Introduction: The purpose of this experiment was to standardize a thiosulfate solution using a potassium iodate primary standard. With this standardized thiosulfate solution, the weight of vitamin C (ascorbic acid) in a commercial vitamin C tablet.
IO3- + 8I- + 6H+ ↔ 3I3- + 3H2O
I3- + 2S2O32- ↔ 3I- + S4O62-
The first step of part one, the preparation and standardization of thiosulfate solution, water (500mL) was set to boil. For the approximately 0.07M thiosulfate solution, amounts of sodium thiosulfate pentahydrate (8.7096g) and sodium carbonate (0.0533g) were measured out and then dissolved in the freshly boiled water. The thiosulfate solution was transferred to a Nalgene bottle, cooled in an ice bath, and then stored within the lab drawer when not in use.
The second step was the preparation of the approximately 0.01M potassium iodate primary standard. Potassium iodate (0.9998g) was measured out and dissolved with distilled water in a volumetric flask (500mL).
The final step of part one was the standardization of the thiosulfate solution. Potassium iodide (approximately 2g) and the potassium iodate primary standard (50.00mL) were added to four flasks labeled one through four. The thiosulfate solution was placed in a buret and the initial volume recorded. Before the titration began, 0.5M sulfuric acid (approximately 10mL per flask) and starch indicator (approximately 2mL per flask) were measured out. The sulfuric acid was added to a flask and immediately titrated. When the color was a very faint yellow, the starch indicator was added, which turned the solution a dark blue. The titration continued until the solution turned colorless. This procedure was repeated a second time with another four trials.
The second part of this procedure was the analysis of vitamin C. Two commercial vitamin C tablets (approximately 500mg per tablet) were measured. The tablets were crushed with a mortar and pestle. The crushed tablets (approximately 200mg) were measured out and placed in five flasks labeled one through five. To dissolve the crushed tablets, approximately 0.3M sulfuric acid (approximately 60mL) was added to each flask. Potassium iodide (approximately 2g) was added to each flask. The thiosulfate solution was added to a buret and the initial volume recorded. As before, starch indicator (approximately 2mL) was measured before titrating. Potassium iodate primary standard (50.00mL) was added via volumetric pipet and the vitamin C solution immediately titrated. As in part one, starch indicator was added when the solution was a very faint yellow, which turned the solution a dark blue. The titration continued until the solution turned colorless.
The results are displayed within the submitted images.
The results of interest are the molarity of the thiosulfate solution and the milligrams of vitamin C per tablet. The molarity of the thiosulfate solution was found to be 0.07389M, with a standard deviation of ±0.00014M. The interval of molarity at a confidence level of 95% was 0.07375M – 0.07403M. The weight of vitamin C per tablet was found to be 419.6mg, with a standard deviation of ±0.2mg. The interval of weight at a 95% confidence level was 419.3mg – 419.9mg.
When calculating the molarity of the thiosulfate solution, some data was not used. The first trial was accidentally over-titrated and reached the end point before the indicator was added. The fourth trial was also over-titrated, and was removed. While running the vitamin C trials, the potassium iodate primary standard ran out before the last trial could be done, so no data was gathered.
As with any experiment, sources of error must be acknowledged. Random errors include titrating drop-wise and by half-drops. While a drop may have a standard volume, half-drops are just determined relatively. Not all half-drops will be the same volume, and therefore possibly yield inaccurate results. Attempting to use the same sized half-drop would increase precision.
This lab was done in an analytical chemistry lab at the Universtity of Wisconsin-Green Bay with the use of a general lab procedure distributed to the class by Dr. Zorn.