Purification of a DIY test reagent

[UK_Pete]

Hi Randy, this is a bit off topic for a reef forum but I'm pulling my hair out and am about to give up so I thought I would see if you had any ideas. Ifs its too OT please feel free to delete.

I'm trying to do the standard ascorbic acid test but I think I have a problem with a reagent. I've got ammonium molybdate, potassium antimony tartrate ACS (sigma 11126), very pure HCl, and ascorbic acid of unknown purity (apparently '100%' - pharm grade). I've also got a salifert PO4 test kit.

Problem is as soon as I mix the HCl and ammonium molybdate and either ascorbic acid of my own source, or the powder reagent in the salifert kit which I assume is ascorbic acid, I get a blue developing fairly quickly. Thats without adding the potassium salt even (which is a catalyst in this reaction). I've concluded through a few tests that the ammonium moly might be the problem. I dissolved a bit in DI water today and had a residue (despite it being sold to me as analytical grade, but no sigma packaging so I don't know).

I was hoping to recrystalise it but its really soluble and I think theres too little solvent (water) left after this to really purify it much. I found a reference to a group who are doing phosphate analysis in oceans measuring very low levels of phosphate, nanomolar (they are using long photometer cells to get more sensitivity). They say (link below):

'Ammonium heptamolybdate tetrahydrate was recrystallized from ethanol to remove possible remaining phosphate and other impurities.'. So I thought I can do this. But I can't find any reference to ammonium moly solubility in ethanol apart from 'insoluble', it seems pretty insoluble to me after I put 5 mg in 10cc of ethanol and heated it. Do you think they are talking about a binary solvent with water and ethanol? A drop of water containing 1g amm moly in 20 ml water added to 10 ml of ethanol instantly turned the ethanol milky and has not settled so far. Adding a total of 10ml of water to the milky ethanol has not cleared the solution.

This 'recrystalised from ethanol' - do you have any ideas what it might mean? Ive scoured the web for that phrase but found nothing. Perhaps I should put a container of saturated amm moly solution in a larger beaker of ethanol and let the ethanol vapor diffuse into the water. Or add ethanol to a water solution of the amm moly until I get a trace of precipitation and then add enough water to turn it clear again, and let it evaporate? Any ideas would be hugely apprecicated as I was really looking forward to doing several tests a day as I bring my po4 down and then monitor (I want to try this capiliary method too for better sensitivity).

Edit: I mean, I know what 'recrystalised from ethanol' would usually mean, in the context of something soluble in ethanol. But with something insoluble in ethanol what does it mean. A solvent mix with the water part being so obvious they did not bother to mention it because any chemist knows they mean a mix with water? Etc.

Thanks, Pete

http://mel.xmu.edu.cn/upload_paper/201255151615-hykFl9.pdf
pg 152 section 2.1, 'reagents and solutions'.

 

[Randy Holmes-Farley]

Recrystallized from ethanol would normally mean 100% ethanol, and it is ok if it is insoluble in room temp ethanol if it dissolves in boiling ethanol.

But, they nave have used some sort if denatured version without bothering to mention it. Adding a little water may be ok, but just enough to dissolve the material when boiling. Then cool very slowly.

 

[UK_Pete]

Hi Randy,

Well I spent much of yesterday trying various things, even a few specks of the ammonia moly compound would not dissolve in 30 ml of boiling ethanol, even with extended time. So I started adding water, in the end it too about a 50% mix of water and ethanol to dissolve those few specks. Once I got to that point I added half a gram of the amm moly, and continued adding water until it dissolved (that only took another few cc of water strangely). I stopped short of enough water to dissolve all the amm moly, so there was a little powder left. After another 10 mins gentle boiling there was still a residue so I was satisfied I had a crystal seed. But on cooling it the residue dissolved! So it must be more soluble in cold than hot ethanol / water mix!

If I leave it to evaporate the alcohol will evap faster, so the salt will become more soluble. I could saturate the cold solvent and then heat it, maybe that would result in some crystalisation, but if I boil any solvent off it will again become richer in water (weaker in alcohol) so I wouldent get much end product.

I was wondeing about something I read on wiki, about letting one solvent (alcohol) diffuse in to the liquid. That way the salt should slowly become less soluble. Never done this though, so I wondered if you had any advise about it - is it too tricky to do?

I am chasing up sources to get hold of the reagent in a more pure form so that might save me.

EDIT: Actually I just thought, maybe I can put the beaker of water / ethanol / barely dissolved amm moly in a larger beaker filled with calcium chloride dihydrate, cover it, and let the calcium chloride absorb the moisture from the air above the solvent mixture, drawing it out of the beaker. Would that work? Or would the alcohol hold on to the water more strongly than the calcium chloride would absorb it?

Thanks, Pete