Mastersizer 2000 MU (Malvern Instruments, GB) was used for measurements of particle size distribution. The device uses dispersion of the light falling on the particles and it enables evaluation of measured signal either based on Mie’s theory or Frauenhoffer diffraction. The particle size distribution was evaluated with red light (HeeNe laser with 633 nm wavelength) and blue light. The solids were ultrasonically homogenized for 90 s and measured in a solution with Na4P2O7 (c ¼ 0.15 mol dm3).