## 1 Introduction  
Iron oxide nanoparticles (NPs) are interesting in view of their applications ranging from biomedical field to environmental remediation. Among the several synthetic procedures developed to prepare magnetite (Fe\({}_{3}\)O\({}_{4}\)) and maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)) NPs, co-precipitation is one of the most commonly used methods. The various co-precipitation procedures are variants of the Massart method [1]. This preparation procedure, in which magnetite NPs are precipitated by adding a base to aqueous solutions of Fe(II) and Fe(III) salts, is not experimentally demanding since it does not require high temperatures, toxic precursors or solvents. The reaction mechanism leading to the formation of magnetite NPs, involving several iron oxides and oxyhydroxide intermediates, is not completely clarified [2; 3; 4; 5]. Reaction conditions, such as the concentration of the reactants, pH, temperature, reaction temperature and time rate of the alkali addition, and the types of counterions of the Fe(II) and Fe(III) salts, determine to a large extent the average size, the size distribution of the individual iron oxide NPs and, thus, their magnetic properties [6; 7; 8; 9]. The co-precipitation in aqueous environment leads to the formation of aggregates (which can reach the micrometric size) consisting of individual NPs. The ionic strength and pH of the reaction mixture influence the aggregation process. NPs of magnetite prepared by co-precipitation are negatively charged since the co-precipitation occurs at pH (10-12) well above the point of zero charge of Fe\({}_{3}\)O\({}_{4}\) particles [10]. The electrostatic repulsion between the negatively charged NPs is reduced by the relatively high ionic strength of the solution and aggregation of NPs occurs. Hence, the formation of clusters of individual NPs limits the colloidal stability of magnetite NP dispersions and influences the magnetic propertiesof these clusters in which the NPs are in close proximity. For most applications the surface of the NPs must be modified in order to prevent or at least reduce their aggregation. A reach literature exists on the surface coating by adsorption of ions and polymers to stabilize dispersions of magnetic NPs in various environments [11; 12; 13; 14], just to cite a few reviews. Citrate ions adsorbed at the surface of NPs are used to stabilize aqueous dispersions of metallic [15; 16] and metal oxide [17] NPs, including those of iron oxides [18]. Citrate ions are chemisorbed via coordination bonding of one or two carboxylate groups with iron ions at the surface of the NPs. At pH above 4 the negatively charged surface provides electrostatic stabilization, at least at relatively low ionic strength [19; 20; 21]. A reliable determination of the size and size distribution of the individual NPs as well as of their aggregates is fundamental for the improvement of the synthetic procedures and to understand the relationships between size and magnetic properties. In the present work, we investigated by means of X-ray Diffraction (XRD), Dynamic Light Scattering (DLS), magnetization measurements and Electron Paramagnetic Resonance (EPR) the size and magnetic properties of magnetite NPs prepared by co-precipitation without and with citrate under various conditions. The aim of this investigation was to clarify the effect of citrate ions on the size of Fe\({}_{3}\)O\({}_{4}\) NPs and on their magnetic properties. In previous studies it was found that the presence of citrate during the co-precipitation leads to a reduction in the size of the NPs [22; 23]. However, there are many aspects of this system to be investigated more deeply. In particular no information about the particle size distribution (PSD) was reported. A study of the particle size, PSD, aggregation of the NPs and their effect on the magnetic properties of magnetite NPs is of fundamental relevance to tailor the preparation procedures in view of the applications of these NPs.