lugol's solution

[Rene]

Hallo to all, can someone more chemically minded give me an answer:

Iodine (I₂) does not dissolve well in water, but mixed with iodide I^- it forms [I₃]^-

Standard protocols dissolve 5g iodide + 10g potassium iodide (KI) in 100 ml water (originally: dissolve to 85ml solution, in order to get 150mg iodine per ml). To that mixture I add 10 ml acetic acid for the standard marine preservative, which is used in the conc of 4ml lugol per 1 liter sample.

Now generally not all iodine dissolves in the stock solution, also, after a year or two I regularly come across small deposits of (I think) iodine in my samples.

I wonder whether I am adding enough potassium iodide. Mol for mol, there seems enough KI to dissolve the iodine (5g I₂ per 100ml is 0.2M, 10 g KI per 100ml gives a 0.6M solution). Is there some equilibrium so that I need to add more KI? Or could it be an effect of the addition of acetic acid?


Many thanks,

René

[treinisch]

Hallo to all, can someone more chemically minded give me an answer:

Iodine (I₂) does not dissolve well in water, but mixed with iodide I^- it forms [I₃]^-

Standard protocols dissolve 5g iodide + 10g potassium iodide (KI) in 100 ml water (originally: dissolve to 85ml solution, in order to get 150mg iodine per ml). To that mixture I add 10 ml acetic acid for the standard marine preservative, which is used in the conc of 4ml lugol per 1 liter sample.

Now generally not all iodine dissolves in the stock solution, also, after a year or two I regularly come across small deposits of (I think) iodine in my samples.

I wonder whether I am adding enough potassium iodide. Mol for mol, there seems enough KI to dissolve the iodine (5g I₂ per 100ml is 0.2M, 10 g KI per 100ml gives a 0.6M solution). Is there some equilibrium so that I need to add more KI? Or could it be an effect of the addition of acetic acid?

Hallo René,

of course there is an equilibrium at work, as always. I did not find a value for the equilibrium constant, but when
calculating using the relevant thermodynamical data I get a value of around 700. Of course experimental values will be different, but as far as I know, the iodine/iodide equilibrium is used as one of the very well working examples in lab courses.

Using the concentrations given by you I calculate a free concentration of Iodine of around 1µM which quit an amount!

br

Timm

[Rene]

Thanks Timm, so can you tell me what the safe amount of KI then is that I need to add in order to make sure that all of my iodine dissolves? I know the Windermere labs (FBA) use a 1:10 ratio of iodine vs potassium iodide, which I always thought was rather excessive (expensive too for the ca. 1000x 1 liter samples that we preserve for our analyses).

Regards, René

[treinisch]

Thanks Timm, so can you tell me what the safe amount of KI then is that I need to add in order to make sure that all of my iodine dissolves? I know the Windermere labs (FBA) use a 1:10 ratio of iodine vs potassium iodide, which I always thought was rather excessive (expensive too for the ca. 1000x 1 liter samples that we preserve for our analyses).

Hallo Rene,

times ten KI should yield around 0.2µM I₂ if all the assumptions are right. I wonder, whether the equilibrium is the problem, maybe KI gets oxidized by O2 or reacts with dust and stuff to form a precipitate?

The following diagram gives the free concentration of iodine and the amount of iodide needed to achieve it, given a total concentration of 0.2M for iodine, if all the assumptions are right and I did not make an mistakes, of course.

kr

Timm

[treinisch]

Thanks Timm, so can you tell me what the safe amount of KI then is that I need to add in order to make sure that all of my iodine dissolves? I know the Windermere labs (FBA) use a 1:10 ratio of iodine vs potassium iodide, which I always thought was rather excessive (expensive too for the ca. 1000x 1 liter samples that we preserve for our analyses).

Hallo Rene,

times ten KI should yield around 0.2µM I₂ if all the assumptions are right. I wonder, whether the equilibrium is the problem, maybe KI gets oxidized by O2 or reacts with dust and stuff to form a precipitate?

The following diagram gives the free concentration of iodine and the amount of iodide needed to achieve it, given a total concentration of 0.2M for iodine, if all the assumptions are right and I did not make any mistakes, of course.

kr

Timm

[Rene]

Thanks again Timm, nice to see old bits of theory floating back in my memory...

However, I'm not sure that the problem is with this equilibrium. Shifting it one way or the other will not cause the iodine to precipetate out. At least that is my understanding. The stuff on the bottom after pouring off the dissolved solution surely looks like leftover iodine. I have no access to the chemical standard works, did you come across anything about maximum dissolution amounts? I don't seem to be able to find a reference on google easily...

Regards, René

[treinisch]

Thanks again Timm, nice to see old bits of theory floating back in my memory...

However, I'm not sure that the problem is with this equilibrium. Shifting it one way or the other will not cause the iodine to precipetate out. At least that is my understanding. The stuff on the bottom after pouring off the dissolved solution surely looks like leftover iodine. I have no access to the chemical standard works, did you come across anything about maximum dissolution amounts? I don't seem to be able to find a reference on google easily...

Hi Rene,

according to http://wapedia.mobi/en/Iodine?t=8. The solubility of elementary iodine should be around 2.3µM at 20°C. Not that bad. So probably the equilibrium itself is not the thing to worry about.

kr

Timm