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Lime sulfur

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Lime sulfur is a vital chemical mixture used in horticulture to protect plants. It is primarily composed of calcium polysulfides and thiosulfate. These substances are created by reacting calcium hydroxide with elemental sulfur. The resulting liquid is reddish-yellow in color. It also possesses a very distinctive, offensive odor of hydrogen sulfide, which smells like rotten eggs. This mixture serves as a powerful tool for controlling various pests.

Creating lime sulfur requires a specific chemical process involving heat and stirring. To prepare it, a person must create a suspension of calcium hydroxide and solid sulfur in water. A small amount of surfactant is often added to help these solids disperse more easily. The mixture must be boiled for one hour while being stirred constantly. During this hour, small amounts of water may be added to replace what evaporates. Once boiling is complete, the liquid undergoes flocculation, decantation, and filtration to remove residual solids. The final product is an aqueous solution.

The chemistry of lime sulfur is quite complex and involves several different reactions. When calcium hydroxide is used, the initial pH of the solution is very alkaline at 12.5. After the reaction, the final pH typically settles between 11 and 12. The synthesis involves the reduction of elemental sulfur, though the exact mechanism is not fully understood. One way to view the reaction is through a disproportionation reaction, also known as dismutation. This process produces various by-products, including sulfite and sulfate. The presence of thiosulfate occurs when sulfite reacts with elemental sulfur or polysulfides.

Different recipes exist depending on the materials available to the maker. A recipe from the New York State Agricultural Experiment Station suggests a specific ratio. It calls for 80 lb. of sulfur and 36 lb. of quicklime in 50 gallons of water. This creates a weight ratio of about 2.2:1 for the sulfur and quicklime. This specific ratio helps achieve the highest proportion of calcium pentasulfide. If a maker uses hydrated lime instead of quicklime, they may need to increase the amount by one-third or more. The purity of the lime also affects how much is needed to ensure a successful reaction.

Lime sulfur has a very long and significant history in agriculture. It is believed to be the earliest synthetic chemical ever used as a pesticide. In the 1840s, farmers in France used it to combat powdery mildew on grape vines. This was critical because the fungus had reduced wine production by 80 percent. Later, in 1886, it was used in California to control the San Jose scale. By the 1920s, almost all commercial orchards in western countries used regular spraying. However, by the 1940s, synthetic organic fungicides began to replace it because they were less risky to foliage.

Today, lime sulfur is used in several specialized ways across different fields. In large-scale agriculture, it is sprayed to control fungi, bacteria, and insects. It is approved for use on organic crops in the United Kingdom and the European Union. Bonsai enthusiasts use a different method by painting undiluted lime sulfur onto deadwood. This process bleaches the wood to a bone-white color to make it look aged. This must be done with great patience to avoid touching the living leaves. Additionally, very diluted solutions are used as a dip to treat pets for ringworm or mange.

Because lime sulfur is a strong chemical, safety must be the highest priority. The solution is strongly alkaline and can cause serious injuries like blindness if it touches the eyes. It is also corrosive to the skin. The reduced species of sulfur in the mix can cause stress corrosion cracking and pitting corrosion. Even stainless steel can be corroded by contact with this substance. When handling it, one must always wear impervious gloves and safety goggles. Furthermore, lime sulfur reacts with strong acids to produce highly toxic hydrogen sulfide gas.

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