Standardization of 0.01 N Iodine Titrant
This operation determines the conversion factor (CF) for the iodine titrant which, when multiplied times the mL titrant used, will convert it to mg vitamin C. (Remember that a conversion factor = desired units/given units.) Here, 10.0 mg of vitamin C is divided by the mL of iodine required to react with it. This yields a CF with the units mg vitamin C/mL iodine. Note that this standardization procedure is required only for the initial step of vitamin C determinations, but not when performing a determination on an unknown sample. (See other protocols for specifics of technique, and reagent preparation.)
Here is a spreadsheet template for recording the standardization data and computing the conversion factor. (the link also has the data sheet for titrating vitamin C in fruit juice.)
Set up your bench with the equipment to perform the titration:
Close the stopcock. Close the stopcock on the buret.
Use a funnel to fill the buret. CAUTION: Add only enough iodine to keep some in the funnel. If you over fill, it will spill all over your desk.
Open the stopcock wide to flush the air out of the spout. If air reappears, pull off the stopcock, apply a thin layer of stopcock grease as a sealant, and replace the spout.
There is no air in the spout.
Deliver 10 mL of Rxn Mix into each of three 250 mL flasks (a repipet simplifies the operation.)
The steps to using a repipet are:
1) Set the volume desired to be delivered. (either 10 mL for clear samples to be titrated, or 2x 7.5 mL for turbid specimens (fruit juice, etc).
2) Place a waste container under the spout.
3) Lift the grip all the way up, and press all the way down. Repeat until you get a steady stream of liquid (no bubbles). Discard the liquid.
4) Place the receiving container under the spout.
5) Raise the grip all the way up.
6) Press the grip all the way down as the liquid is delivered into the receiving container.
Set a displacement pipet to 1.000 mL (1000 uL)
Withdraw 1.00 mL of 1.00 % Vit C (corresponds to 10.0 mg of vitamin C).
Deliver the 1.00 mL of 1.00% Vitamin C into each of three flasks to which reaction mix was previously added.
Read and record the starting volume in the buret to the nearest 0.01 mL
Titrate the first flask with 0.01 N Iodine solution until the solution just turns faint blue.
Repeat the titration for the other two flasks.
5. Read and record the finishing volume in the buret, also to the nearest 0.01 mL.
6. Repeat the titration for the other two flasks.
7. Determine the volume of iodine solution used in each titration, determine the mean volume required for the three flasks.
8. Calculate the conversion factor for the iodine by dividing 10 mg Vit C by the mean mL iodine used, giving the conversion factor: