NOVELTY - Reaction chamber comprises: a reaction chamber that houses a solution which comprises an ammonium ion, a metal ion and a buffer; and an anion selective electrode in the solution to monitor the metal ion, where the anion selective electrode measures the metal ion during metal ammonium complex formation, metal-thiourea complex formation, metal sulfide composition formation and/or metal sulfide layer formation.    USE - The reaction chamber is useful for: monitoring a metal ion in solution during the formation of a metal sulfide layer on a substrate; forming a metal sulfide layer on a solar cell substrate; fabricating a semiconductor; identifying cadmium sulfide film heterogeneous growth and cadmium sulfide powder homogenous precipitate; and monitoring cadmium sulfide layer deposition on the substrate (all claimed).    ADVANTAGE - The methods allow: cadmium ion molarity to be monitored in real time for the first time which allows for the first time; and real time control and suppression of homogeneous precipitation of cadmium sulfide during heterogeneous nucleation and ion by ion growth of dense cadmium sulfide film.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) forming a metal sulfide layer on a solar cell substrate or fabricating a semiconductor composition comprising: (a) providing the reaction chamber in communication with a solution; (b) providing a substrate in communication with the reaction chamber and the solution; (c) combining an ammonium ion, a metal ion and a buffer in the solution; (d) monitoring the metal ion in the solution with anion selective electrode; (e) forming a metal-ammonium complex in the reaction chamber; (f) decomplexing the metal-ammonium complex; (g) titrating with a thiourea composition to form a metal-thiourea complex; (h) forming a metal sulfide composition from the metal-thiourea complex; and (i) depositing a metal sulfide layer on a solar cell substrate in the reaction chamber; and    (2) identifying cadmium sulfide film heterogeneous growth and cadmium sulfide powder homogenous precipitate or monitoring cadmium sulfide layer deposition on the substrate comprising: steps (a), (b) and (c) as above per se, (d1) monitoring the cadmium ion in the solution with an anion selective electrode to identify the heterogeneous cadmium sulfide film growth or the homogenous cadmium sulfide powder precipitate; (e1) forming a cadmium-ammonium complex in the reaction chamber; (f1) decomplexing the cadmium-ammonium complex; (g1) titrating with a thiourea composition to form a cadmium-thiourea complex; (h1) forming a metal sulfide composition from the cadmium-thiourea complex; and (i1)depositing a cadmium sulfide layer on a solar cell substrate in the reaction chamber.