Re: slight turbidity in solutions

From: Farooq (farooq_w_at_hotmail-dot-com.no-spam.invalid)
Date: 09/14/04 

Date: 14 Sep 2004 17:37:32 -0500

> Wilco Oelenwrote:
Hello,
>
> Usually, when soluble chemicals are dissolved in water, the
solution
> is completely clear, when the solid has dissolved completely. Some
> chemicals, however, do dissolve easily, but the solution is not
> completely clear. I know that some compounds hydrolyse to some
extent,
> forming hydroxide-compounds, which may make the solution a little
> cloudy. Examples of this are Fe(III)-salts and Cu(II)-salts. Much
> stronger examples of this are Sn(II) salts and Sb(III) salts.
>
> However, some chemicals also show the cloudy effect, while they are
> not hydrolysing to (as far as I know) insoluble compounds. Even on
> acidification the solutions remain cloudy.
>
> Examples of such chemicals are:
> sodium fluoride
> ammonium persulfate
> sodium persulfate
> nickel sulfate
>
> A solution of thiourea also is somewhat cloudy. This, however, on
> acidification becomes completely clear.
>
> Yet another example:
> The compound Fe(NH4)2(SO4)2.6H2O (Mohr's salt) dissolves easily and
> completely, but the solution at first is very cloudy, even when all
> solid has dissolved. Slowly, however, the cloudyness disappears and
> after a few minutes the solution is completely clear.
>
> I do not think, that the cloudyness is due to being close to
> saturation. If I add some solid to a solution, which is somewhat
> cloudy, then the solid does dissolve and the liquid remains
somewhat
> cloudy.
> I did the tests with distilled water, so I also want to rule out
> formation of insoluble calcium or magnesium salts.
>
> Is this cloudyness due to impurities in the chemicals or is there
> another effect, which I am not aware of. Is there something I can
do
> against it?
>
> If someone has any idea, I would be glad to read about it.
>
> Thanks,
>
> Wilco

Wilco,
Few thoughts on the turbidity of sodium and ammonium salts :
Did you check out the solubility data of sodium fluoride, ammonium
persulfate and sodium persulfate which I think are not highly soluble
in water. Secondly you might not be using the extra pure forms of
these salts, some manufacturers do label the percentage of insoluble
matter in their chemicals. BTW I would not dare to acidify sodium
fluoride solution.

As far as nickel sulfate is concerned, it does hydrolyse in water. The
degree of turbidity of transition metal solutions also depends on
thier concentration (solublity product principle-assuming a hydroxide
is formed), eg. a highly concentrated solution of copper nitrate will
begin to show cloudness after few minutes of its preparation, however
if the very same cloudy solution is diluted 100 times, cloudiness
disappears...the insoluble hydroxide now dissolves due to the
shifting of equilibrium towards the left:
Cu(2+) + OH(-) from water <-----> Cu(OH)2 (solid)

Distilled water kept in ordianry glass for a long time may have its pH
changed towards the basic side.

To a very dilute solution of copper containing ( 5 mg copper
nitrate/Liter) addition of a small amount base will not show any
precipitate formation. However after a day or two you might observe
cloudiness.

The disappearance of cloudiness of thiourea on acidfication can be
explained on the assumption that is a weak base. Addition of acid
might increase its solubility. Wilco, do you still work with this
nasty substance? I think it has been proven a human carcinogen.

Just yesterday I was making a standard solution of ferrous ammonium
sulfate Fe(NH4)2(SO4)2.6H2O for analysis but I couldn't obtain clear
solution without addition of an acid. The reason that came to my mind
was that sample was contaminated with some ferric salt (air
oxidation), because ferric salts immediately hydrolyse in solution.
The particles that I could see in the solution were brown...might be
ferric oxide or hydroxide.

However perhaps this can explain your obsevation: Since a solution of
Mohr slat is fairly acidic, any hydrolysis product might dissolve
after few minutes.

I don't think calcium and magnesium are contributing to cloudiness,
until and unless your water supply provides hard water, if this is
true then cloudiness of sodium fluoride can be attirbuted to the
presence of insoluble calcium fluoride.

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