Sodium hydrosulfide

Sodium hydrosulfide is the chemical compound with the formula NaSH. Other names include sodium bisulfide, sodium sulfhydrate, sodium hydrogen sulfide and Nash. This species is the product of the half -neutralization of hydrogen sulfide with a sodium-derived base. NaSH is a useful reagent for the synthesis of organic and inorganic sulfur compounds. It is a colorless solid that typically smells like H2S due to hydrolysis by atmospheric moisture. In contrast with Na2S, which is insoluble in organic solvents, NaSH, being a 1:1 electrolyte, is more soluble. Alternatively, in place of NaSH, H2S can be treated with an organic amine to generate an ammonium salt. Solutions of SH- are sensitive to oxygen, converting mainly to polysulfides, indicated by the appearance of yellow.

Crystalline NaHS undergoes two phase transitions. At temperatures above 360K, NaHS adopts the NaCl structure, which implies that the HS- behaves as a spherical anion due to its rapid rotation leading to equal occupancy of eight equivalent positions. Below 360K, a rhombohedral structure forms, and the HS- sweeps out a discoidal shape. Below 114K, the structure becomes monoclinic. RbHS and KHS behave similarly.

NaHS has a relatively low melting point of 350 °C. In addition to the aforementioned anhydrous forms, it can be obtained as two different hydrates, NaHS.2H2O and NaHS.3H2O. These three species are all colorless and behave similarly, but not identically.

Preparation
The usual laboratory synthesis entails treatment of NaOMe with hydrogen sulfide:
 * NaOMe +  H2S →  NaHS  + MeOH

Industrially, NaOH is employed as the base. The quality of the NaHS can be assayed by iodometric titration, exploiting the ability of HS- to reduce I2.

Applications
Thousands of tons of NaHS are produced annually. Its main uses are in paper manufacture as a makeup chemical for sulfur used in the Kraft process, as a flotation agent in copper mining, and also in the leather industry for the removal of hair from hides.