making some potassium sodium tartrate

[KOLM]

Hi,

 

I have some tartaric acid crystal. Can anyone help me determine if my calculations are correct for making the rochelle salt? I figured for every mole in weight of the tartaric, I add each a mole of sodium hydroxide and potassium hydroxide, and stir in DI water. Maybe check for pH. Does that make sense? 

Thanks for your advice and expertise.

 

[chenbeier]

Sounds ok

[exchemist]

11 hours ago, KOLM said:

Hi,

 

I have some tartaric acid crystal. Can anyone help me determine if my calculations are correct for making the rochelle salt? I figured for every mole in weight of the tartaric, I add each a mole of sodium hydroxide and potassium hydroxide, and stir in DI water. Maybe check for pH. Does that make sense? 

Thanks for your advice and expertise.

 

Hmm, not sure about this. How do you know you will get the mixed metal salt,, rather than a random mixture of sodium and potassium tartrates? According to WIKI, Rochelle salt is made by adding NaOH to potassium bitartrate, i.e. potassium hydrogen tartrate, up to a pH of 8. This suggests you need a 2 step process, first generating the K bitartrate and then treating with NaOH. 
 

Generating the bitartrate suggests to me a process of gradually adding dilute KOH solution and stopping while the mixture is still acid, to make sure you only neutralise one end of the molecule. I have not looking into what pH that would be but no doubt you can look up the pKas for tartaric acid. The usual source of K bitartrate is the crystals naturally deposited in white wine barrels. There is potassium in the grapes and the acidity of wine is appropriately mild. 

Edited Thursday at 07:25 AM by exchemist

[chenbeier]

That is what he is doing.

In solution you have Na+, K+ and the [OOCCHOHCHOHCOO]2 - present.

It doesn't matter you add NaOH or KOH first to the tartratic acid.

Also possible you mix 1 mol sodium tartrate with 1 mol potassium tartrate in solution.

[exchemist]

2 hours ago, chenbeier said:

That is what he is doing.

In solution you have Na+, K+ and the [OOCCHOHCHOHCOO]2 - present.

It doesn't matter you add NaOH or KOH first to the tartratic acid.

Also possible you mix 1 mol sodium tartrate with 1 mol potassium tartrate in solution.

Quite right I was talking nonsense, once the acid is fully neutralised it’s just a solution with equal numbers of Na and K cations. Serves me right for trying to do chemistry with brain fog from my ‘flu’. 
 

But will crystallisation produce just Rochelle salt, or a mixture of Na/Na, K/K, and Na/K tartrates, and if just Rochelle salt why? Is it a happy accident of thermodynamics that the mixed salt is preferred? My instinct would be that either Na or K would fit the lattice better and the mixed salt might be disfavoured relative to the one with the better fit.

[KJW]

4 hours ago, exchemist said:

But will crystallisation produce just Rochelle salt, or a mixture of Na/Na, K/K, and Na/K tartrates, and if just Rochelle salt why? Is it a happy accident of thermodynamics that the mixed salt is preferred? My instinct would be that either Na or K would fit the lattice better and the mixed salt might be disfavoured relative to the one with the better fit.

This is reminiscent of the crystallisation of a racemic mixture, which can either crystalise as the racemate or as a mixture of enantiomeric crystals. This is especially relevant to this thread because the first resolution of a racemic mixture was of sodium ammonium tartrate by Louis Pasteur who manually separated the individual enantiomeric crystals into separate piles. It is said that he was quite fortunate to have found a racemic mixture that crystallises as a mixture of enantiomeric crystals because most racemic mixtures crystallise as the racemate.
 

 

[Ammaniya]

Haha, I guess your calculations seem generally correct! Just a comment: when mixing tartaric acid with sodium and potassium hydroxides, ensure the pH is carefully monitored. It's crucial for proper Rochelle salt formation. I'm quite surprised by how straightforward the process can be!
Good luck!

[John Cuthber]

4 hours ago, exchemist said:

My instinct would be that either Na or K would fit the lattice better and the mixed salt might be disfavoured relative to the one with the better fit.

The rochelle salt lattice only really works with equal numbers of both cations (you can substitute some NH4 or Rb in place of K if you like).
 

[KOLM]

Thanks, members. So helpful. I will get back to yous for more detailed numbers..........

[exchemist]

38 minutes ago, John Cuthber said:

The rochelle salt lattice only really works with equal numbers of both cations (you can substitute some NH4 or Rb in place of K if you like).
 

Aha, so that would I suppose indicate that formation of the rochelle salt is in fact favoured thermodynamically, compared to the the two unmixed salt structures. Interesting. 

And of course what it is known for is its piezo-electric properties. I'll have to look up how all that arises.

(I'm just old enough to have had at one time a  hi-fi amplifier with 2 inputs for record player cartridge inputs, one for magnetic cartridges and one for crystal or ceramic ones.  Presume rochelle salt may have been use in the latter.)