Re: Crystals in PMDD

[Paul Sears <psears at nrn1_NRCan.gc.ca>]

> From: "Jamie Johnson" <jjohnson at davisfloyd_com>
> Subject: Re: Crystals in PMDD
> 
> Tom was asking about crystals in his PMDD:
> 
> > From: "Tom Brennan" <brennans at ix_netcom.com>
> > What is crystallizing out of my PMDD in the refrigerator? 
> > Mixed with 1/2 liter of water,
> >        1 tablespoon chelated trace mix (Homegrown Hydroponics)
> >        1 tablespoon MgSO4+7H2O
> >        2 tablespoons K2SO4
> >        1 tablespoon KNO3  (more or less, depends on nitrate)
> 
> Tom, I would suspect the K2SO4. MgSO4 has a high solubility, so 
> it's probably not that, and the KNO3 has a pretty high solubility of 
> 35.7g/100ml so you're probably safe there. K2SO4 has a maximum 
> solubility @ 25C of ~12g/100ml, which would be 60g in your 500ml 
> volume. I weighed two semi-heaping Tlbs of K2SO4 and it came 
> out to 35g. This is still lower than the max of 60g, but with the 
> combo of all the MgSO4 and KNO3, the saturation point of your 
> solution may have been reached.

	Ionic solids begin to precipitate when the solubility product
of the ions is exceeded.  For K2SO4, that means that it will come down
when the critical value of [K+][K+][SO4--] is exceeded.  If there is just
K2SO4 in the solution, solubility can be expressed as "12g per 100 mL", but if
you increase the K+ concentration (let's say by a factor of 2) then the
maximum (SO4--) concentration will fall (in that case by a factor of 4).
You have put K2SO4 in the solution, but you have also added more K+ (as
nitrate) and more SO4-- (as magnesium salt).  Exceeding the solubility
product of K2SO4 wouldn't be difficult, especially if the solution is
refrigerated.

-- 
Paul Sears        Ottawa, Canada

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