[tver2007]

I followed this method for producing lactic acid (https://pubs.rsc.org/en/content/articlelanding/2017/gc/c6gc02443b/unauth), through this production pathway: (https://pubs.rsc.org/image/article/2017/GC/c6gc02443b/c6gc02443b-s1_hi-res.gif). How would we go about extracting the lactic acid from this mixture, as there is also glyceraldehyde, fructose, barium hydroxide and glucose. After the reaction, a white layer of solid sediment formed on the bottom of the flask, which judging by the volume we think is the barium hydroxide. Any help is appreciated.

[rolnor]

That sounds not so easy, kan ju make a insoluble salt from it? Maybe a calcium salt? Is it really barium hydroxide together with lactic acid, do they not react to form barium lactate?
Calcium lactate is very soluble in methanol, insoluble in ethanol, you can use these properties to purify the calcium lactate, then you add some dilute sulphuric acid to form gypsum and lactic acid.

[tver2007]

correct, the method said the add an acid in order to make the lactic acid from the bariumlactate, but won't the acid react with the hydroxide ions from the barium hydroxide added?

[Aldebaran]

I would expect dilute sulfuric acid to react with any barium ions present and form a precipitate of barium sulfate which is pretty much insoluble. But maybe I’m missing something.

[rolnor]

The calcium lactate needs to be purified first, thats why I suggested to use its solubility properties eg MeOH/EtOH But I think you already have the lactic acid in its barium salt form and that makes thing more complicated. You cab liberate it first by adding dil. H2SO4 Barium sulphate is insoluble in almost everything

[tver2007]

We've added a lactic acid solution and a calcium hydroxide solution together, but no precipitate formed. We've also noticed the calcium hydroxide didn't dissolve well in water at all. We had to be careful not to saturate the solution. The lactic acid solution has a concetration of 88% and the calcium hydroxide solution ended up being saturated.
So, does anyone have any idea as to why the lactic acid didn't form a precipitate with calcium? 

[Aldebaran]

@OP
As alraedy stated calcium hydroxide is not very soluble. However it does dissolve to some extent (used as limewater to test for CO₂ for example). When mixed with lactic acid calcium lactate will be formed which is quite soluble so you would not expect to see a precipitate.
I must say I find your train of thought a bit confusing. Your original post seems to be about purifying the lactic acid formed from the given experimental procedure which seems to be quite specific about using barium hydroxide and liberating the lactic acid by adding hydrogen ions. If the acidification was done using sulfuric acid (which is not actually specified in your post) then I would expect your ppt to be barium sulfate thus leaving any excess of sulfuric acid and unwanted organic contaminants to  be dealt with by other means.

[rolnor]

@OP
As alraedy stated calcium hydroxide is not very soluble. However it does dissolve to some extent (used as limewater to test for CO₂ for example). When mixed with lactic acid calcium lactate will be formed which is quite soluble so you would not expect to see a precipitate.
I must say I find your train of thought a bit confusing. Your original post seems to be about purifying the lactic acid formed from the given experimental procedure which seems to be quite specific about using barium hydroxide and liberating the lactic acid by adding hydrogen ions. If the acidification was done using sulfuric acid (which is not actually specified in your post) then I would expect your ppt to be barium sulfate thus leaving any excess of sulfuric acid and unwanted organic contaminants to  be dealt with by other means.

You completely missunderstod me, you need to use the solubility of calcium lactate in MeOH and its non-solubility in EtOH to purify the calcium lactate, I just gave you a hint how to do this, its not simple but a way to solve the problem. And, as I also said, You have to consider the barium hydroxide.  And of course calcium hydroxide has little solubility in water, yes. You can not just add calcium salts to the solution

[tver2007]

could calciumchloride potentially be used in stead of calciumhydroxide to form calciumlactate?

[rolnor]

Hydrocloric acid would be liberated, and thats a much stronger acid then lactic acid, so this does not work, but calcium chloride is soluble in alcohols, so if you add a weak base, like treathylamine to a solution of lactic acid and calciumchloride in ethanol, then calcium lactate might precipitate. But what is the pH of you soltuíon? If you have free lactic acid it should be acidic, I suspect that you have pH above 7 because you ran the reaction in presence of barium hydroxide?

[tver2007]

yes the pH is now above 7. We have filtered out most of if not all of the solid Bariumhydroxide that was left behind, so  the next step is adding sulfuric acid. Would we have to add acid until neutralised or do we have to calculate the amount needed to free up all the barium lactate that formed?

[rolnor]

If you get it to pH 2 then most of the lactic acid will be i free form. You probably get a precipitate from barium sulphate

[tver2007]

thanks, that helps. How exactly did you arrive at a pH of 2?
And how would use ethanol and methanol to isolate the calcium lactate?

[rolnor]

pKa is 3,86 for lactic acid and at pH 2 there will be mostly free acid in you mixture.
If you have the mixture at pH 2, you can evaporate it (a lot of work, tedious to evaporate water) then you can dissolve the solid residue in MeOH, and filter this and evaporate again. Then you can wash this solid with EtOH and since calcium lactate is insoluble in EtOH, all impurities should be dissolved and the remaining solid should be calcium lactate (in the best scenario).
Then you can liberate your lactic acid with dilute sulfuric acid and the calcium sulphate formed will precipitete (its gypsum) and can be removed by filtration. Here you need to use exactly 2 equivalent sulphuric acid, otherwise you get lactic acid contaminetad with sulphuric acid Calcium lactate is really calcium dilactate. There is a risc that these filtations will be problematic because very fine particles are formed, so you can use Celite to facilitate the filtration. To extract a polar molecule like lactic acid from a bioorganic mixture like this is often problematic and takes long time, it would be best if you had a published procedure.