However, the Cl− concentration in the slurry of titania precursor
is found to fall less than that predicted by the first formula after
washing 5 times, but follow the relation ln C =
−0.45n + 6.59. This
is probably due to the fact that after 5 washing times, the concentration
of Cl− ions in the slurry is below 100 ppm, and further
washing may lead to the destruction of the double electric layers
surrounding the particles in the slurry. As a result, the Cl− ions
adsorbed on the surface of particles could be released, making
extra contribution to the concentration of Cl− in the slurry. This
explains why after 5 washings, the reduction in Cl− concentration
with washing time is below the prediction by the formula based on
the concentrating–diluting principle, though the product could be
cleaned up to as low as 10 ppm after 10 washings, which could not
be reached by the traditional plate-and-frame filter process