Chem - Cu2+ and KI

I have a solution of aqua Cu2+ complex, I add KI. It went murky yellow, then I added sodium thiosulphate and I think there was a white precipitate and the solution looked kinda blue/green. What was going on? I know the yellowness was from I2, and the precipitate is CuI (I think)...

Reply 1

charco

Study Forum Helper

Sparkling_Jules

Quite correct it's a redox reaction

2Cu2+ + 2I- --> 2Cu+ + I2

The copper I sulphate formed is a white insoluble salt (that slowly disproportionates in aqueous conditions 2Cu+ --> Cu2+ + Cu - but too slowly to affect this reation scheme)

Reaction with sodium thiosulphate reduces the iodine formed back to iodide ions and the yellow colour (iodine in aueous solution) disappears

2S2O3(2-) + I2 --> S4O6(2-) + 2I-

Reply 2

Mafer

why blue green?

Reply 3

Mafer

I thought it was only blue, isn't it?

Reply 4

charco

Study Forum Helper

if there are some copper II ions remaining after addition of the I- then they will be blue hexaaqua ions but if there is some yellow iodine formed then this would give the solution a green tinge