54 Keywords: Synthesis, Magnetite, Characterization, Reverse Coprecipitation Article history: Received 02 October 2020 Accepted 12 December 2020 Published 31 December 2020 Abstract In this study, the synthesis of magnetite was carried out through the reversed co-precipitation method with ammonium hydroxide (NH 4 OH) as precipitating agent. The aim of the study was to obtain the most appropriate moles ratio of Fe(III) and Fe(II) in obtaining the best characteristics of magnetite, considering that Fe(II) was easily oxidized to Fe(III). Characterization of synthesized magnetite was performed using a Fourier Transform Infrared (FTIR) spectrophotometer and X-Ray Diffraction (XRD). The results showed that the moles ratio of Fe(III) and Fe(II) that produced magnetite with high yield, FTIR spectra absorption and diffractogram peaks with high intensity was magnetite synthesized using a ratio of Fe(III) and Fe(II)= 1.5: 1. Yield of magnetite synthesized in this condition was 81%. Based on the stability test, magnetite was stable at pH 2-10. * Coresponding Author : maya.rahmayanti@uin-suka.ac.id INTRODUCTION In recent years, the adsorption process using magnetite compounds and modified magnetite (Fe 3 O 4 ) compounds has been developed. Applications of magnetite compounds include metal adsorption (Muniroh et al., 2019; Rahmayanti et al., 2016a; Rahmayanti et al., 2016b; Rahmayanti et al., 2019, Rahmayanti, 2020), and dye adsorption (Latifah et al. al., 2020; Liu et al., 2019; Rahmayanti et al., 2020; Putri et al., 2020). Magnetite compounds or modified magnetite compounds can adsorpt the target compound, if an external magnetic field was applied, the adsorbent can be separated from the solution rapidly. Furthermore, the adsorbate can be separated using a solvent or without a solvent. Magnetite was widely used because it was a semiconductor compound that has a small particle size, low toxicity, and was paramagnetic in nature (Jingjing et al., 2011). Magnetite was known as black iron oxide or ferrous ferrite, which was the ferrous metal oxide with the strongest magnetic properties. In an oxidizing atmosphere, magnetite can be oxidized to maghemite or hematite according to the reaction presented in equation 1. Meanwhile, in a reducing atmosphere, magnetite can be reduced to wustite or iron as shown in equations 2 and 3 (Petrova et al., 2011 ). 4Fe 3 O 4(s) + O 2 (g) 6Fe 2 O 3 (s) (1) Fe 3 O 4(s) + C (s) 3FeO (s) + CO (g) (2) Fe 3 O 4(s) + 4C (s) 3Fe (s) + 4CO (g) (3) The magnetite surface contains hydroxyl groups which were protonated at a pH below pH PZC magnetite and deprotonated at a pH above pH PZC magnetite. The condition of pH with the same group concentration and on the surface was called the point of zero charge (PZC). Generally, it was at pH Vol. 09, No. 02: 54 — 58 December 2020 DOI: 10.22487/25411969.2020.v9.i3.15298 Original article Synthesis of Magnetite Nanoparticles Using Reverse Co-precipitation Method With NH4OH as Precipitating Agent and Its Stability Test at Various pH Maya Rahmayanti* Department of Chemistry, State Islamic University of Sunan Kalijaga, Jl. Marsda Adi Sucipto, Yogyakarta, Indonesia.