Old chemicals absorbing water - heating to reverse?

[taricha]

Can Sodium Iodide crystals in a bottle become a hydrate, and then be heated on a coffee warmer back to pure NaI?
I was going to make some stock Iodide solution to re-check some earlier calculations. I pull some out of a several year old bottle of NaI out and weigh it. It looks a little clumpy, so I decide to put it on a drying dish over my coffee heater, and make sure it's totally dry.
The crystals melt(?) then dry out.

Mass before = 0.2469g
Mass after 4hours heating = 0.1956g

trying to make sense of this, I find I am nowhere near melting point of NaI 600+C, but NaI does have a dihydrate version, and NaI is hygroscopic - can absorb water from air.

The sample before weighed 1.26 times as much as the 4hr heated sample.
NaI dihydrate has a molar mass of 1.24 times the molar mass of the anhydrous NaI.

Is this just a coincidence, or is that actually likely what happened, and now I can be pretty sure I actually have NaI?

 

[sghera64]

I think what you are trying to do should work and should be safe (NaI, KI) but don’t try this with other iodide salts like ammonium.

What might be happening is the heat is not enough to compete with the thermodynamic equilibrium that the NaI has with water . I did not know that NaI had hydrates. I think it might just be acting like a desiccant.

Might need more heat and a drier environment over the salt.

 

[sghera64]

Here is a reference that might be helpful:

https://link.springer.com/article/10.1134/S0020168509110235

I read you need to get above 180C to break the dihidrate. They used a microwave. Be careful using a microwave without enough “stuff” in it to absorbs the energy. Might want to put a potato in a towel in there with the salt if you go down that route.

 

[Randy Holmes-Farley Verified Community Expert]

I'm not sure if sodium iodide will hydrate on exposure to air, but your experiment is certainly consistent with you having the hydrate and driving off the moisture.

Normal sodium iodide does not decompose on heating before melting at many hundreds of degrees, but in air you may get some conversion of iodide to iodine that may come off, leaving behind sodium oxide. I doubt that happens especially rapidly in the solid form.

 

[taricha]

This morning, the heated NaI crystals left out overnight became a puddle of liquid.

Reading a bit, NaI is deliquescent (can dissolve in absorbed water). Looks like it's got some drawbacks as a solid used to make an iodide stock.

Looks like KI may be a better choice for making accurate stock?
"[KI] is less hygroscopic than sodium
iodide, making it easier to work with. Potassium iodide is stable in dry air but slightly hygroscopic in moist air. Aged and impure samples are yellow because of oxidation of the iodide to iodine..."

 

[taricha]

Yesterday I did it again, heated NaI, and KI side-by side on coffee warmer for 2-3 hours.
NaI: 0.4385 -> 0.3497 or 125% of the "dried" weight.
KI: 0.1569 -> 0.1522 or 103% of the "dried" weight

It looks like KI is indeed much better.
wikipedia on iodide titrations

Standard iodine solution is prepared from potassium iodate and potassium iodide, which are both primary standards

KI being a primary standard probably means a fresh bottle of KI is about the best one can do for accuracy.

Makes me wonder about nitrate when trying to make an accurate NO3 stock, if NaNO3, KNO3 or other are most stable/preferred.