Analysis of Ascorbic Acid by Redox Titration, Lab Report Example

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Lab Report

Introduction

Ascorbic acid (C6H8O6), more commonly known as vitamin C, is a vital vitamin in mammals. In fact, the recommended daily intake of Vitamin C is 75-90 mg per day.  Vitamin C is important because it functions as a cofactor in the synthesis of collagen, protein metabolism, iron absorption and the healing of wounds. (Lab Manual)  Although vitamin C is an important substance, it is not produced by the body and must be ingested through diet and nutrition. Vitamin C is found in many foods; however, it is destroyed during cooking and therefore, fruits are the most reliable source of Vitamin C.  In this lab, Vitamin C was determined in juice though oxidation-reduction reactions.  The oxidation-reduction reaction using iodine was conducted. The solubility of iodine is triiodide. The triiodide oxidizes vitamin C to dehydroascorbic acid. The purpose of this experiment was to take unknown samples and perform the oxidation-reduction reactions in order to analyze the amount of ascorbic acid contained in the sample and compare it to the amount of ascorbic acid in juice.

Formulas and Equations

C6H8O6 + I3 – + H2O ® C6H6O6 + 3I- + 2H+ (2)

Formulas and Equations

vitamin C dehydroascorbic acid

  1. Moles of Vitamin C oxidized= Equvilane point/ml
  2. Concentration of Vit C= Molecular Weight VitC *Molarity Iodine
  3. Grams Vit C used: Concentration Vit C x Volume Unknown
  4. Ka from pH of the half neutralization point (equation 4)
  5. Moles Vit C Oxidized= Molarity Iodine x Volume Iodine used

 Procedure

  • Unknown sample obtained.
  • 150 ml iodine solution poured into 250 ml Erlenmeyer flask.
  • One stir bar, 50 ml buret, 10 ml pipet obtained.
  • 10ml of unknown transferred to 250ml beaker with 25ml distilled water.
  • 10 drops of 2% starch added to solution. Starch used visual indicator used to follow the reaction.
  • pH meter used to test pH of substance.
  • Unknown sample titrated and total volume added versus mV readings recorded.
  • Carefully watched for mV readings to change rapidly for the approaching of the endpoint.
  • White sheet of paper placed underneath the sample to see endpoint.
  • After the unknown samples analyzed, 25 ml of fruit juice titrated.
  • Ascorbic acid concentration in the juice sample calculated.
  • Waste flushed down the drain, due to “Green Chemistry” practice.

Data and Observations

Table 1. Unknown Titration Data

Unknown Titration #1
Volume of Vit C unk solution 10.00ml
ml of iodine used (visual/mV) 17.43/270  (.065)
Moles of Vit C oxid. .174 mol
Grams of Vit C oxid .0176g
Conc of Vit C unknown (g/L) 1.76g/L
Average Conc Vit C (visual/mV) 27.3ml/305.1mV  (.0894)

Table 2.  Orange Juice Titration Data

Fruit Juice Titration Orange Juice
Volume of Vit C unk solution 25.0ml
ml of iodine used (visual/mV) 17.38ml
Moles of Vit C oxid.  
Grams of Vit C oxid  
Conc of Vit C unknown (g/L)  
Average Conc Vit C (visual/mV)  

Table 3. Data from buret reading. Initial buret reading was 12.43 ml and initial mV was 270mV

Buret Reading (ml) Volt Meter Reading Buret Reading Volt Meter Reading
18.3 264.2 28.5 245.0
18.5 265.8 28.72 245.0
19.0 265.3 28.98 246.6
19.5 266.4 29.22 247.9
20.0 265.5 29.38 282.9* Equivalence point
20.5 265.0 29.77 286.0
21.0 263.6 30.00 295.0
21.5 267.3 30.5 360.0
22.0 261.4 30.7 362.4
22.5 259.7 30.9 369.2
23.0 256.8 31.1 373.9
23.2 254.6 31.3 379.3
23.4 253.8 31.4 388.0
23.6 252.9 31.6 391.4
23.8 251.8 31.8 391.7
24.0 250.9 32.0 393.1
24.2 250.2 32.2 394.0
24.6 249.4 32.4 394.8
24.8 248.2 32.6 395.3
25.0 247.6 32.8 396.3
25.5 247.5 33.0 397.0
26.0 247.2 33.2 397.3
26.5 246.6 33.4 398.0
27.0 246.0 33.6 398.8
27.5 245.6 33.8 398.9
28.0 245.3 34.0 399.5
Avereage      

Table 4. Average Vitamin C concentration in Juices

Type of Juice Average Concentration of Vitamin C in Juice
Pink Grape Juice 0.324 g/L
Orange Juice 0.349 g/L
Apple Juice 0.178 g/L
Pineapple Juice 0.696 g/L

Calculations

  1. Concentration of Vitamin C= Molecular Weigh Vit C * Molarity Iodine
    1. 1g/mol x 0.010 mol/L = 1.76 g/L
  2. Grams of Vit C oxidized= Concentration of Vitc C x Volume of unknown
    1. 76g/L x 10.00ml /1000ml=.0176g
  3. Moles of Vitamin C oxidized =.0100 mol/ml x 17.43ml= .174mol

Graphs

 Figure 1. ph (mV) and ml of base used in the experiment for the unknown sample.

ph (mV) and ml of base used in the experiment for the unknown sample

 Discussion

This experiment analyzed an unknown substance using an oxidation reduction reaction in order to determine the concentration of ascorbic acid.  In addition, the concentration of ascorbic in orange juice was determined.  The experiment did go well. As seen in Figure 1, as the milliliters of base increase, so did the millivolts. This indicates that the ascorbic acid was oxidized.   Our results did indicate that there was enough Vitamin C in our sample that is necessary for the recommended daily intake of 75 mg.  Our results indicated that there was .0176 grams  of Vitamin C (17.6mg). This is not enough for the daily intake.  This illustrates how much food and drink that one would actually need in order to get the daily dosage that is recommended.

In this experiment, there were, however, some areas where there could have been human error. One, the volt meter might not have been calibrated properly and the results may have not been accurate. The meters were given to us already calibrated, therefore, we had to trust that the meters were properly calibrated.  In addition, there was one person recording the readings while during the titrations.  It is possible that some of the readings were not properly recorded by the lab partner.

This experiment was a “Green Chemistry” experiment. The point of Green Chemistry is that there are no harmful or carcinogenic toxic chemicals being released into the environment.  The previous experiment used for this oxidation reduction lab used chromium. This chemical is toxic and is known as a carcinogen. Releasing this product into the environment can therefore have detrimental effects on living organisms. If chromium is used, it needs special waste disposal so that it is not released into the environment.  This can lead to costly removal services for the school. The current experiment has left no need for a costly waste disposal and is not carcinogenic.  For instance, the oxidized ascorbic acid made during this experiment is identical to the oxidized ascorbic acid that is excreted from humans on a daily basis and is not a toxic chemical. In addition, the iodine used in this laboratory served as a preventative measure for infections, thereby resulting in no waste problem as well.  Furthermore, the final solution that was prepared during this experiment is very low on the acidity level. In fact, it is less than the acidity of the soda-pop, Coca-Cola.

Overall, this experiment illustrated how an oxidation reduction reaction occurred and it showed how chemistry can be “Green” even though you are working with chemicals.

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