The acidic texturisation process is an important and widely used step in the fabrication of mulitcrystalline silicon solar cells. The resulting silicon surface structure mainly depends on the concentrations of the hydrofluoric acid (HF), nitric acid (HNO3) and hexafluorosilicic acid (H2SiF6). Therefore, a continuous dosing of concentrated acids is necessary to remain a constant etching rate and a constant surface structure. In state of the art process tools, this dosing is triggered by a certain number of processed wafers. Small inaccuracies in the silicon entry or the dosed volume lead to significant drifts of the acid concentrations. We present an automated dosing algorithm using NIR-spectroscopy for the concentration measurement, which is able to keep the bath composition in an optimal process window. A theoretical approach suggests, that the used algorithm is able to keep the etching rate constant in a range of ±2 μm/min. First experiments using this algorithm in an inline process tool show, that the mechanism finds the optimal concentrations from a starting point, where both, HFand HNO3-concentrations are too low.