Sciencemadness Discussion Board

Testing purity of Potassium Iodide

gdflp - 26-3-2014 at 07:45

I recently obtained about 4oz of KI for a good price in a one time auction. The seller claimed the iodide was ACS reagent grade, but I highly doubt this based on the price. Is there any way to test the KI for impurities(It doesn't have to be to ACS standards, but I'm looking for a quantitative test so I can know it's around 98% or better purity.)

Galinstan - 26-3-2014 at 08:12

take a small sample of your KI and convert it to iodine then dissolve it in an excess of a KI solution to form the triiodide ion and titrate against sodium thiosulphate or other equivalent reducing agent to work out the percentage of your KI that is actually KI.

MrHomeScientist - 26-3-2014 at 08:29

Quote: Originally posted by Galinstan  
take a small sample of your KI and convert it to iodine then dissolve it in an excess of a KI solution to form the triiodide ion and titrate against sodium thiosulphate or other equivalent reducing agent to work out the percentage of your KI that is actually KI.

This. I just did this recently myself, actually. Be aware that KI is somewhat reactive towards air, and solutions are slowly but easily oxidized to iodine by oxygen. I found this out when leaving a clear KI + starch solution out over the weekend, and returned to find it brownish.

gdflp - 26-3-2014 at 08:31

Wouldn't that require known KI to do? I'd rather not drop $10 for an ounce of good KI, and have to wait a few weeks to get it and titrate the unsure KI.

Zyklon-A - 26-3-2014 at 08:36

Quote: Originally posted by gdflp  
Wouldn't that require known KI to do?

No. Re-read Galinstan's reply.

Paddywhacker - 26-3-2014 at 17:40

It would need some form of known standard unless you had laboratory standardised sodium thiosulfate solution. Potassium iodate is the usual laboratory primary standard because it can be obtained very pure and it dries in the oven without decomposition and cools in the desiccator without absorbing atmospheric moisture.