Re: slight turbidity in solutions

From: Wilco Oelen (photo_at_woelen.nl)
Date: 09/15/04 

Date: 15 Sep 2004 06:27:21 -0700

farooq_w@hotmail-dot-com.no-spam.invalid (Farooq) wrote in message news:<414772ac$1_2@127.0.0.1>...> 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.
Ammonium persulfate and sodium persulfate are soluble very well.
Sodium fluoride is moderately soluble (3 to 4 g/l, depending on
temperature). Because you do not mention any other possible cause of
the cloudiness, I assume it is due to insoluble impurities.
Unfortunately I have no data sheets with the chemicals, I just bought
them in sealed plastic bags without any labeling or data besides the
name of the chemical, and hence I do not know the manufacturer. I have
no reagent grade chemicals to compare with.
As far as toxicity of HF is concerned, I know of the risks of this. I
do not think, however, that adding some acid to a dilute NaF solution
is dangerous. The liquid does not fume or smell in any way. Of course
I do not touch it and do not prepare or put it in a glass container.

>
> 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.
OK, I did not know that nickel also exhibited this behaviour. I'll
check whether the cloudiness disappears when some acid is added.

>
> 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.
Do you mean with this statement that weak bases dissolve a little
cloudy in many cases? I also observed similar behaviour with other
weak bases, such as sodium percarbonate, sodium carbonate and borax.
After a while, however, these compounds form clear solutions, while
thiourea remains cloudy. Sodium percarbonate may produce very small
bubbles of oxygen, but these look definitely different.

> Wilco, do you still work with this
> nasty substance? I think it has been proven a human carcinogen.
Yes, but not frequently. Maybe twice a year. It is used as a
brown/sepia toner in photography. It is a non-smelly alternative to a
toner bath, made of a solution of sodium sulfide. The principle behind
the thiourea toner is that it hydrolyses in alkaline solutions,
forming small amounts of free sulfide, sufficient for toning prints.
I've read the MSDS *** for thiourea, but sometimes I think MSDS
sheets are exaggerating. Of course, if one works in industry every
day, 8 hours/day with a certain chemical, then I think the MSDS's are
right, but for hobby use, just a few times a year, I don't think it is
that bad.

I wonder what MSDS's would say about cigarettes and tobacco if these
were introduced now ;-)

>
> 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.
In my solutions, I do not see brown particles, they are white (or very
light green). The salt I have is very pale green/blue. With pre-boiled
ferrocyanide solution it gives a nice light off-white precipitate and
not a blue one, indicating that indeed it hardly has any ferric ions
as contaminant.
As far as the hydrolysis products is concerned, why do they at first
appear? If the iron dissolves, then I assume it does this together
with the ammonium, and then I would not expect formation of hydrolysis
products, which disappear lateron. Or does Mohr's salt dissolve, first
the iron-part and lateron the ammonium part (I however, do not think
so)?

>
> 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.
We have hard water. A solution of sodium fluoride in this water
becomes cloudy at once, much more than what I described in the
original post. For this reason, I hardly do any experiments, nor any
photo work with tap water, except rinsing.

Having cloudy solutions is very annoying in some experiments and for
photo prints it also is bad. Glossy surfaces tend to look dull and
irregular if small particles have deposited on them. That's why I do
quite some effort in getting clear solutions and that's why I asked
the question about the chemicals I mentioned.

Wilco

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