I am going to establish the pH of montmorillonite or bentonite dispersion at a previously set ionic strength (e.g., 5 mM NaClO4) in order to study their aggregation subsequently over a period of several days. However, once I adjust the pH with 0.1 M HCl/NaOH, immediately afterwards it changes, e.g., from 8 to 7.5 within a few minutes in the case of montmorillonite, or from 9, 7 and 5 to 8.3, 7.6, and 6.4, respectively, within several days in the case of bentonite.
The use of standard buffer solutions may not be appropriate for these experiments, since the buffer solution can change the surface characteristics of colloids as well as having no significance to the real environment.
I would highly appreciate it if somebody could suggest a simple and fast way to maintain the pH for this kind of experiment.
Hi Peyman,
This is not an uncommon phenomenon as we also observe changes in pH when engineered nanomaterials are dispersed in aqueous media. The underlying mechanisms have been discussed in the literature and you can read up on that. This is why we often use buffers when studying aggregation to keep pH as constant as possible. You are right in that buffers themselves may affect the surface properties of your colloids but you may not be totally right to say buffers have no relevance in the real environment. It is well known that natural water systems are typically well-buffered (e.g. carbonate buffer system). My advise is that you carefully choose a buffer that does not change the surface properties of your colloids too much and is relevant to the real environment. Or you could keep adjusting the pH of your clay materials till you reach equilibrium but that way you must have also changed their surface properties significantly.
Best wishes.
Hi Adeyemi,
Thank you very much for your informative answer. I agree that clay particles have buffering capacity by itself, especially an important feature of montmorillonite and bentonite is their buffering capability at pH ~6.5 and ~8, respectively. On the other hand, I have not seen any paper in the literature that had used a standard buffer, what I exactly meant in my question, for clay suspensions. The reasons based on my understanding can be what I mentioned already. In addition, if I use a standard buffer solution which should certainly be at least 10 mM, because of the intrinsic buffering quality of the dispersion, then what will be the ionic strength?
The common practice is to keep adjusting the pH until it will be stabilized over a period of one week. Nevertheless, it is not practical for my experiments as there are many cases need to be adjusted. Moreover, as you kindly pointed out that this method also cannot guarantee not to change the surface characteristics as well as, to me, the ionic strength.
Very best
It looks like you already have natural buffers. pH's of 6.4, 7.5 and 8.3 are typical for carbonate buffers. Maybe you should work at these pH's or else try to remove the CO2 from your samples?
I tend to agree with Mr Zijlstra : if you are in contact with the atmosphere, you would only be measuring the dissolution of CO2 into your solution. In this case, I would try to add some calcite solid or bicarbonate in your system, the pH should stabilize around 8.5. But.... you may observe some Na/Ca exchange
In the case you want to measure or use specific value of pH, you have to work in a glove box to avoid this contact with atmospheric CO2, or to adapt you vessel so that between each measurement, it is tight to the atmosphere.
Depending of the range of pH you want to study, borate (BBS) may be a good candidate for slightly alkaline pH buffering.
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