Lime sulfur
mixture of calcium polysulfides and thiosulfate used as pest control agent
In horticulture, lime sulfur (lime sulphur in British English; see American and British English spelling differences) is mainly a mixture of calcium polysulfides and thiosulfate (plus other reaction by-products such as sulfite and sulfate), formed by reacting calcium hydroxide with elemental sulfur, and is used in pest control. It can be prepared by boiling a suspension of poorly soluble calcium hydroxide (lime) and solid sulfur in water, together with a small amount of surfactant to facilitate the dispersion of these solids. After elimination of residual solids (flocculation, decantation, and filtration), it is normally used as an aqueous solution, which is reddish-yellow in color and has a distinctive offensive odor of hydrogen sulfide (H2S, rotten eggs).
Synthesis reaction
The exact chemical reaction leading to the synthesis of lime sulfur is generally written as:
Ca(OH)2 + x/8 S8 → CaSx + by-products (S2O2−3, SO2−3, SO2−4)
as reported in a document of the US Department of Agriculture (USDA).
This reaction is poorly understood because it is vague and involves the reduction of elemental sulfur, and no reductant appears in the 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 the range 11–12, typical for sulfides, which are also strong bases.
When the hydrolysis of calcium sulfide is taken into account, the individual reactions for each of the by-products are:
1/2 S8 + H2O + 2 Ca(OH)2 → 2 H2S + CaS2O3
3/8 S8 + H2O + 2 Ca(OH)2 → 2 H2S + CaSO3
1/2 S8 + 2 H2O + 2 Ca(OH)2 → 3 H2S + CaSO4
However, elemental sulfur can undergo a disproportionation reaction, also called dismutation. The first reaction resembles a disproportionation reaction. The inverse comproportionation reaction occurs in the Claus process, which is used for desulfurization of oil and gas products in the refining industry:
H2S + 3/2 O2 → SO2 + H2O
By rewriting the last reaction in the inverse direction, one obtains a reaction consistent with what is observed in the overall lime sulfur reaction:
3/8 S8 + 2 H2O → 2 H2S + SO2
In alkaline conditions, it yields:
3/8 S8 + 2 H2O + 6 OH− → 2 S2− + SO2−3 + 5 H2O
and after simplification, or more precisely recycling of water molecules in the above reaction:
3/8 S8 + 6 OH− → 2 S2− + SO2−3 + 3 H2O
Adding back 6 Ca2+ cations from hydrated lime for the sake of electroneutrality, one obtains the overall reaction.
This last reaction is consistent with the overall lime sulfur reaction mentioned in the USDA document.
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