The Indirect Iodimetric Analysis Ascorbic Acid

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-