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Vitamin C is a familiar name to chemistry students, and simple experiments that determine the amount of vitamin C in pharmaceutical prescriptions are important in the general chemistry laboratory (1, 2). This procedure involves an ascorbic acid redox titration with iodine solution using starch as an indicator:
The end point of this titration is determined by the first excess of iodine in the reaction medium that reacts with starch, forming a complex with an intense dark blue-violet color (3).
In conventional analytical iodometric experiments, most of the experimental work is involved with the standardization of the iodine solution. This procedure determines the iodine concentration using a solution of sodium thiosulfate (Na^sub 2^S^sub 2^O^sub 3^), which is itself standardized against a primary standard such as potassium dichromate or potassium iodate in an acid aqueous media (4). Both of these compounds produce iodine in a redox reaction that is titrated with a thiosulfate solution using a starch solution as indicator (5). The concentration of the thiosulfate solution is then calculated considering the number of moles of the primary standard used. From an analytical point of view this is not very accurate, because the titration errors are cumulative.
The purpose of this article is to show that ascorbic acid is an excellent compound to use for the standardization of iodine solutions. Using ascorbic acid, the standardization of iodine solutions is fast, accurate, and easy compared with the conventional methods. During a single laboratory period, the students in a general chemistry course are able to standardize an iodine solution and determine, with good precision, the amount of vitamin C in some pharmaceutical tablets.
Experimental Procedure
Glassware and Reagents
All reagents were of analytical grade or better. A buret (50 mL) and a microburet (5 mL) were used without calibration. A pipet (10 mL) was calibrated with water (5 replicates), giving (9.970 +/- 0.004) mL.
Preparation of Solutions
Iodine (3.8 g) and 20 g of potassium iodide (KI) were completely dissolved in 20 mL of distilled water and the mixcure was diluted to 1 L. This iodine solution (0.015 mol L^sup -1)^ was stored in a dark flask for further standardization.
Twenty-five grams of sodium thiosulfate (Na^sub 2^S^sub 2^O^sub 3^ * 5H^sub 2^O) and 0.1 g of sodium carbonate (Na^sub...