Hi,

  Zetasizer measurements are very much dependent on sample preparation. the sample should be in colloidal state. you can use some surfactant.

do not use ordinary bath sonicator. use probe sonicator and you can vary the power it is not properly dispersing.

If there is 'no reading' this could have two reasons:

a) If the sample is clear, all material was filtered out and is likely stuck to the filter membrane. You should still obtan some scattering intensity from the Zetasizer, just no decent correlation function. You may be able to use a larger pore size filter, and keep in mind that realistically DLS is ideal for the range of ~1nm to ~1micron. Since you indicate there is strong sedimentation without filtration, this may mean that your particle size is too large for DLS. In Zeta potential if the sample is clear you may not have a strong enough scattering signal to get data.

b) If the sample is black, there may be too much absorption in the sample, and very few photons actually reach the detector. Try to dilute the sample to such an amount that it appears hazy and you can almost read through the cuvette.

If neither of these are applicable, send a data file to Malvern's help desk, making sure that you checked the "Allow results to be saved containing correlation data only" option in the setup of the size measurement. This may help with trouble shooting.

Dispersibility of carbon black in water is really really poor. Therefore you do not have a dispersion which can be characterized by Zeta sizer. Sonication does not help because attraction between particles is much higher than interaction with water. You may use dispersant like anionic surfactant, however, carbon black dispersion will be much different from the macrodispersion in only water.