Lab 5m63a

Lab #6- The Determination of Ascorbic Acid in Vitamin C Tablets Lucus Turner February 18, 2015

OBJECTIVE: The purpose of this experiment is to determine the amount of ascorbic acid in Vitamin C tablets by titrating samples with 0.10 N iodine solution (the USP method, and seeing if the final result yields within USP guidelines).

METHOD: 5 Vitamin C tablets (C6H8O6) Starch test solution Retort stand

Carbon dioxide-free water

2 N Sulfuric acid (H2SO4)

0.1 N Iodine solution (I2)

3 250 mL Erlenmeyer flasks Burette clamp

50 mL Burette

Beakers

Mortar

Pestle

H2SO4- can cause severe damage to skin and clothing, wear eye protection. If in , immediately wash the area with large amounts of water.

I2- can stain skin and clothing, if in , wash with plenty of water immediately.

For a full description of this experiment, refer to Chemistry 2131 Laboratory Manual, experiment #6, The Determination of Ascorbic Acid in Vitamin C Tablets, p. 43.

The average weight of a single tablet was calculated using five tablets instead of ten.

OBSERVATIONS: Vitamin C tablets used were Kirkland 500 mg and five tablets were weighed out to be 0.6096 g, 0.6029 g, 0.5981 g, 0.6057 g, and 0.6025 g, with the average of these five tablets coming to 0.6038g.

These tablets were crushed using a mortar and pestle and then weighed to approximately 400 mg each (samples respectively being 409.2 mg, 402.2 mg, and 405.3 mg). Each of these samples were dissolved in a mixture of 100 mL carbon dioxide-free water and 25 mL 2 N sulfuric acid in separate 250 mL Erlenmeyer flasks.

The samples were than titrated at once with the 0.1 N iodine solution until the yellow remained for a moment before disappearing once titration was stopped and 3 mL starch test solution was added. The titration was continued until the endpoint (now a dirty purple because of the addition of starch test solution) was reached and held for 90 seconds.

Table 6.1- Determination of Ascorbic Acid in Vitamin C Tablets

Sample 1 Sample 2 Sample 3

Initial Volume (mL) 0.00 mL 0.35 mL 0.05 mL

Final Volume (mL) 39.10 mL 38.20 mL 38.80 mL

Volume Dispensed (mL) 39.10 mL 37.85 mL 38.75 mL

CALCULATIONS: Amount of Ascorbic Acid in Each Sample For each 1 mL of 0.1 N iodine solution titrated is equivalent to 8.806 mg of ascorbic acid. This result is calculated by multiplying the amount of iodine solution titrated into the sample by 8.806.

1) 39.10 mL x 8.806 mg = 344.3146 mg 2) 333.3071 mg 3) 341.2325 mg

Determination of Ascorbic Acid per Tablet The amount of ascorbic acid per tablet was calculated by a ratio of grams of ascorbic acid in sample over grams of sample equaling amount of ascorbic acid per table (the unknown variable) over the average weight of the five tablets.

1) 0.3443146 g = x 0.4092 g 0.6038g x= 0.3443146 g x 0.6038 g 0.4092 g x= 0.5080576 g = 508.0576 mg 2) x= 500.3750 mg 3) x= 508.3548 mg

These amounts were than calculated to average 505.5958 mg, which can be round to be 506 mg.

%Ascorbic Acid= 506 x 100 500 = 101.2%

DISCUSSION: The USP guidelines state that Vitamin C tablets must contain no less than 90% and no more than 110% the labelled amount (Chemistry 2131 Laboratory Manual), which in this experiment was 500 mg. In the lab it was calculated the average amount of ascorbic acid in Kirkland Vitamin C tablets was 506 mg, falling at 101% in the tolerance which meets USP guidelines.

A possible source of error in this experiment was it was easy to over titrate past the endpoint, possibly causing the calculations to be a bit inaccurate.

CONCLUSION: The purpose of this lab was to determine the amount of ascorbic acid in Vitamin C tablets using the USP method, and to see if the final result fell within USP guidelines. In this experiment, the average amount of ascorbic acid calculated to be in Kirkland 500 mg Vitamin C tablets was 506 mg. This gives a percentage of 101% which falls in USP guidelines as it does not exceed 110% ascorbic acid.

POST LAB QUESTIONS: 1) (12 M) (1 mL) = (C2) (100 mL) C2= (12 M) (1 mL) (100 mL) C2= 0.12 M (0.12 M) (10 mL)= (C2) (100 mL) C2= (0.12 M) (10 mL) (100 mL) C2= 0.012 M

(0.012 M) (1 mL)= (C2) (100 mL) C2= (0.012 M) (1 mL) (100 mL) C2= 0.00012 M

≈ 0.0001 M To prepare a 100 mL solution of 0.0001 M sulfuric acid from 12 M sulfuric acid requires a serial dilution to obtain, in this case a 1/100 dilution followed by a 1/10 dilution, and then another 1/100 dilution. First, 1 mL of the stock 12 M is pipetted into a 100 mL volumetric flask (a quarter to half full so water isn’t being directly added to acid) and then topped to the line with deionized water and mixed. The new solution, with a concentration of 0.12 M, has 10 mL pipetted into a 100 mL flask and then topped with deionized water. This solution, 0.012 M, has 1 mL pipetted into a 100 mL volumetric flask and topped to the line. This final solution is the 0.0001 M sulfuric acid.

REFERENCES: White, J. (n.d.). Experiment #6- The Determination of Ascorbic Acid in Vitamin C Tablets, Chemistry 2131 Laboratory Manual, p. 43.