In principle, the solubility of base metal sulfides can be enhanced by the formation of such species as CaHS (super +) , MgHS (super +) , NH 4 HS (super degrees ) , etc. in ore-transporting fluids. The possibility that these species exist has never been investigated. A previously developed electrochemical method employing three ion-selective electrodes in two combinations has been used to seek evidence for these species in 2 m NaCl below 50 degrees C. No evidence is found for CaHS (super +) or MgHS (super +) ; an upper limit for the logarithm of the stoichiometric association constant (K (super *) ) in each case is --0.4. The same experiments suggest that an upper limit for log K (super *) of CaS (super degrees ) and MgS (super degrees ) is -- 0.1. In contrast, a distinct but weak interaction occurs with NH (super +) 4 . Assuming the associated species to be NH 4 HS, the logarithm of the stoichiometric association constant is roughly +0.2 at 25 degrees C. On the basis of electrostatic theory, association of monovalent NH (super +) 4 is somewhat surprising when no association occurs with divalent Ca (super 2+) and Mg (super 2+) . It is likely that NH 4 HS is partially stabilized by hydrogen bonding. These findings, combined with the available composition data on natural brines, suggest that bisulfide ion-pairs will rarely, if ever, account for a significant fraction of the total reduced sulfur in low-temperature brines. Furthermore, general knowledge of the effect of temperature on association constants suggests that this is likely to be true up to perhaps 150 degrees C. On the other hand, at much higher temperatures, where even alkali chlorides are partially associated, bisulfide ion-pairing could be important.

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