handling iodine and finally to use a fume cupboard in order to avoid inhaling iodine vapour.
According to figure 1, it demonstrates that as temperature at which the Vitamin C solution is
heated at increases, the concentration of vitamin C decreases. This suggests that excess heat
degrades/ denatures vitamin C. Not only this, but further supported by figure 2 which indicates that
the heating time has a significant effect on the vitamin C content in the solution, as the heating time
increases, the percentage loss of vitamin C increases too, and this is due to vitamin C being easily
destroyed by excessive heat, as well as exposure to air and that time at which the vitamin C is
heated at is equally as important as the temperature.
Relating this to food processing and nutritional value is that heat helps to sterilise food by
killing harmful bacteria and other microorganisms, and it increases the availability of nutrients.
However, using incredible heat about 75oC for longer than about 15 seconds will reduce the
nutritional value; so, as result there will be a loss of major nutrients like vitamin C.
On the one hand, it could be argued that increasing the temperature as well as the time the
content is heated at decreases vitamin C content within the nutritional value. Having said that, on
the other hand this experiment was a small scale one using only 100ml which contains 56g of acid at
75oC for 10 seconds, which means that temperature and time does not have an affect on the
amount of acid as 60mg at 250ml is the wanted amount. Thus, one can suggest that this may not be
as reliable, an so cannot be applicable to the real life event.
Conclusion
To sum up, the hypothesis is accepted as the temperature does affect vitamin C when it
increased above about 75oC, but what is more interesting about this is the time that comorbid with
this because the temperature could be increased but held on that temperature for much more less
time than the usual otherwise the vitamin C content will hinder vastly. The method displayed various
and continuous results that could be taken into account by the Drinks Company when pasteurising
the fruit and vegetable juices as it is important not only for destroying bacteria and other harmful
microorganisms but for nutritional values of the product.
Referencing
1.
125BMS Experimental design Lab Book (2019). ‘Vitamin C concentration by redox
titration using iodine solution, 2019’ Coventry University
2.
AW. Munyaka (2010). Thermal stability of L-ascorbic acid and ascorbic acid oxidase in
broccoli (Brasica oleracea var. italica). PubMed. Available from:
https://www.ncbi.nlm.nih.gov/pubmed/20546391 Updated: May, 2010.
3.
L. Petruzzi et al. (2017). Wiley Online Library. Thermal treatments for fruit and
vegetable juices and beverages: A literature overview. Available from:
https://onlinelibrary.wiley.com/doi/full/10.1111/1541-4337.12270 Updated: 30th May,
2017.
4.
Y. Kim et al (2013). ‘vitamin C is an essential factor on the anti-viral immune responses
through the production of interferon-a/b at the initial stage of influenza A virus (H3N2)
infection’. Immune network. PMC. Available from:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3659258/ updated: 13th Apr, 2013
5.
(n.n) (n.d). Iodine- 125 safety Precautions. Available from:
https://ehs.uky.edu/radiation/isotopes/Iodine.html
6.
(n.n) (2003). Pasteurisation. Science Direct. Available from:
https://www.sciencedirect.com/topics/food-science/pasteurization Updated: 2003
7.
(n.n) (2018). Vitamin C Fact sheet. National institutes of health. Available from:
https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/ updated: 18th Sept, 2018