Standardization of the sodium thiosulfate solution:
Begin this procedure by rinsing the calibrated buret with deionized water and small portions of the
Na2S2O3 solution. Then rinse the 25 mL pipet with deionized water and the KIO3 solution. Fill the buret with the Na2S2O3
solution and record volume. Next, pipet 25 mL of the KIO3 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 H2SO4. 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 volume. 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 Na2S2O3 solution. Fill buret with the Na2S2O3 solution and record volume. Next, rinse the 25 mL pipet with deionized
water followed by a few mL of the KIO3 solution. Set aside. In the flask contain the vitamin c powder, add approximately
50 mL of deionized water. Next add 2 mL of 6M H2SO4 and mix thoroughly. Solution will be turbid. Add approximately 2g
of KI. Using the pipet, transfer 25 mL of the KIO3 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 volume 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 KIO3 used (g)
Volume of deionized
water used (mL)
Molarity of KIO3 solution
2.1001
500
1.963x10-2
Table I shows the data for the preparation of the KIO3 solution. The mass of KIO3 used, the molar mass of KIO3
(214.001g/mol) and 500 mL of deionized water was used to calculate the molarity of the solution (i).
Table II
Trial
Initial Volume (mL)
Final Volume (mL)
Total Corrected Volume (mL)
Molarity of S2O32Average M of S2O32Standard Deviation
RSD (ppt)
Volume of KIO3 used (mL)
Molarity KIO3 used
1
0.00
48.37
48.34
6.092x10-2
6.086x10-2
0.00007
1.2
25.00
1.963x10-2
2
0.00
48.40
48.37
6.088x10-2
3
0.00
48.49
48.46
6.078x10-2
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 KIO3
solution was found in a previous experiment. All buret volume measurements were corrected by -0.03 using the
correction graph created from data found in Lab #2. It is displayed on page 23 of the laboratory notebook. The pipet
volume was not corrected. The pipet correction factor determined by experimentation was found to be 0.01. The
manufacturers mark is accurate to ±0.03 so a correction was not needed. The molarities of S2O32- 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 S2O32- and multiplied by 1000 to find the RSD. This was done to determine the precision of the S2O32molarity.
Table III
Trial
Mass of tablet used (g)
Mass of powder used (g)
Corrected volume of S2O32- used (mL)
Actual mass of vit. C (g)
Standardized mass of vit. C
Standardized average mass of vit. C
Standard deviation of mass vit. C
% actual mass vit. C in powder
Average % mass vit. C in powder
Standard deviation of % actual mass
M of S2O32- used
Volume of KIO3 used (mL)
M KIO3 used
1
0.9629
0.9306
28.83
0.113
109 mg
110 mg
1.00
11.28%
11.46%
0.175
6.086x10-2
25.00
1.963x10-2
2
0.9577
0.9275
28.30
0.116
111 mg
3
0.9556
0.9493
28.10
0.115
110 mg
RSD (ppt)
11.63%
11.47%
RSD (ppt)
15.3
9.1
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 volume measurements were corrected by -0.03 using the correction graph
created from data found in Lab #2. It is displayed on page 23 of the laboratory notebook. The pipet volume was not
corrected. The pipet correction factor determined by experimentation was found to be 0.01. The manufacturers mark is
accurate to ±0.03 so a correction was not needed. The volume of KIO3 and molarity of KIO3 was used to find the moles of
I3- present in the KIO3 solution(iii). The corrected volume and molarity of the S2O32- solution was used to calculate the
moles of I3- present in the S2O32- 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:
IO3- + 5I- +6H+  3I2 + 3H2O
I2 +I-  I32S2O32- + I3-  S4O62- + 3I-
Iodometric Analysis of Vitamin C
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)
2.1025
0.500
0.01965
Mixing a dried sample of 2.1025 grams of KIO 3, 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 KIO 3 solution was then able to be
calculated, and was found to be 0.01965 molar.
Calculation I
Molarity of KIO3
Using the amount of KIO3 in solution, and the calculated molecular weight of KIO 3 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, 0.500 L used to prepare the solution;
the molarity of the KIO3 solution was calculated to four significant figures and found to be 0.01965 molar.
Iodometric Analysis of Vitamin C
Table II
Standardization of Sodium Thiosulfate
Trial
Initial
apparent
volume
S2O32-
Initial
corrected
volume
S2O32-
Final
apparent
volume
S2O32-
Volume
correction
value (mL)
Volume
dispensed
S2O32-(mL)
Corrected
volume of
KIO3 (mL)
Mass of
KI (g)
(mL)
(mL)
(mL)
1
0.00
0.00
41.42
0.13
41.55
25.00
2.07
2
0.00
0.00
41.40
0.13
41.53
25.00
2.12
3
0.00
0.00
41.44
0.13
41.57
25.00
2.09
Through three separate trials, data was collected from the titration of sodium thiosulfate with
potassium iodate, shown in Table II above. The apparent volume 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 volume correction value as shown above in Table II. The pipet used to
deliver the KIO3 solution was also previously calibrated and the volume deliver by the pipet was found to be
within the manufacturer’s specification of 25.00 +/- 0.01 mL. Therefore the amount of KIO3 solution drawn
into the pipet to the 25 mL mark is an actual volume of 25.00 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.
Equations I
Chemical Formulas
1) IO3- + 5I- + 6H+ → 3I2 + 3H 2O
2) I2 + I- → I33) 2S2O32- + I3- → S4O62- + 3I4)
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
Iodometric Analysis of Vitamin C
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 S2O32- 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
volume 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
S2O32-. The division of the moles of S2O32- by the corrected volume of S2O32- 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 Trials and RSD
Trial
Molarity Calculated for
S2O32- (mol/L)
Average Molarity of S 2O32for Three Trials
Sx of Three
Trials
RSD (ppt) for
Three Trials
1
0.07094
0.07094 M
0.000035
0.493
2
0.07097
-
-
-
3
0.07090
-
-
-
Iodometric Analysis of Vitamin C
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 0.000035, with a relative
standard deviation of 0.493 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
0.07094 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
volume
S2O32-
Initial
corrected
volume
S2O32-
Final
apparent
volume
S2O32-
Volume
correction
value (mL)
Volume
dispensed
S2O32-(mL)
Corrected
volume of
KIO3 (mL)
Mass of
vitamin
C tablet
(g)
Mass of
vitamin C
powder (g)
(mL)
(mL)
(mL)
1
0.00
0.00
24.58
0.00
24.58
25.00
1.2506
1.2418
2
0.00
0.00
23.56
0.00
23.56
25.00
1.2609
1.2514
3
0.00
0.00
24.42
0.00
24.42
25.00
1.2675
1.2628
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
volume 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 volume
correction value, at these particular volumes 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 volume deliver by
the pipet was found to be within the manufacture’s specification of 25.00 +/- 0.01 mL. Therefore the amount
of KIO3 solution drawn into the pipet to the 25 mL mark was delivered at the precise amount of 25.00 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 S2O32- 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.
Iodometric Analysis of Vitamin C
Calculation III
Sample calculation of Ascorbic Acid in Vitamin C Tablet
Using the stoichiometry ratios discussed in Equation I section, and the volume 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 KIO3 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 S2O32- 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
Iodometric Analysis of Vitamin C
titration of S2O32- 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 176.12 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 Trials, RSD, and Comparison
Trial
Vitamin C
mass
(mg)
Average
Vitamin
C mass
(mg)
Sx of
Three
Trials
RSD of
Three
Trials
(ppt)
% of
Vitamin C
in each
Tablet
Average %
of Vitamin C
in each
Tablet
Sx of
Three
Trials
RSD of
Three
Trials
(ppt)
1
107
109
3.464
31.78
8.54%
8.66%
0.2765
0.0319
2
113
-
-
-
8.98%
-
-
-
3
107
-
-
-
8.47%
-
-
-
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
31.78 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 0.2765, with a relative standard deviation of 0.0319 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.