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Concept
Lime sulfur
Summary
In horticulture, lime sulphur (American spelling lime sulfur) is mainly a mixture of calcium polysulfides and thiosulfate (plus other reaction by-products as sulfite and sulfate) formed by reacting calcium hydroxide with elemental sulfur, used in pest control. It can be prepared by boiling in water a suspension of poorly soluble calcium hydroxide (lime) and solid sulfur together with a small amount of surfactant to facilitate the dispersion of these solids in water. After elimination of any residual solids (flocculation, decantation and filtration), it is normally used as an aqueous solution, which is reddish-yellow in colour and has a distinctive offensive odour of hydrogen sulfide (H2S, rotten eggs).
The exact chemical reaction leading to the synthesis of lime sulfur is poorly known and is generally written as:
as reported in a document of the US Department of Agriculture (USDA).
This vague reaction is puzzling because it involves the reduction of elemental sulfur and no reductant appears in the above mentioned equation while sulfur oxidation products are also mentioned. The initial pH of the solution imposed by poorly soluble hydrated lime is alkaline (pH = 12.5) while the final pH is in range 11–12, typical for sulfides which are also strong bases.
When the hydrolysis of calcium sulfide is accounted for, the individual reactions for each of the by-products are:
However, elemental sulfur can undergo a disproportionation reaction, also called dismutation. The first reaction resembles a disproportionation reaction. The inverse comproportionation reaction is the reaction occurring in the Claus process used for desulfurisation of oil and gas crude products in the refining industry:
→ +
By rewriting the last reaction in the inverse direction one obtains a reaction consistent with what is observed in the lime sulfur global reaction:
2 → 2 +
In alkaline conditions, it gives:
and after simplification, or more exactly recycling, of water molecules in the above reaction:
adding back 6 Ca2+ cations from hydrated lime for the sake of electroneutrality, one obtains the global reaction.
Official source
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