Hi,
I am interested in performing a colloid titration, as a back-titration, for an anionic sample of bentonite clay and water (1g/L clay suspension). The reason being, to estimate the difference in charge magnitude between samples of clay-water suspensions.
I am using methyl glycol chitosan (MeGch - N/200) and polyvinyl alcohol sulfate (PVSK - N/400) as the titrants. And Toluidine Blue as the indicator.
Numerous papers on the topic state that if the sample's charge is neutralized by the added polyelectrolytes of the opposite charge, the polyelectrolytes tend to associate and eventually precipitate. This is the first step of the titration, where in my case I've been adding MeGch to the clay-water sample before adding Toluidine Blue and PVSK. However, I have not been able to see any precipitation of MeGch in the sample. Therefore, I am unsure if I've reached the correct point of neutralization to proceed with the rest of the titration steps.
I suspect that perhaps, the sample concentration of clay-water is too high and that of MeGch is too low for the reaction scheme.
What might be the reason behind the absence of MeGch precipitate formation? And what might I troubleshoot for to correct for any possible error?
And if anyone knows of a better method (or alterations to this method) to determine charge differences in anionic colloidal solutions without the use of special apparatus/meters, I would love to hear your suggestions.
All feedback is greatly appreciated!
Thank you
- Yana
Hi Yana Snisarenko , it sounds like you may not be neutralizing the charge. Are your concentrations close to the ones in the publications? Do you have access to an instrument to measure the zetapotential of your sample? This might help to see the effective charge. It is also possible that the titration went "too far" and re-stabilized your sample at the opposite charge....
Hi Ulf Nobbmann , thank you so much for your input!
Unfortunately my lab does not have any instrument to measure zeta potentials.
My selected concentration of clay was chosen because I wanted to simulate natural concentrations of this compound in water. As a result, the concentration of my colloid was slightly higher than literature values. So, I was expecting to use more MeGch in order to neutralize my particular sample charge.
In further trials with a lower clay concentration I have now been able to get precipitation of MeGch. However, the step that I am facing some difficulty with now, seems to be my indicator solution. Where now, despite the precipitation of MeGch (which should indicate neutralization of my colloidal clay sample) the addition of Toluidine Blue produces a purple rather than blue solution colour.
And so I am in the process of further troubleshooting this next step.
Again, thank you for your help. It's much appreciated!
Dear Yana,
When your sample has Toluidine blue in is purple color, it means the indicator is in its metachromatic state i.e. in a stacked conformation bound to an anionic charged species. Toluidine blue is able to stack on surfaces or species which have anionic charges not further apart than 0.45 nm. When it is in this stacked state its color is purple. Free Toluidine blue is indeed blue. So, it seems that you did not neutralize all anionic charges. The fact that you still see precipitate from the MeGch could indicate that in stead of neutralizing all charges on your clay species you had a pH shift causing the MeGch to become uncharged (and hence precipitated). I would measure the pH at all stages of the process to see which changes occur that may lead to your observations. Next, please also measure conductance to get an idea of the ionic strength you are working at. Low ionic strength is preferred for Toluidine blue to work well. Feel free to check out my publications in my profile. I worked a lot with Toluidine blue.
Hoping this helps you understand what is going on.
Kind regards, Leo
Dear Leo Vleugels ,
Thank you for your response, it's given me some great suggestions for further investigation.
I was definitely unsure about the particular effects of pH on MeGch, so I did not think a pH shift could have also caused the precipitation of the titrant. And I will look into the ionic strength of my samples as well, that is another important factor I did not consider.
I should have looked further into the chemistry of all my reagents, and I really appreciate the input you've provided as guidance for troubleshooting my titration.
Thank you again,
Yana