In this work, we present a modification of the sol-gel based, Stober, Fink and Bohn's (SBF) procedure for the preparation of nanometric spheres of silica with a monomodal distribution of size and controlled diameter. In the SBF method, tetraethyl orthosilicate (TEOS) is used as a precursor molecule and hydrolysis and condensation reactions to form chains and branches of Si-O bonds are carried out. The novelty of this work consists in the use of an aqueous ammonium hydroxide solution as ammonia source. Graphics of particle size as a function of reagent contents are presented (reaction yield control) and differences to the results from the original SBF method are discussed. This procedure is the very first step for the development of a sol-gel synthetic opals preparation process. As a result, the best route to prepare monodisperse spheres in the whole range of 250 to 850 nm is that of keeping constant the TEOS and NH3 contents at 0.28:1M varying water contents in the range of 8 - 12.5 M.