At the isoelectric point (IEP) of a suspension, where the ionic interface charge of a particle is zero, coagulation occurs, solubility is zero, no protonisation or de-protonisation and no dissociation occurs. In many applications it is important to determine the IEP, one of them being to produce particles with an IEP that will produce a stable suspension. Up to now, it was difficult to measure the IEP of powder suspensions. Usually, sedimentation is the limiting factor.
The Stabino® Charge Titration Method
In the Particle Metrix Stabino® 10 ml of a suspension are constantlyagit ated during measurement. This movement guarantees that the sample stays homogeneous whilst the measurement of the Stabino® potential is in action. The monitoring signal during the addition of charge modifying liquids (titration) is the electro-kinetic potential. This arises from displacing the ionic cloud (double layer) at the particles interface, when the liquid is passing the particles. The titration steps are at intervals of between 3 to 30 seconds. The amount of titrand solution added per step is between 10 and 100 µl, depending on the signal change and the selected titration program.
The software and instrument configuration are designed to characterize the ionic surface in an environment close to the end product.
The diagram shows pH - charge titrations with NaOH and HCl on 6 suspensions. Their size distributions range from 0.02 to 100 µm. Two of the powders were selected as being representative (red and blue). Their particle size distributions were measured with the Microtrac “BlueWave” Mie Scattering particle size distribution analyser.
In Figure 2A is shown the titration curve from the measured streaming (ψ) as shown in Figure 2B approximated by a mathematical function. By calculating the first and second derivative of this function a local minimum from -284 mV and 0.43 mL can be calculated.
Summary and Benefits
Usually, suspensions of 0.01 to 10% concentration are measured in a 10 ml Teflon® cylinder. The sample is constantly agitated as the titrand is added to the sample. The upper limits are concentration at 50%vol, viscosity at 50 mPa.s, conductivity at 50 mS/cm, all dependent on the sample and the application. The duration of one titration depends on the concentration of the sample and the titrand. In one hour, 5 titration curves can be easily acquired. No sample parameters are needed, making the Stabino® method very efficient and simple.