Sodium Hydrosulphide and Sodium Thiosulfate

Sodium Hydrosulphide
Sodium Hydrosulphide (NaHS) finds use in ore flotation (30 percent), dyestuff processing (20 percent), as a leather depilatory (20 percent), the manufacture or rayon and organic chemicals (10 percent) and in the metallurgical industry. These were five merchant producers who operated six plants in 1981. Total demand was about 18,000 t/year. Wow very big amount.

Sodium Thiosulfate
Sodium Thiosulfate crystallizes in large, transparent, extremely soluble prisms with five molecules of water. It is a mild reducing agent, like sodium sulfite. It is employed as an antichlor following the bleaching of cellulose products and as a source of sulphur dioxide in the bleaching of wool, oil, and ivory. In photography, which accounts for 90 percent of current use, it is used to dissolve unaltered silver halogen compounds from negatives and prints, where it is commonly called “hypo”. It is a preservative against fermentation in dyeing and serves in the preparation of mordants. Other minor uses of sodium thiosulfate are in the reduction of indigo, in the preparation of cinnabar, in the preparation of silvering solutions, and in medicine.

Use of the sodium salt is slowly deelining, especially in the photography business, where gains are being made by ammonium thiosulfate.

Manufacture:
Of the several methods for the production of sodium thiosulfate business, where important is form sodium sulfite and free sulphate.

Na2SO3 + S → Na2S2O3

The resulting liquor is concentrated and crystallized. The crystals formed (Na2S2O3. 5H2O) are immediately packed in airtight containers to prevent efflorescene. A second method of preparation is from sodium sulphide. Sulphur dioxide is passed into a solution of sodium sulphide and sodium carbonate of low concentration (not more than 10% each).

Na2SO3 + 2 Na2S + 4 SO2 → 3 Na2S2O3 + CO2

Sodium thiosulfate is obtained by evaporation and crystallization.

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