If all salts were weighed correctly and precisely, pH does not need to
be adjusted. That's the whole point of those PBS recipes.
pH being off target is an indication that amounts of salts were
incorrect. A common problem is not taking into account the exact
crystalline form of phosphates - they are available as anhydrous or
several water-containing forms. The amounts to use depend on water
content. You may be having bigger problem than just precipitation
because molar concentrations you have may be incorrect since the pH is off.
The explanation for your precipitation: the 10M concentration of NaOH
that you use to adjust the pH is too high in case you have Ca or
especially Mg salts in the solution. No matter how fast you stir the
solution, the uniformity does not happen instantaneously. It takes at
least several seconds for the NaOH solution to mix uniformly. While this
is happening, there are some local areas where pH is still highly
alkaline (theoretical pH of 10M NaOH, assuming full dissociation, is
~15, in practice, apparent pH is close to 14, which means alkaline
concentration is seven orders of magnitude higher than your target!). At
this level of alkalinity, calcium and, especially magnesium hydroxides
precipitate. Calcium hydroxide dissolves relatively easy upon mixing,
magnesium - not so much. It forms very complex forms which dissolve very
slow at neutral pH. Furthermore, to complicate things even more,
magnesium phosphate, especially basic phosphate are very insoluble at
neutral pH, but they are very easily formed in the presence of phosphate
(regardless where it comes from) and hydroxide (regardless where it
comes from). And this is your precipitation which at neutral pH is
irreversible.
So, you have a buffer with wrong concentration of phosphate, wrong pH,
almost no magnesium, and probably much less calcium that it should be
(calcium phosphate is also insoluble, same concept as with Mg, but just
a bit more labile, but don't count on it for practical reasons).
- Check your phosphates first for hydration forms, and if they are
correct, you should not need to adjust pH, if it's off - your salts are bad.
- Confirm that containers where you keep phosphates are very tightly
closed, as exposure to air changes composition due to moisture in the
air, especially if original form was anhydrous.
- If you absolutely need to adjust pH, do NOT use NaOH with
concentration higher than 1M, and titrate VERY slowly, one very small
drop at a time and wait till pH stabilizes completely before adding
another drop.
This is all supposed to be very common basic knowledge in
inorganic/analytical chemistry which is a prerequisite for buffer
preparation.
Alternatively, there are plenty of commercial sources of PBS
================================
Dmitry Bochkariov, Ph.D.
Principal Scientist
Advansta Inc.
1505 Adams Drive, Suite B1
Menlo Park, CA 94025
(650) 325-1980 x530
================================
On 7/12/2017 5:21 AM, najib abdellaoui wrote:
hello
i was trying to make a PBS as usual and the component of it as following to
make 500 ml of it:
-Na2HPO4 0.575 g
-NaCl 4g
-KH2PO4 0.1 g
-KCl 0.1 g
-CaCl2-2H2O 0.065g
-MgCl2 0.0235
normally we dissolve Na2HPO4 first and we add the remaining components.
we check the pH which was around 6.8 and we added 10 M of NaOH however, we
observed formation of cloudy precipitate.
can someone explain why this formation of white precipitate?
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