pH drift method for determining PZC of adsorbent
Farooq W
zero charge (the point at which the net charge on surface is zero) of
adsorbent. I tried to repeat the procedure below with my adsorbent
(carbonized corncobs), yet what I observed that the trend is that the
final pH was always lower than the initial one, hence the curve so
obtained by plotting initial pH versures final pH does not cross x=y
line.
Is there a possible explanation?
Langmuir 2004, 20, 6736-6741
"The points of zero charge (pH PZC values) of WC and AC were determined
by the pH drift method. The method, originally developed for activated
carbon, provides a quick yet reliable
determination of pHPZC. The method is presumed to be suitable for WC
because any potential side effects on the pH drift from the dissolution
processes of salts in wheat char have been eliminated during the WC
enrichment. A solution of 0.005 M CaCl2 was boiled to remove dissolved
CO2 and then cooled to room temperature. The pH was adjusted to a value
between 2 and 10 using 0.5 M HCl or 0.5 M NaOH. AC or WC (0.06 g) was
added into 20 mL of the pH-adjusted solution in a capped vial and
equilibrated for 24 h. The finalpH was measured and plotted against the
initial pH. The pH at which the curve crosses the
pHinitial ) pH final line is taken as pH PZC."
Bob
>Does anybody know other (simple) methods of determining the point of
>zero charge (the point at which the net charge on surface is zero) of
>adsorbent. I tried to repeat the procedure below with my adsorbent
>(carbonized corncobs), yet what I observed that the trend is that the
>final pH was always lower than the initial one, hence the curve so
>obtained by plotting initial pH versures final pH does not cross x=y
>line.
>Is there a possible explanation?
>
The naive explanation is that your material is highly acidic. Do you
know for sure otherwise?
It would be useful to know the kinetics of the pH drift. There is
nothing magic about their choice of 24 hours. Presumably they found it
suitable for their material (and convenient). But it need not be
appropriate for all. More importantly, kinetics may reveal if
something else is going on (e.g., biphasic kinetics, possibly
indicating a chemical reaction). I suppose I would explore by taking 3
pHs (lo, medium, hi), and measuring the pH at intervals as convenient
-- short intervals at first, then longer once you get a feel for what
the data look like.
The effect of temperature might also be revealing if there is a
reaction going on.
bob
Farooq W
Bob wrote:
> On 27 Dec 2005 06:32:38 -0800, "Farooq W" <faro...@gmail.com> wrote:
>
> >Does anybody know other (simple) methods of determining the point of
> >zero charge (the point at which the net charge on surface is zero) of
> >adsorbent. I tried to repeat the procedure below with my adsorbent
> >(carbonized corncobs), yet what I observed that the trend is that the
> >final pH was always lower than the initial one, hence the curve so
> >obtained by plotting initial pH versures final pH does not cross x=y
> >line.
> >Is there a possible explanation?
> >
>
> The naive explanation is that your material is highly acidic. Do you
> know for sure otherwise?
Indeed it is acidic (treated with H2SO4) but I had washed so many times
with water (approx with 300g material 15 L) that I assumed all free
acid has been removed.
Finally, I have thought of a last resort. I have treated the suspension
with dilute NaOH to neutralize any free remaining acid (the pH of
slurry was 10-11) and then with dilute HCl to restore acid
functionalaties on its surface. I would wash it again and again over a
Buchner funnel till the washing give no ppt with AgNO3. Any residual
acidity then, would be due to material only and fortunately HCl is
volatile. Drying under partial vacuum conditions would further remove
any acid-presumably. Let's see if this pretreatment gives sensible
results. This is what the following authors have done except they did
not treat the wheat carbon with base.
rek...@gmail.com
point (pI or IEP) or pH where zeta potential = 0. Are you specifically
trying to measure it for carbon or are you just trying to demonstrate
the concept? This sounds like a simplified version of this type of
measurement (zeta potential is usually directly measured with various
electrokinetic techniques, which can require some semi-complicated
setups).
Your carbon is probably pretty dirty and could be slowly releasing
residue salts into solution. It doesn't necessarily have to be
something acidic leaching out, because the presence of salts will
change the surface charge properties by providing ionic species that
can adsorb other than H+ and OH-. The authors of the cited paper
specifically have to qualify that the method can be used for carbonized
wheat because of 'enrichment', whatever that is, to eliminate pH drift
from dissolution of salts.
Farooq W
rek...@gmail.com wrote:
> I'm assuming "point of zero charge" is the same as the isoelectric
> point (pI or IEP) or pH where zeta potential = 0. Are you specifically
> trying to measure it for carbon
Yes for the carbon made from corncobs.
>or are you just trying to demonstrate
> the concept? This sounds like a simplified version of this type of
> measurement (zeta potential is usually directly measured with various
> electrokinetic techniques, which can require some semi-complicated
> setups).
I am not aware of eletrokinetic techniques. Are there any common ones
which can be easily done? But the pH drift method is quite common with
those who are into adsorption studies, since most of authors who have
reported PZC have utilized this method. Some call it suspension tests.
Even the suspension time is variable, some soak for 24 hours some for 6
hours.
>
> Your carbon is probably pretty dirty and could be slowly releasing
> residue salts into solution.
It is indeed dirty. I assume this is residual sulfuric acid leaching
in, the corncobs wer carbonized with sulfuric acid and heated at 150
oC.
>It doesn't necessarily have to be
> something acidic leaching out, because the presence of salts will
> change the surface charge properties by providing ionic species that
> can adsorb other than H+ and OH-. The authors of the cited paper
> specifically have to qualify that the method can be used for carbonized
> wheat because of 'enrichment', whatever that is, to eliminate pH drift
> from dissolution of salts.
Yes, this is what they have said.
rek...@gmail.com
look up methods of doing this. I think a multistage water extraction
(under reflux?), then filtering the carbon, might be a good thing to
try. But stuff sticks to carbon pretty strongly, and carbons tend to
have huge surface areas with inumerable little pores, so I don't know a
good way of dealing with it.
The electrokinetic techniques are things like electrophoretic light
scattering, viscous flow effects through parallel charged plates, etc.
Typically you have a colloidal dispersion, and you're just measuring
the mobility of the particles as a function of pH. The mobility will
be directly proportional to surface charge. There are things out there
called 'ZetaProbe' etc, which beam a laser through your sample and
count particles passing in front of a photodiode with time, under an
applied electric field. This gives you an average particle velocity,
therefore a mobility. So a relatively complex setup. None of this is
really necessary for what you're doing.