Re: pH drift method for determining PZC of adsorbent

On 27 Dec 2005 06:32:38 -0800, "Farooq W" <farooq.w@xxxxxxxxx> 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?

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


>
>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."
.

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