The Indirect Iodimetric Analysis Ascorbic Acid

Topics: Iodine, Redox, Titration Pages: 9 (1553 words) Published: March 24, 2014
Oxidation Reduction Titrations: The Indirect Iodimetric Analysis Ascorbic Acid Unknown Number 69
I: Purpose: To determine the percent weight of Ascorbic Acid in unknown sample. This was to be done by the means of an indirect iodimetric analysis. In an iodometric analysis, the oxidizing agent can be determined by a different means than titrating directly with standard iodide, because a high concentration of I- is needed to form the I3- complex. In this type of analysis, excess iodide is added to the oxidizing agent, and the triiodine is titrated with stand thiosulfate. This indirect analysis finds the number of moles of ascorbic acid based on the known number of moles of IO3- and subtracting half the amount of moles of the thiosulfate solution. II: Equations:

Iodate with Iodide:
IO3- + 8I- + 6H+  3I3- + 3H2O
Thiosulfate with Triiodide
2S2O32- + I3-  S4O62- + 3I-
Ascorbic Acid with Triiodide
C6H8O6 + I3- +2H2O  C6H6O6 +3I- + 2H3O+
III: Sample Calculations:
A) Concentration Iodate:
1.9502g KIO3 * * = 0.01823 M KIO3
B) Concentration Thiosulfate (S2O32-)
0.0250481 L KIO3 * = 0.00045663 mole KIO3
0.00045663 moles IO3- * * = 0.0027366 M
C) First and Second Derivative
V(mL)
E(mV)
∆V(mL)
∆E
∆E/∆V
ml

∆(∆E/V)
∆(∆E/∆V)/∆
Vav
14.536
265

0.021
-181.25
-8629.05
14.5305

0.01
-4
-400
14.541

14.546
261

0.011
-100
-9090.9
14.5465

0.012
-6
-500
14.552

14.558
255

0.047
500
10638
14.5555

First Derivative: On the y-axis plot ∆E/∆V (column 5) and on the x-axis plot
ml (column 6). The slope is steepest at the end point.
Second Derivative: On the y-axis plot ∆(∆E/∆V)/∆ (column 9). And on the x-axis plot Vav (column 10), where the graph crosses zero is the end point.

D) % Weight Ascorbic Acid (fully automated)
-Moles Ascorbic Acid = moles I3- - ½ (moles S2O32-)
-Mols IO32- = 0.025048 L IO3- * * = 0.001370 mols I3-
-Mols S2042- 0.014872L S2032- *0.07151 M S2O42- = 0.00106 mols S2O42- -Mols Ascorbic Acid = 0.001370 moles I3- - ½ (1.00106 moles S2O62-)= 0.000838 moles Ascorbic Acid - 0.000838 moles Ascorbic Acid * = 0.1476 g AA

- % weight = *100 = 50.18%
IV: Tables and Graphs: See attached for first and second derivative and chart used to obtain first and second derivative.
Semi-Manual Titration

mL
mV

mL
mV

mL
mV
0
3040

14.342
287

14.632
164
2.030
339

14.390
284

14.642
156
4.026
336

14.484
275

14.660
152
5.970
334

14.494
274

14.676
147
8.010
330

14.504
272

14.692
144
10.004
326

14.536
265

14.706
140
12.026
318

14.546
261

14.736
137
12.650
314

14.560
255

14.764
133
13.186
310

14.570
247

14.796
131
13.428
308

14.578
241

14.816
128
13.912
301

14.580
236

14.864
126
14.106
296

14.600
193

14.930
123
14.154
295

14.612
174

15.054
120
14.296
289

14.622
168

15.868
112

V. Conclusion: After completing three separate methods to determine the percent weight of ascorbic acid in a sample, in the manual titration the percent weight was 51.45%. The semi-manual method gave a percent weight of 51.40%, and the automatic titration gave a percent weight of 50.18%. Averaged together, giving a percent weight of 51.01% and a standard deviation of 0.719 and an RSD of 14 ppt. A possible systematic error that could have occurred would have been while doing the manual titration to not wipe the tip of your 25 mL pipet. This would have caused the calibrated amount to not have been correct and you would have added a falsely high amount of potassium iodate. This would have given a value of moles I3- higher than what was actually given. This is turn would create a larger value for the moles of ascorbic acid and you would get a higher percent weight. Another possible systematic error would be if the automatic titrator was improperly calibrated...