The Indirect Iodometric Analysis of Ascorbic Acid
Essay by Darby Monagle • March 20, 2017 • Lab Report • 412 Words (2 Pages) • 1,168 Views
The Indirect Iodometric analysis of Ascorbic Acid
Unknown number 78
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I. Purpose: To determine the weight percent of ascorbic acid in an unknown sample by indirect iodometric analysis. This is done by reacting ascorbic acid with triiodide and then titrating with standardized thiosulfate solution.
II. Theory: A known concentration of potassium iodate and excess potassium iodide is used in titration to standardize the thiosulfate solution. This process with calculations will give the concentration of standardized thiosulfate solution which is used to titrate the ascorbic acid unknown sample. To titrate the unknown, potassium iodate solution and excess potassium iodide are mixed to form triiodide. This reacts with Ascorbic Acid to form dehydroascorbic acid. The excess triiodide is then titrated with standardized thiosulfate and ascorbic acid moles can be calculated from the known moles of iodate and the volume and concentration of triiodide.
III. Sample Calculations:
Concentration of the KIO3
[pic 1]
Net Ionic equations
a) the reaction of iodate with iodide
IO3- + 8I- + 6 H+ → 3I3- + 3 H2O
b) the reaction of triiodide with thiosulfate
2S2O3-2 + I3- → S4O6-2 + 3 I-
c) the reaction of ascorbic acid with triiodide
C6H8O6 + I3- + H2O → C6H6O7 + 3I- + 4H+
Concentration of the standardized thiosulfate solution
[pic 2]
[pic 3]
Weight percent of Ascorbic Acid in Unknown
[pic 4]
[pic 5]
[pic 6]
[pic 7]
[pic 8]
Ascorbic acid concentration
0.00256 M Ascorbic Acid[pic 9]
IV. Data and Results
Mass Na2S2O4 1.7050 g
Mass KIO3 0.2020
Concentration KIO3 0.009439 M
- Standardization of Thiosulfate[pic 10]
Flask | KI (g) | Thiosulfate added (L) | M Thiosulfate |
2 | 0.4995 | 0.003790 | 0.074715 |
3 | 0.5052 | 0.003865 | 0.073265 |
4 | 0.4986 | 0.003880 | 0.072982 |
- Titration of Ascorbic Acid
Flask | Unknown amount (g) | KI (g) | Thiosulfate titrant added (L) | I3- Reacted (mol) | Ascorbic Acid (g) | Ascorbic Acid M | Weight % Ascorbic Acid |
1 | 0.1527 | 0.4936 | 0.005910 | 0.000207119 | 0.036478 | 0.002559 | 23.889 |
4 | 0.1551 | 0.5020 | 0.005451 | 0.000224056 | 0.039461 | 0.002785 | 25.442 |
3(auto) | 0.1532 | 0.4993 | 0.006022 | 0.000203028 | 0.035757 | N/A | 23.340 |
[pic 11]
FW Ascorbic Acid 176.12 g
Titration curve Graph 1
[pic 12]
[pic 13]
[pic 14]
IV. Conclusion: The unknown provided an ascorbic acid content of 24% by weight. The autotitrator stirrer got stuck for the first trial so it had to be repeated. The second trial was kept. However, since a significant amount of thiosulfate solution was used in this process, the final trial (flask 4) of the manual titration needed 1mL of another thiosulfate solution. This could have provided contamination error. This could have provided error in the weight percent of ascorbic acid. It is possible that some measured dry solids were not fully transferred or entirely dissolved in solution which could have provided error.
- Autotitration Data and Calculations
Volume V (mL) | Voltage E (V) | ΔV (mL) | ΔE (V) | Δ E /Δ V (V/mL) | ▽ (mL) | ▽Δ (mL) | Δ (Δ E/Δ V) (V/mL) | Δ (Δ E/Δ V)/ Δ▽ (V/mL2) | Vavg (mL) |
0 | 0.3888 | 0.01 | -0.0002 | -0.02 | 0.005 | 0.01 | 0.01 | 1 | 0.01 |
0.01 | 0.3886 | 0.01 | -0.0001 | -0.01 | 0.015 | 0.01 | 0.01 | 1 | 0.02 |
0.02 | 0.3885 | 0.01 | 0 | 0 | 0.025 | 0.193 | -0.00027 | -0.00138 | 0.1215 |
0.03 | 0.3885 | 0.376 | -0.0001 | -0.00027 | 0.218 | 0.599 | -0.00752 | -0.01255 | 0.5175 |
0.406 | 0.3884 | 0.822 | -0.0064 | -0.00779 | 0.817 | 0.716 | 0.003851 | 0.005379 | 1.175 |
1.228 | 0.382 | 0.61 | -0.0024 | -0.00393 | 1.533 | 0.656 | -0.00048 | -0.00073 | 1.861 |
1.838 | 0.3796 | 0.702 | -0.0031 | -0.00442 | 2.189 | 0.697 | -0.00136 | -0.00196 | 2.5375 |
2.54 | 0.3765 | 0.692 | -0.004 | -0.00578 | 2.886 | 0.674 | -0.00047 | -0.0007 | 3.223 |
3.232 | 0.3725 | 0.656 | -0.0041 | -0.00625 | 3.56 | 0.648 | -0.00172 | -0.00265 | 3.884 |
3.888 | 0.3684 | 0.64 | -0.0051 | -0.00797 | 4.208 | 0.619 | -0.0039 | -0.00631 | 4.5175 |
4.528 | 0.3633 | 0.598 | -0.0071 | -0.01187 | 4.827 | 0.535 | -0.01016 | -0.01899 | 5.0945 |
5.126 | 0.3562 | 0.472 | -0.0104 | -0.02203 | 5.362 | 0.361 | -0.02077 | -0.05752 | 5.5425 |
5.598 | 0.3458 | 0.25 | -0.0107 | -0.0428 | 5.723 | 0.178 | -0.07607 | -0.42735 | 5.812 |
5.848 | 0.3351 | 0.106 | -0.0126 | -0.11887 | 5.901 | 0.063 | -0.07113 | -1.12908 | 5.9325 |
5.954 | 0.3225 | 0.02 | -0.0038 | -0.19 | 5.964 | 0.02 | -0.095 | -4.75 | 5.974 |
5.974 | 0.3187 | 0.02 | -0.0057 | -0.285 | 5.984 | 0.018 | -0.17125 | -9.51389 | 5.993 |
5.994 | 0.313 | 0.016 | -0.0073 | -0.45625 | 6.002 | 0.014 | -0.83542 | -59.6726 | 6.009 |
6.01 | 0.3057 | 0.012 | -0.0155 | -1.29167 | 6.016 | 0.011 | -4.70833 | -428.03 | 6.0215 |
6.022 | 0.2902 | 0.01 | -0.06 | -6 | 6.027 | 0.01 | 4.1 | 410 | 6.032 |
6.032 | 0.2302 | 0.01 | -0.019 | -1.9 | 6.037 | 0.01 | 1.21 | 121 | 6.042 |
6.042 | 0.2112 | 0.01 | -0.0069 | -0.69 | 6.047 | 0.01 | 0.32 | 32 | 6.052 |
6.052 | 0.2043 | 0.01 | -0.0037 | -0.37 | 6.057 | 0.031 | 0.204615 | 6.600496 | 6.0725 |
6.062 | 0.2006 | 0.052 | -0.0086 | -0.16538 | 6.088 | 0.077 | 0.095777 | 1.243854 | 6.1265 |
6.114 | 0.192 | 0.102 | -0.0071 | -0.06961 | 6.165 | 0.151 | 0.036108 | 0.239125 | 6.2405 |
6.216 | 0.1849 | 0.2 | -0.0067 | -0.0335 | 6.316 | 0.262 | 0.01529 | 0.058359 | 6.447 |
6.416 | 0.1782 | 0.324 | -0.0059 | -0.01821 | 6.578 | 0.386 | 0.006603 | 0.017106 | 6.771 |
6.74 | 0.1723 | 0.448 | -0.0052 | -0.01161 | 6.964 | 0.493 | 0.003057 | 0.006201 | 7.2105 |
7.188 | 0.1671 | 0.538 | -0.0046 | -0.00855 | 7.457 | 0.406 | -0.00167 | -0.00411 | 7.66 |
7.726 | 0.1625 | 0.274 | -0.0028 | -0.01022 | 7.863 | ||||
8 | 0.1597 |
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