This is a lab that I did a while ago. It took over a week to get this one to work. It’s potentiometry- the electrochemical determination of a titration endpoint.

We’re determining the amount of ascorbic acid in vitamin C tablets and comparing it to the advertised amount. I also had to check how stable the titrant (sodium thiosulfate) was over a period of 5 weeks.

First what we do, is we standardize the sodium thiosulfate. Standardizing something in this case is using a known amount of triiodide ion (the stuff in the beaker) and dripping sodium thiosulfate (the stuff in the buret) into the triiodide ion to react with it and determine the concentration of sodium thiosulfate. We can tell when all of the triiodide ion has been reacted by reading the voltage that the computer gives us. When the voltage is stable, we know everything has been reacted.

After we get the concentration of the sodium thiosulfate, we can move on to the analysis of ascorbic acid in vitamin c tablets.

What we do, is we take vitamin c tablets and crush them up to put all the ascorbic acid into solution. Then, we oxidize the ascorbic acid with a known amount of the triiodide ion, but we add the triiodide ion in excess because if we don’t, the oxidation takes a really long time. After all the ascorbic acid has been oxidized, there is excess triiodide ion. Using the sodium thiosulfate, we can “back titrate” the triiodide ion (react the triiodide ion with sodium thiosulfate, as before), and determine how much of the excess triiodide ion had to be back titrated.

Since the triiodide ion and the ascorbic acid react in a 1:1 ratio, we can determine how much ascorbic acid was in the tablet by taking the initial amount of triiodide ion added, and then subtracting the amount of excess that needed to be back titrated. The resulting number is the amount of triiodide ion it took to react with the ascorbic acid, and transitively we get the amount of ascorbic acid in the tablet.

Ta-da!

Problems we had:

“stuff” building up on one of the electrodes.

not enough triiodide ion, so there wasn’t any excess to back titrate

binders in the tablet getting in the way of the titration

computer crashing

10/08/12 at 5:38pm
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  1. hood32 posted this