A method of calibrating the zero point of an apparatus which is used to determine the quantity of silica contained in a silica solution sample to be analysed, using a colorometric method. The colorometric method used by the apparatus consists of successively introducing the following components into the sample: a molybdate solution, a developer and a reagent. In order to determine the zero point, the developer is first introduced into the sample of silica solution to be analyzed, a first measurement is taken, followed by addition of the molybdate solution and the reducing agent to the sample, where a second measurement is take and the zero point is calculated.