SrCl2 hydrolysis
Kurt Grosshans
experiment on the hydrolysis of salts and the subsequent affect
on pH, I found that strontium chloride was not behaving the way
I thought it should. The hydorlysis of this salt should yield a
weak base and a strong acid thus lowering the pH right? Why
then are we coming up with a neutral pH? I have tried different
SrCl2 samples and have measured the pH with both paper and a
probe hooked up to an IBM computer. I keep getting a reading of
about 6.8 to 7.1. Any solutions? (Forgive the pun).
---
Kurt Grosshans
Kgro...@radford.vak12ed.edu
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"Sometimes the light's all shinin' on me.
Other times I can barely see.
Lately it occurs to me,
What a long, strange trip its been."
The Grateful Dead.
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Stephen Cheng
>While having my first year chemistry students perform an
>experiment on the hydrolysis of salts and the subsequent affect
>on pH, I found that strontium chloride was not behaving the way
>I thought it should. The hydorlysis of this salt should yield a
>weak base and a strong acid thus lowering the pH right? Why
>then are we coming up with a neutral pH? I have tried different
>SrCl2 samples and have measured the pH with both paper and a
>probe hooked up to an IBM computer. I keep getting a reading of
>about 6.8 to 7.1. Any solutions? (Forgive the pun).
I believe that the acidity of the aquated metal ions, especially for the
main group metals, follow the trend that the acidity decreases down the
group. I think that Sr, being an alkali earth, really has not much
tendency to follow the reaction:
Sr(H2O)n --> Sr(OH)2(H2O)n-2 + 2H+ (this is actually an
equilibrium lying far to the left)
For something like TiCl4, there is a much greater tendency to form
Ti(OH)4. Try throwing a drop or two into water and see what happens...
Stephen Cheng (sch...@unixg.ubc.ca)
gozdz,antoni s
>While having my first year chemistry students perform an
>experiment on the hydrolysis of salts and the subsequent affect
>on pH, I found that strontium chloride was not behaving the way
>I thought it should. The hydorlysis of this salt should yield a
>weak base and a strong acid thus lowering the pH right? Why
Wrongo.
pKb of SrOH+ is 0.7, and is very sensitive to ionic
medium. So you're dealing with a salt of a strong acid
and a strong base... Check the CRC Handbook of Chemistry
and Physics, p 8-39 (72nd ed.) for more details.
>then are we coming up with a neutral pH? I have tried different
>SrCl2 samples and have measured the pH with both paper and a
>probe hooked up to an IBM computer. I keep getting a reading of
>about 6.8 to 7.1. Any solutions? (Forgive the pun).
>
>---
>Kurt Grosshans
>Kgro...@radford.vak12ed.edu
Tony Gozdz
to...@cc.bellcore.com
Michael Moroney
>While having my first year chemistry students perform an
>experiment on the hydrolysis of salts and the subsequent affect
>on pH, I found that strontium chloride was not behaving the way
>I thought it should. The hydorlysis of this salt should yield a
>weak base and a strong acid thus lowering the pH right? Why
>then are we coming up with a neutral pH? I have tried different
>SrCl2 samples and have measured the pH with both paper and a
>probe hooked up to an IBM computer. I keep getting a reading of
>about 6.8 to 7.1. Any solutions? (Forgive the pun).
Merck Index states that SrCl2 forms a neutral solution, which is in
line with what you're seeing. I believe Sr is a strong base, not
a weak one.
-Mike
Kurt Grosshans
> World Public Access UNIX, Brookline, MA
> References: <1993May20.0...@radford.vak12ed.edu>
> Date: Thu, 20 May 1993 21:29:25 GMT
> Lines: 17
>
> kgro...@radford.vak12ed.edu (Kurt Grosshans) writes:
>
> >While having my first year chemistry students perform an
> >experiment on the hydrolysis of salts and the subsequent affect
> >on pH, I found that strontium chloride was not behaving the way
> >I thought it should. The hydorlysis of this salt should yield a
> >weak base and a strong acid thus lowering the pH right? Why
> >then are we coming up with a neutral pH? I have tried different
> >SrCl2 samples and have measured the pH with both paper and a
> >probe hooked up to an IBM computer. I keep getting a reading of
> >about 6.8 to 7.1. Any solutions? (Forgive the pun).
>
> Merck Ind
I would like to thank everyone who responded. As usual, my
first step should have been to dive for the CRC. Instead, I
opted to believe the lab manual which was incorrect.
Thanks again.
Martin Meints
> While having my first year chemistry students perform an
> experiment on the hydrolysis of salts and the subsequent affect
> on pH, I found that strontium chloride was not behaving the way
> I thought it should. The hydorlysis of this salt should yield a
> weak base and a strong acid thus lowering the pH right? Why
> then are we coming up with a neutral pH? I have tried different
> SrCl2 samples and have measured the pH with both paper and a
> probe hooked up to an IBM computer. I keep getting a reading of
> about 6.8 to 7.1. Any solutions? (Forgive the pun).
>
The lowering of pH is the result of the reaction
Sr2+ + H2O ----> Sr(OH)+ + H+.
But this reaction is not of real importance. SrCl2 has a small
activity coefficient (0.511 at 0.1 M and 0.461 at 1.0 M at 25øC)
so it is not completly diluted. And Sr(OH)+ is a stronger base
as you might expect. I only could find the pKb of Ca(OH)+; it is
1.40 at 30øC, so Ca(OH)+ is a strong base. And Sr(OH)+ is even
stronger.
For this infomation look for example at R. C. Weast (ed.),
"Handbook of Chemistry and Physics", 67th edition, CRC Press,
Boca Raton 1985, p. D-174 and D-165.
Did you look at the pH of your water? Sometimes it is not 7. If
you didn't use destilled or demineralized water you often find
a pH between 7.3 and 7.8 cause of soluted CO32- and HCO3-. And
demineralized water has often a pH around 6.5.
Try Mg(Cl)2, you will find a stronger lowering of the pH.
Martin Meints