Sodium hydrosulfide


Sodium hydrosulfide is the chemical compound with the formula NaHS. This compound is the product of the half-neutralization of hydrogen sulfide with sodium hydroxide. NaHS is a useful reagent for the synthesis of organic and inorganic sulfur compounds, sometimes as a solid reagent, more often as an aqueous solution. Solid NaHS is colorless, and typically smells like H2S owing to hydrolysis by atmospheric moisture. In contrast with sodium sulfide, which is insoluble in organic solvents, NaHS, being a 1:1 electrolyte, is more soluble. Alternatively, in place of NaHS, H2S can be treated with an organic amine to generate an ammonium salt. Solutions of HS− are sensitive to oxygen, converting mainly to polysulfides, indicated by the appearance of yellow.

Structure and properties

Crystalline NaHS undergoes two phase transitions. At temperatures above 360 K, NaHS adopts the NaCl structure, which implies that the HS− behaves as a spherical anion owing to its rapid rotation, leading to equal occupancy of eight equivalent positions. Below 360 K, a rhombohedral structure forms, and the HS− sweeps out a discoidal shape. Below 114 K, the structure becomes monoclinic. The analogous rubidium and potassium compounds 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 sodium methoxide with hydrogen sulfide:
Industrially, sodium hydroxide is employed as the base. The quality of the NaHS can be assayed by iodometric titration, exploiting the ability of HS− to reduce I2. A method for the preparation of anhydrous sodium hydrosulfide involves reaction of clean solid sodium metal with hydrogen sulfide gas in a water and oxygen free environment.

Applications

Thousands of tons of NaHS are produced annually. Its main uses are in cloth and paper manufacture as a makeup chemical for sulfur used in the kraft process, as a flotation agent in copper mining where it is used to activate oxide mineral species, and in the leather industry for the removal of hair from hides.