I am doing liquid-liquid extraction experiments to measure the zeta potential of the aqueous phase (Synthetic solution with deionized water and 1300 ppm K and 25 ppm Li) as a function of pH at different concentrations of KCl to increase its electrical conductivity (50 and 100 mS/cm), all this because the increase in electrical conductivity improves the phase separation times, however, the zeta potential values do not agree with the literature, since we should have zeta potentials that are more neutral or close to the isoelectric point, and yet the values at high concentrations of KCl (100 mS/cm) give us very negative Zeta Potential values (up to values of -500 mV) Is there any way to explain these results without contradicting the scientific advances in this area?
Dear friend Jorge Romero
It is uncommon to observe such negative values of zeta potential in liquid-liquid extraction systems, especially at high concentrations of KCl where one would expect the surface charge to be close to neutral or at the isoelectric point. However, there could be several factors contributing to the unexpected results.
One possibility could be the formation of double layers with high charge density due to the high ionic strength of the KCl solution, leading to an increase in the negative zeta potential values. Additionally, the presence of other electrolytes in the solution or impurities in the synthetic solution could also affect the zeta potential values.
It is important to ensure that the experimental conditions are controlled and reproducible, including the pH of the aqueous phase, the concentration of KCl, and the measurement techniques used to determine the zeta potential. It may be helpful to repeat the experiments with different synthetic solutions or to consult with experts in the field to troubleshoot the experimental setup.
In summary, while the observed negative zeta potential values at high KCl concentrations may be unexpected, there could be several factors contributing to the results. Further investigation is necessary to determine the cause of the discrepancy between the experimental results and the literature.
What instrument are you using to estimate zeta potential and what is the molar concentration of the KCl? It might be that the salt concentration is too high for the instrument and is causing problems such as thermal convection and electrolysis at the electrodes.
John Francis Miller I am using an electrophoretic cell with two electrodes and a power source, the molar concentrations of KCl range from 0.1 to 1.2 M to achieve conductivity values of 100 mS/cm, indeed many bubbles are observed in one of the electrodes that increase As I condition the solution to higher conductivity values, I also assume that the use of the organic extractant p507 provides a negative charge but not enough for those values that I observe in my graphs.
Jorge Romero Thank you for the information. How are measuring the zeta potential with that arrangement? Most commercial instruments will struggle to make meaningful measurements at those KCl concentrations. Electrolysis products can adsorb onto your particles. Also, if you are using light scattering, the presence of bubbles may be interfering. i.e., the instrument may be detecting light dominated by that scattered by bubbles.
John Francis Miller The way in which it is measured is through the electrophoretic speed of the extractant drops that have already absorbed Li and K, applying a voltage they move to one of the electrodes and their movement is recorded by video, to determine their speed and later with the Eq. from Smoluchoswky, its electrophoretic mobility is obtained and thus the zeta potential is estimated.
The cell has an intermediate deposit and two electrodes on the sides (Where the bubbles can be seen) in theory the bubbles are not seen in the measurements, however it is observed that when the bubbles burst inside the cell, a sudden movement occurs in the measurements which makes these completely incorrect, so very low voltages have to be handled to delay this effect and to be able to measure its speed.
You need to be careful with interpreting your observations. Unless you are using platinized platinum (platinum black) for your electrodes, you will very likely have considerable electrode polarization. This means that the electric field that the particles experience is less than that you would calculate based on the voltage and electrode separation. If you are using a DC voltage, you will very quickly reach maximum polarization. Based in my experience, the voltage decrease due to polarization is constant (i.e., it is the same amount irrespective of the voltage applied).
Thank you very much for your observations, they are very useful, I always carry out my measurements with constant voltage, I never use constant current, I am going to check the electrodes, and see if I can find some way to measure a standard and adjust data.
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