Determination of percent ascorbic acid in a commercially prepared vitamin C tablet:

Transcription

1 Standardization of the sodium thiosulfate solution: Begin this procedure by rinsing the calibrated buret with deionized water and small portions of the Na 2S 2O 3 solution. Then rinse the 25 ml pipet with deionized water and the KIO 3 solution. Fill the buret with the Na 2S 2O 3 solution and record. Next, pipet 25 ml of the KIO 3 solution into a 250 ml Erlenmeyer flask. Using a graduated cylinder, add 25 ml of deionized water to the flask. Then, using the top load scale, add approximately 2g of KI. Finally, add 1 ml of H 2 SO 4. Titrate immediately. The solution should change from a brown-red color to a pale yellow. At this point, add starch indicator. This will turn your solution a blue-black color. Complete titration by titrating drop wise until solution turns clear. Record your final titrant. Perform two more trials. Determination of percent ascorbic acid in a commercially prepared vitamin C tablet: Start by weighing a vitamin c tablet. Record the mass. Then grind the tablet using a mortar and pestle. Weigh the powder on a weigh paper and record the mass. Next, transfer the powder to a dry 250 ml Erlenmeyer flask. Do not get powder wet until you are ready to titrate. Rinse calibrated buret with deionized water and small portions of the Na 2S 2O 3 solution. Fill buret with the Na 2S 2O 3 solution and record. Next, rinse the 25 ml pipet with deionized water followed by a few ml of the KIO 3 solution. Set aside. In the flask contain the vitamin c powder, add approximately 50 ml of deionized water. Next add 2 ml of 6M H 2SO 4 and mix thoroughly. Solution will be turbid. Add approximately 2g of KI. Using the pipet, transfer 25 ml of the KIO 3 solution into the flask. Swirl to mix and immediately titrate. The vitamin c tablet has a starch based binder so the use of a starch indicator is not needed. Titrate until solution turns clear and record the of titrant used. Perform two more trials. Results: The data obtained through titration demonstrated that the amount of vitamin C that a commercially prepared tablet contains is on average 110 mg. This is higher than the expected 100 mg content that was listed on the bottle. All three titrations produced different results. Table I Mass of KIO 3 used (g) Volume of deionized water used 500 Molarity of KIO 3 solution 1.963x10-2 Table I shows the data for the preparation of the KIO 3 solution. The mass of KIO 3 used, the molar mass of KIO 3 ( g/mol) and 500 ml of deionized water was used to calculate the molarity of the solution (i). Table II Trial Initial Volume Final Volume Total Corrected Volume Molarity of S 2O x x x10-2 Average M of S 2O x10-2 Standard Deviation RSD (ppt) 1.2 Volume of KIO 3 used Molarity KIO 3 used 1.963x10-2

2 Table II, on the previous page, shows the data obtained from three trials. The titration of sodium thiosulfate solution into the potassium iodate solution was performed for the purpose of standardization. The molarity of the KIO 3 solution was found in a previous experiment. All buret measurements were corrected by using the correction graph created from data found in Lab #2. It is displayed on page 23 of the laboratory notebook. The pipet was not corrected. The pipet correction factor determined by experimentation was found to be The manufacturers mark is accurate to ±0.03 so a correction was not needed. The molarities of S 2O 3 found (ii) and were averaged to find the molarity of the standardized solution. The standard deviation was found using a TI-84 Plus C Silver Edition calculator. The standard deviation was within the acceptable ± 2 SD range. This value was then divided by the average molarity of S 2 O 3 and multiplied by 1000 to find the RSD. This was done to determine the precision of the S 2 O 3 molarity. Table III Trial Mass of tablet used (g) Mass of powder used (g) Corrected of S 2 O 3 used Actual mass of vit. C (g) Standardized mass of vit. C 109 mg 111 mg 110 mg Standardized average mass of vit. C 110 mg Standard deviation of mass vit. C 1.00 RSD (ppt) 9.1 % actual mass vit. C in powder 11.28% 11.63% 11.47% Average % mass vit. C in powder 11.46% Standard deviation of % actual mass RSD (ppt) 15.3 M of S 2O 3 used 6.086x10-2 Volume of KIO 3 used M KIO 3 used 1.963x10-2 Table III consists of data collected from three trials performed to determine the milligrams of ascorbic acid in a commercially prepared vitamin C tablet. All measurements were corrected by using the correction graph created from data found in Lab #2. It is displayed on page 23 of the laboratory notebook. The pipet was not corrected. The pipet correction factor determined by experimentation was found to be The manufacturers mark is accurate to ±0.03 so a correction was not needed. The of KIO 3 and molarity of KIO 3 was used to find the moles of I 3- present in the KIO 3 solution (iii). The corrected and molarity of the S 2 O 3 solution was used to calculate the moles of I 3- present in the S 2O 3 solution (iv). The standardized mass of vitamin C was found using the actual percent mass of vitamin C in powder, mass of powder used, the molar mass of ascorbic acid (176.12g/mol) and the mass of the tablet used (v). The standardized mass of vitamin of the three trials was averaged to produce a more accurate result. The standard deviation for the actual percent of vitamin C in the powder and the standardized mass of vitamin C data was calculated using a TI-84 Plus C Silver Edition calculator. The standard deviation was recorded and fell within the accepted ± 2 SD range. These values were then divided by the average actual percent of vitamin C and multiplied by 1000 to find the RSD. The RSD value for both the standardized mass and percent actual mass were high. Sample Calculations: IO I - +6H + 3I 2 + 3H 2 O I 2 +I - I 3-2S 2O 3 + I 3- S 4O 6 + 3I -

3 Results Through the standardization the concentration of potassium iodate was able to be determined with a high degree of accuracy. The amount of ascorbic acid present in a tablet of vitamin C was then able to be determined using these standardized solutions. From the data found during the analysis of vitamin C, the amount of ascorbic acid was able to be compared to the advertised amount of 100mg per tablet, found in the manufacturer s nutritional information, to the actual amount found in each tablet. Table I Molarity of KIO3 Mass of KIO3 (g) Volume of DI H2O (L) Molarity of KIO3 (mole/l) Mixing a dried sample of grams of KIO3, and a precise amount of deionized water of 500 ml, as seen above in Table I, a solution of precisely known concentration was able to be determined for later use in the analysis of ascorbic acid in a vitamin C tablet. The molarity of the KIO3 solution was then able to be calculated, and was found to be molar. Calculation I Molarity of KIO3 Using the amount of KIO3 in solution, and the calculated molecular weight of KIO3 the number of moles of KIO3 was able to be determined as seen above in Calculation I. From the division of the calculated amount of moles of KIO3, by the precisely measured amount of deionized water, L used to prepare the solution; the molarity of the KIO 3 solution was calculated to four significant figures and found to be molar.

4 Table II Standardization of Sodium Thiosulfate Trial Initial apparent Initial corrected Final apparent Volume correction value Volume dispensed S2O3 Corrected of KIO3 Mass of KI (g) S2O3 S2O3 S2O Through three separate trials, data was collected from the titration of sodium thiosulfate with potassium iodate, shown in Table II above. The apparent delivered by the buret was then adjusted using the buret value previously found for the particular buret used during these trials. The correction value is represented on the column labeled correction value as shown above in Table II. The pipet used to deliver the KIO3 solution was also previously calibrated and the deliver by the pipet was found to be within the manufacturer s specification of / ml. Therefore the amount of KIO3 solution drawn into the pipet to the 25 ml mark is an actual of ml as seen above in Table II. From the data collected and the stoichiometric analysis of the chemical reaction equations as seen in Equations I below the concentration of sodium thiosulfate was able to be calculated, see Calculations II below for molarity calculation. 1) IO I - + 6H + 3I2 + 3H2O 2) I2 + I - I3-3) 2S2O3 + I3 - S4O6 + 3I - 4) Equations I Chemical Formulas For the iodometric analysis of vitamin C tablet the reaction equations above shown in Equations I, were used to perform the calculations of the molarity of sodium thiosulfate, and in the determination of the amount of ascorbic acid in a vitamin C tablet. During the iodometric titrations, the iodide ion is added in excess to the analyte, and is shown in Equations I, numbered 1. During the standardization of the sodium

5 thiosulfate solution a known molarity of potassium iodate solution was used, the mole ratio and reaction are shown in equation numbered 1 occurs. The potassium iodate is in excess so the iodate ion becomes the limiting reactant, and dictates the amount of I2 produced. The excess iodide ion reacts with the I2 formed, and tri-iodide ion is produced, shown in Equation I, number 2. This solution is then titrated with sodium thiosulfate to determine the concentration of the sodium thiosulfate solution, as shown in Equation I, number 3. Finally using both the KIO3 solution and the S2O3 solution the amount of ascorbic acid in a vitamin C tablet is able to be determined using the mole ratio, shown in Equation I, number 4, of ascorbic acid and the excess I 3- ion. With these calculated amounts, the amount of ascorbic acid in a vitamin C tablet can be calculated. Calculation II Sample Calculation of molarity of Na2S2O3 The molarity of the sodium thiosulfate was able to be calculated, as seen above in Calculation II. The of KIO3 solution and the molarity previously calculated was use along with the stoichemetric mole ratios of the chemical reaction equations, as seen in Equations I above, to determine the number of moles of S2O3. The division of the moles of S2O3 by the corrected of S2O3 in liters, as seen in Table II above, resulted in the molarity of the sodium thiosulfate solution. A similar calculation was performed using data from all three trials, shown in Table II above, and the average molarity was determined as shown below in TableIII. Table III Average of Three and RSD Trial Molarity Calculated for S2O3 (mol/l) Average Molarity of S 2 O 3 for Three S x of Three RSD (ppt) for Three M

6 The concentration of sodium thiosulfate solution was determined using the data collected during the three separate trials as seen above in Table III. The molarity of each trial was calculated using the calculation technique as seen in Calculations II above. Each molarity was then determined for each trial and averaged. A standard deviation was then determined for the three trials and was found to be , with a relative standard deviation of parts per thousand; this value was then used as a tolerance value of the molarity of the sodium thiosulfate solution. The molarity of the sodium thiosulfate solution was determined to be molar +/- 0.5 ppt. This value can then be used in the determination of ascorbic acid in a vitamin C tablet. Table IV Determination of ascorbic acid in Vitamin C Tablet using prepared solutions Trial Initial apparent S 2O 3 Initial corrected S 2O 3 Final apparent S 2O 3 Volume correction value Volume dispensed S 2O 3 Corrected of KIO 3 Mass of vitamin C tablet (g) Mass of vitamin C powder (g) During three trials, as seen above in Table IV, data was tabulated and used in subsequent calculations to determine the amount of ascorbic acid in a vitamin C tablet, as seen below in Calculation III. The apparent delivered by the buret in each trial, was adjusted using a correction value previously determined for the buret used during this experiment. The correction value is represented on the column labeled correction value, at these particular s the correction value was found to be 0.00, as shown above in Table IV. The pipet used to deliver the KIO3 solution was also previously calibrated and the deliver by the pipet was found to be within the manufacture s specification of / ml. Therefore the amount of KIO3 solution drawn into the pipet to the 25 ml mark was delivered at the precise amount of ml as seen above in Table IV. During each trial the mass in grams of each vitamin C tablet was measured and recorded, these values are represented in the column labeled mass of vitamin C tablet. The tablet was then ground into a fine powder and the mass was tabulated in the column labeled mass of vitamin C powder. Using the previously calculated molarities of the KIO3 and S2O3 solution, and the data collected during the three titrations shown in Table IV, the amount of ascorbic acid in a vitamin C tablet was able to be determined, see Calculation III below.

7 Calculation III Sample calculation of Ascorbic Acid in Vitamin C Tablet Using the stoichiometry ratios discussed in Equation I section, and the and molarity of the potassium iodate solution, the percent of ascorbic acid in each tablet tested, as well as the amount of vitamin C in milligrams was able to be determined, as represented in a sample calculation seen above in Calculation III. Using a precise amount and molarity of KIO 3 solution and the mole ratio of the reactions seen in Equation I, the number of moles of I3 - from IO3 - was calculated, then using the amount of S2O3 used in the titration trial of the vitamin C powder, and mole ratio seen in Equation I, number 3, the number of moles I3 - from the

8 titration of S 2 O 3 was calculated as shown in Calculations III. This value was then subtracted from the amount of I3 - from IO3 - to determine the number of moles of I3 - reacted with the absorbic acid. The stiochimetric ratio is a one to one ratio, as seen in Equation I, number 4, using this mole ratio the number of moles of ascorbic acid was calculated. Using the number of moles of ascorbic acid and its molecular weight of grams per mole the amount in grams of ascorbic acid was determined, as seen above in Calculation III. This amount of ascorbic acid was then divided by the number of grams of powdered vitamin C, to find the pecentage of vitiamin C in each tablet. Furthermore this value was then multiplied by the mass of the tablet prior to being powered to calculate the amount of ascorbic acid in each tablet in milligrams, as seen in Calculation III above. Using a similar calculation, the pecentage of vitiamin C in each tablet and the mass of vitamin C in milligrams for each trial was able to be determined. The amount of ascorbic acid was then able to be compared to the advertised manufacture s amount, seen below in Table V. Table V Average of Three, RSD, and Comparison Trial Vitamin C mass (mg) Average Vitamin C mass (mg) S x of Three RSD of Three (ppt) % of Vitamin C in each Tablet Average % of Vitamin C in each Tablet S x of Three RSD of Three (ppt) % 8.66% % % From the calculation technique shown above in Calculation III, the mass of vitamin C in a tablet used for each trial, and the percentage of vitamin C in each tablet, was determined as shown above in Table V. The average amount of vitamin C in each tablet was able to be calculated, as seen in Table V above, as well as the average percent of vitamin C in each tablet, also shown in Table V above. A standard deviation for the mass of vitamin C was determined for the three trials and was found to be 3.464, with a relative standard deviation of parts per thousand. A standard deviation for the percent of vitamin C in each tablet was determined for the three trials and was found to be , with a relative standard deviation of parts per thousand. Using the data collected during the trials, shown in Table V, the average amount found for each tablet was then able to be compared to the amount in milligrams advertised by the manufacturer of 100mg per tablet, found in the nutritional information of the pill bottle, to the actual average amount of 109 mg found in each tablet. Discussion During this laboratory experiment the amount of ascorbic acid in a tablet of vitamin C was able to be determined, and then compared to the amount specified by the manufacturer of 100mg per tablet. The success of the preparation and standardization of various solutions with a high degree of accuracy was achieved. The concept of molar ratios were used in the calculations required to standardized the solutions being used during this experiment, these ratios were also used in the determination of ascorbic acid in a vitamin C tablet.