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In horticulture, lime sulfur ( lime sulphur in British English, see American and British English spelling differences) is mainly a mixture of calcium 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 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 odor of hydrogen sulfide (H2S, rotten eggs).
as reported in a document of the US Department of Agriculture (USDA).
This vague reaction is poorly understood, because it involves the reduction of elemental sulfur and no reductant appears in the equation while sulfur oxidation products are also mentioned as products. 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 .
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 desulfurization 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:
In alkaline conditions, it gives:
and after simplification, or more exactly recycling, of water molecules in the above reaction:
adding back cations from hydrated lime for the sake of electroneutrality, one obtains the global reaction.
This last reaction is consistent with the global lime sulfur reaction mentioned in the USDA document. However, it does not account of all the details, a.o., the production of thiosulfate and sulfate amongst the end-products of the reaction. Meanwhile, it is a good first order approximation and it usefully highlights the overall lime sulfur reaction scheme because the chemistry of reduced or partially oxidized forms of sulfur is particularly complex and all the intermediate steps or involved mechanisms are hard to unravel. Moreover, once exposed to atmospheric oxygen and microbial activity, the lime sulfur system will undergo a fast oxidation and its different products will continue to evolve and finally enter the natural sulfur cycle.
The presence of thiosulfate in the lime sulfur reaction can be accounted by the reaction between sulfite and elemental sulfur (or with sulfide and polysulfides) and that of sulfate by the complete oxidation of sulfite or thiosulfate following a more complex reaction scheme. More information on calcium thiosulfate production has been described in a patent registered by Hajjatie et al. (2006).
Hajjatie et al. (2006) wrote the lime sulfur reaction in various ways depending on the degree of polymerisation of calcium polysulfides, but the following reaction is probably the simplest of their series:
They also managed to successfully control this reaction to achieve the conversion of elemental sulfur in a quasi-pure solution of calcium thiosulfate.
Bonsai enthusiasts use undiluted lime sulfur to bleach, sterilize, and preserve deadwood on bonsai trees while giving an aged look. Rather than spraying the entire tree, as with the pesticidal usage, lime sulfur is painted directly onto the exposed deadwood, and is often colored with a small amount of dark paint to make it look more natural. Without paint pigments, the lime sulfur solution bleaches wood to a bone-white color that takes time to weather and become natural-looking. In the very specific case of the bonsai culture, if the lime sulfur is carefully and very patiently applied by hand with a small brush and does not enter into direct contact with the leaves or needles, this technique can be used on evergreen bonsai trees as well as other types of green trees. However, this does not apply for a normal use on common trees with green leaves.
Diluted solutions of lime sulfur (between 1:16 and 1:32) are also used as a dip for Pet to help control ringworm, mange and other dermatoses and parasitism. Undiluted lime sulfur is corrosive to skin and eyes and can cause serious injury like blindness.
Safety goggles and impervious must be worn while handling lime sulfur. Lime sulfur solutions are strongly (typical commercial concentrates have a pH over 11.5 because of the presence of dissolved sulfides and hydroxide anions), and are harmful for living organisms and can cause blindness if splashed in the eyes.
The corrosion nature of lime sulfur is due to the reduced species of sulfur it contains, in particular the sulfides responsible for stress corrosion cracking and the thiosulfates causing pitting corrosion. Localized corrosion by the reduced species of sulfur can be dramatic, even the mere presence of elemental sulfur in contact with metals is enough to corrode them considerably, including so-called .
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