Vitamin C (Ascorbic acid) is a necessary nutrient in the human diet. Fruit juices are among the most common consumer items taken to meet our daily requirements of ascorbic acid.
Vitamin C can be determined in food by use of an oxidation-reduction reaction. The redox reaction is preferable to an acid-base titration because a number of other species in juice can act as acids, but relatively few interfere with the oxidation of ascorbic acid by iodine.
This method determines the vitamin C concentration in a solution by a redox titration with potassium iodate in the presence of potassium iodide. The excess iodine is back titrated with thiosulfate (S2O32-). Starch solution is used as indicator. Iodide ions reduce iodate ions producing iodine in an amount equivalent to the iodate. The principal reactions involved are shown by the following equations.
Generation of iodine:
IO3- + 5I- + 6H+ → 3I2 + 3H2O
Titration of iodine with thiosulfate:
I2 + 2S2O32- → 2I- + S4O62-
The reaction of iodine with ascorbic acid is shown in the following equation:
C6H8O6 + I2 → C6H6O6 + 2I- + 2H+
Ascorbic acid is oxidized to dehydroascorbic acid
Apparatus – burette (50 mL), volumetric flasks (250 mL, 100mL), beaker (400 mL), pipette ( 10 mL, 25 mL), Erlenmeyer flasks (250mL)
Reagents – oxalic acid, C2H2O4.2H2O, potassium iodate KIO3, sodium carbonate, Na2CO3, sodium thiosulfate pentahydrate, Na2S2O3.5H2O, sulfuric acid, H2SO4, potassium iodide, KI, starch indicator solution (2% w/v in distilled water), apple juice
Preparation of Na2S2O3 and KIO3 solution
Na2S2O3.5H2O and Na2CO3 were weighed 1.8613 g and 8 mg respectively to prepare exactly 250 mL of 0.03M sodium thiosulfate solution. Dilution of 0.25 g KIO3 in 250 mL volumetric flask was done.
Standardization of Sodium Thiosulfate
2 g of KI and 10 mL of 0.6M H2SO4 were added into 250 mL Erlenmeyer flask after 25 mL of KIO3 was pipette. This solution was titrated with sodium thiosulfate solution until the initial brown color of the solution turns to pale yellow. 2 mL of starch indicator was added and the titration was completed at the disappearance of the blue color. Titration was repeated 2 times.
Preparation of sample
0.1 gm of oxalic acid was added into 50 mL of apple juice after it was pipette then the sample juice was diluted in a 100 mL volumetric flask.
Titration of sample
2 g of KI and 10 mL of 0.6M H2SO4 were added into Erlenmeyer flask after 25 mL of KIO3 and 25 mL of diluted sample were pipette. The solution was titrated with standard Na2S2O3. 2 mL starch indicator was added just before the end point. The titration was repeated 2 times.
RESULT AND DISCUSSION
The molar concentration of standardize solution and vitamin c studied are shown in summary data report sheet and the all calculations are shown in appendixes.
Volumetric analyses based on titrations with reducing or oxidizing agents are very useful for many determinations one of the example is iodometry titration. It was performed using visual indicator that is starch indicator to determine the end point. Starch forms a not very reversible complex with I2 that is a very dark-blue color. The color reaction is sensitive to very small amounts of iodine.
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Iodine is a moderately strong oxidizing agent and a weak reducing agent. When an excess of iodide is added to a solution of an oxidizing agents, I2 is produced in an amount equivalent to the oxidizing agent present. This I2 can therefore, be titrated with a reducing agent and the result will be the same as if the oxidizing agent were titrated directly. The titrating agent used is sodium thiosulfate.
The oxidizing agent not directly titrate with the thiosulfate because strong oxidizing agents oxidize thiosulfate to a oxidation states higher than that of tetrathionate.
The end point is detected with starch. The starch is not added in the beginning of the titration when the iodine concentration is high. Instead, it is added just before the end point when the dilute iodine color becomes pale yellow. The reason for using acid is that reactions between many oxidizing agents and iodide are promoted by high acidity.
Sodium thiosulfate solution is standardized iodometrically against a pure oxidizing agent that is KIO3. from the volume of titrant used to titrate the primary standard, the molar concentration of the titrant can be calculated.
The calculations that involve in these data are base on volumetric analysis which consists of titration and standard solution that means need stoichiometric calculations. The data of standardization of thiosulfate in term of volumes sodium thiosulfate have low accuracy because the differentiation between readings quite obvious compare to volumes of sodium thiosulfate in determination of vitamin C.
The data of the experiment is compared to the true data. The true data state that the content of vitamin C mg per 100 mL is 15 mg but in the experiment is 0.8096 mg / 100 mL. This is because the vitamin C is decompose by heat. The diluted sample is prepared too earlier.
There are several errors that might be occurred in this experiment. One of that is systematic error which means error in burette reading. The sources of systematic error consists of three types ; instrument error, method error, and personal error. Personal error occurred to the operator of equipment especially in sensitivity in color changes. For example in this experiment the color changes involve is brownish to pale yellow. Whereas the method error occurred when the reaction does not complete. Even the instrument also can cause the error especially when the end of burette and pipette are blocked by something.
Instrument errors can minimize by good maintenance of equipment. To observe the color changes when do titration in analyte put the plain white paper at the bottom of conical flask. When taking burette reading always read at the meniscus and the reading of pipette as well.
The error in data can be overcome by applying statistical tests on data for example find the standard division and mean. To apply this test carries out replicate measurements. In this experiment we do triplicate.
The molarity of KIO3 is 4.6728 x 10-3 M. The molarity of standardize thiosulfate is 0.0278 M and the content of vitamin C in mg/100 mL is 0.8096.
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