Determination Of Vitamin C Biology Essay

Vitamin C ( Ascorbic acid ) is a necessary food in the human diet. Fruit juices are among the most common consumer points taken to run into our day-to-day demands of ascorbic acid.

Vitamin C can be determined in nutrient by usage of an oxidation-reduction reaction. The redox reaction is preferred to an acid-base titration because a figure of other species in juice can move as acids, but comparatively few interfere with the oxidization of ascorbic acid by I.

This method determines the vitamin C concentration in a solution by a redox titration with K iodate in the presence of K iodide. The extra I is back titrated with thiosulfate ( S2O32- ) . Starch solution is used as index. Iodide ions cut down iodate ions bring forthing I in an sum equivalent to the iodate. The chief reactions involved are shown by the undermentioned equations.

Coevals of I:

IO3- + 5I- + 6H+ > 3I2 + 3H2O

Titration of I with thiosulfate:

I2 + 2S2O32- > 2I- + S4O62-

The reaction of I with ascorbic acid is shown in the undermentioned equation:

C6H8O6 + I2 > C6H6O6 + 2I- + 2H+

Ascorbic acid is oxidized to dehydroascorbic acid


SSC 2831, SEMESTER 1, 2008/2009


Apparatus – burette ( 50 milliliter ) , volumetric flasks ( 250 milliliter, 100mL ) , beaker ( 400 milliliter ) , pipette ( 10 milliliter, 25 milliliter ) , Erlenmeyer flasks ( 250mL )

Reagents – oxalic acid, C2H2O4.2H2O, K iodate KIO3, Na carbonate, Na2CO3, Na thiosulfate pentahydrate, Na2S2O3.5H2O, sulphuric acid, H2SO4, K iodide, KI, amylum index solution ( 2 % w/v in distilled H2O ) , apple juice

Preparation of Na2S2O3 and KIO3 solution

Na2S2O3.5H2O and Na2CO3 were weighed 1.8613 g and 8 milligram severally to fix precisely 250 milliliter of 0.03M Na thiosulfate solution. Dilution of 0.25 g KIO3 in 250 milliliters volumetric flask was done.

Standardization of Sodium Thiosulfate

2 g of KI and 10 milliliter of 0.6M H2SO4 were added into 250 milliliters Erlenmeyer flask after 25 milliliter of KIO3 was pipette. This solution was titrated with Na thiosulfate solution until the initial brown colour of the solution turns to blanch xanthous. 2 milliliter of amylum index was added and the titration was completed at the disappearing of the bluish colour. Titration was repeated 2 times.

Preparation of sample

0.1 gram of oxalic acid was added into 50 milliliter of apple juice after it was pipette so the sample juice was diluted in a 100 milliliter volumetric flask.

Titration of sample

2 g of KI and 10 milliliter of 0.6M H2SO4 were added into Erlenmeyer flask after 25 milliliter of KIO3 and 25 milliliter of diluted sample were pipette. The solution was titrated with standard Na2S2O3. 2 mL amylum index was added merely before the terminal point. The titration was repeated 2 times.


SSC 2831, SEMESTER 1, 2008/2009


The molar concentration of standardize solution and vitamin degree Celsiuss studied are shown in drumhead informations study sheet and the all computations are shown in appendixes.

Volumetric analyses based on titrations with cut downing or oxidising agents are really utile for many findings one of the illustration is iodometry titration. It was performed utilizing ocular index that is starch index to find the terminal point. Starch forms a non really reversible composite with I2 that is a really dark-blue colour. The colour reaction is sensitive to really little sums of I.

Iodine is a reasonably strong oxidising agent and a weak reduction agent. When an surplus of iodide is added to a solution of an oxidising agents, I2 is produced in an sum equivalent to the oxidising agent nowadays. This I2 can therefore, be titrated with a cut downing agent and the consequence will be the same as if the oxidizing agent were titrated straight. The titrating agent used is sodium thiosulfate.

The oxidising agent non straight titrate with the thiosulfate because strong oxidising agents oxidise thiosulfate to a oxidization states higher than that of tetrathionate.

The terminal point is detected with amylum. The amylum is non added in the beginning of the titration when the I concentration is high. Alternatively, it is added merely before the terminal point when the dilute I colour becomes pale yellow. The ground for utilizing acid is that reactions between many oxidising agents and iodide are promoted by high sourness.

Sodium thiosulfate solution is standardized iodometrically against a pure oxidising agent that is KIO3. from the volume of titrant used to titrate the primary criterion, the molar concentration of the titrant can be calculated.

The computations that involve in these informations are base on volumetric analysis which consists of titration and standard solution that means need stoichiometric computations. The information of standardisation of thiosulfate in term of volumes sodium thiosulfate have low truth because the distinction between readings rather obvious comparison to volumes of Na thiosulfate in finding of vitamin C.

The information of the experiment is compared to the true information. The true informations province that the content of vitamin C milligram per 100 milliliter is 15 milligram but in the experiment is 0.8096 milligram / 100 milliliter. This is because the vitamin C is decompose by heat. The diluted sample is prepared excessively earlier.


SSC 2831, SEMESTER 1, 2008/2009

There are several mistakes that might be occurred in this experiment. One of that is systematic mistake which means mistake in burette reading. The beginnings of systematic mistake consists of three types ; instrument mistake, method mistake, and personal mistake. Personal mistake occurred to the operator of equipment particularly in sensitiveness in colour alterations. For illustration in this experiment the colour alterations involve is chocolate-brown to blanch xanthous. Whereas the method mistake occurred when the reaction does non finish. Even the instrument besides can do the mistake particularly when the terminal of burette and pipette are blocked by something.

Instrument mistakes can minimise by good care of equipment. To detect the colour alterations when do titration in analyte put the field white paper at the underside of conelike flask. When taking burette reading ever read at the semilunar cartilage and the reading of pipette as good.

The mistake in informations can be overcome by using statistical trials on informations for illustration find the standard division and mean. To use this trial carries out replicate measurings. In this experiment we do triplicate.


The molar concentration of KIO3 is 4.6728 ten 10-3 M. The molar concentration of standardize thiosulfate is 0.0278 M and the content of vitamin C in mg/100 milliliter is 0.8096.