[rap35]

I'm having issues dissolving LiOH in deionised water and it won't fully dissolve.

I'm working up a reaction that requires mole ratio values of 1 : 3 for FeSO4.7H2O : LiOH.H2O in 50 mL of water, whilst maintaining the reaction concentration of FeSO4 at 0.5 M. To do this, 3.147  g of LiOH.H2O is needed.

SigmaAldrich states the published water solubility of this to be 200 g/L at 20 °C. I have tried warming the water, leaving it stirring over the weekend, using a different supplier of LiOH.H2O and it still fails to dissolve.

Previously an organic chemist now working on inorganics, not sure if I have missed out something regarding the theory etc?

Any suggestions are greatly appreciated 

[wildfyr]

Potentially some lithium carbonate contamination in there? Alkali hydroxides tend to pick those up over time in an incompletely sealed bottle from CO2 in the air.

Its only soluble 1.29 g/100 mL, and is less soluble as temp goes up.

What color is the precipitate also... no way iron hydroxide or oxides are precipitating out?

[rap35]

Potentially some lithium carbonate contamination in there? Alkali hydroxides tend to pick those up over time in an incompletely sealed bottle from CO2 in the air.

Its only soluble 1.29 g/100 mL, and is less soluble as temp goes up.

What color is the precipitate also... no way iron hydroxide or oxides are precipitating out?

Is that still applicable to the hydrated form?
That's interesting and something I may need to look into further. I have a brand new bottle currently in transit so I will be able to rule that out (if so).

The iron is in the reaction but the part that includes the LiOH.H2O is with Phosphoric Acid (H3PO4).

LiOH.H2O (3.147 g) is dissolved in H2O (50 mL) and H3PO4 (1.454 mL) is introduced dropwise to form Li3PO4. The precipitate is white. Iron sulphate (6.9505 g) is then added to the solution which is stirred under nitrogen.

[wildfyr]

Certainly, the hydrate is fully capable of reacting with CO2 to give the carbonate.

I would think adding acid would help dissolve any carbonate though... have you done that? You'd probably notice some bubbles forming on acid addition if there was significant carbonate.

For an old bottle is entirely possible that its almost all lithium carbonate. So a new bottle is definitely the way to go.

[Babcock_Hall]

IIRC the trilithium form of phosphate is not very soluble.  Is this the white precipitate that you mentioned?  It it is possible to prepare lithium hydroxide that is free of carbonate, but it is a bit of trouble.

[rap35]

Certainly, the hydrate is fully capable of reacting with CO2 to give the carbonate.

I would think adding acid would help dissolve any carbonate though... have you done that? You'd probably notice some bubbles forming on acid addition if there was significant carbonate.

For an old bottle is entirely possible that its almost all lithium carbonate. So a new bottle is definitely the way to go.

I haven't tried adding the acid unless the LiOH has dissolved fully. I have scaled down the reaction by making the FeSO4 concentration lower to account for using less LiOH that does dissolve. I haven't tried dissolving the carbonate upon the addition of acid. I think that would introduce issues with yield and contamination? Perhaps it would be best to not have the carbonate involved at all. I guess the best route would be to use a new bottle, which may take a few weeks to arrive but I will update as soon as it's received! Thank you!

[rap35]

IIRC the trilithium form of phosphate is not very soluble.  Is this the white precipitate that you mentioned?  It it is possible to prepare lithium hydroxide that is free of carbonate, but it is a bit of trouble.

That's correct, the white precipitate isn't very soluble, but the issue arises prior to the formation of the precipitate. I need to add H3PO4 to a fully dissolved solution of LiOH.H2O, however, the LiOH.H2O isn't dissolving fully. The mass I'm using is much lower than the solubility value per L, so there is no reason for it not to dissolve. As Wildfyr suggested, it may potentially be carbonate CO2 contamination. 

[Babcock_Hall]

One can prepare LiOH from degassed water and lithium metal.  I did this some years ago, but I don't believe that I assayed directly for carbonate.

[wildfyr]

That is hella overboard for this application Babcock!

[rap35]

Update: after ordering a new batch of LiOH.H2O I can confirm that the product is now entirely dissolving  I assume Wildyr's suggestion (CO2 contamination resulting in lithium carbonate) was the issue.