We have investigated changes in the etch rate of p-PS with increasing etching time as well as changes of the porosity of buried layers with depth. These effects can be attributed to the influence of chemical etching and variations in the electrolyte composition with depth. To study these changes, first the porosities of layers above and below layers with different thicknesses were determined by a fit of the reflectance spectra of these layer systems using the effective medium theory. Secondly we have measured oscillations in the reflectance during the formation of PS layers caused by the increasing layer thickness. Using these experimental results we are able to give a functional description of the changes in the optical thickness with depth. In addition, the influence of the chemical etching and changes of the HF concentration on the optical thickness can be estimated, As a result a method for changing the current with depth can be given, which can be used to minimize porosity gradients.