Hydrogen sulfide

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Hydrogen sulfide or H2S, is a highly toxic gas with a characteristic foul odor, often recognized as the smell of rotting eggs.

Properties

Chemical

Hydrogen sulfide is a reducing agent, so finds several uses, but use is often limited due to its toxicity. Most of the time however, a replacement is looked for simply because of the very off-putting nature of this gas.

Sodium hypochlorite will react with the gas to produce sodium chloride, water and sulfur. Because of the docile nature of the products, bleach can be used to neutralize the toxic gas.

Hydrogen sulfide will react with sulfur dioxide to produce elemental sulfur and water. Hydrogen sulfide is also highly flammable in the presence of oxygen, with it burning to produce sulfur dioxide and water.

A common detection method for the sulfide is through the use of lead(II) acetate paper. A small amount of hydrogen sulfide will quickly turn the clear solution of lead acetate a grey colour with the formation of insoluble lead(II) sulfide.

The black tarnish on silver often appears due to silver sulfide on the surface, and it is only made from the reaction of small amounts of atmospheric hydrogen sulfide reacting with the silver metal. Silver should be kept outside the lab to prevent this tarnish if hydrogen sulfide may be present.

When dissolved in water, hydrogen sulfide acts like a weak acid.

Physical

Hydrogen sulfide is a colorless gas with a strong unpleasant smell. It has a density at standard conditions of 1.363 g/dm3. Its melting point is −82 °C and boils at −60 °C. It has a solubility in water of 4 g/l. It is also soluble in acetone, chlorobenzene, methanol and THF, but poorly soluble in ethylene glycol.[1]

Availability

Hydrogen sulfide is difficult to acquire due to its strong smell and hazards. It's best to make it yourself.

Preparation

An acid acting on a metal sulfide is one way of generating the gas. For small amounts, iron sulfide is an option as lumps of the FeS2 mineral pyrite is always sold in gemstone/mineral stores. Larger amounts can be made from the hydrolysis of aluminium sulfide, which in turn is made by the almost thermite like combustion reaction between elemental sulfur and aluminium. Another method involves synthesis of ferrous sulfide from elemental iron and sulfur and treating it with acids. The disadvantage of this method is that ferrous sulfide, being an insoluble melt, tends to "goat" test tubes.

Projects

Handling

Safety

This gas is flammable and ideal mixes of O2 and H2S are listed as explosive.

Hydrogen sulfide is very toxic, but the reason it is still able to be used is that it is detectable, via its foul odor, in very low doses, below the extremely harmful level. The foul odor of the gas is a natural deterrent. An amateur chemist is far more likely to be caught off guard by a poisonous gas that either has no odor or has a rather pleasant smell. It should be noted, however, that the smell of hydrogen sulfide is among the most likely substances to alert neighbors or authorities of what you are doing, and the smell may even be mistaken for a gas leak, which could cause serious legal problems. Hydrogen sulfide can be lead into a cold solution containing a base to neutralize a quantity of it, reducing both the smell and chance of injury.

Though this gas is both useful and generally not regarded as an extreme danger, there is, however, a point where breathing in this gas can cause serious harm. A property of H2S is that is can temporarily deaden a person's sense of smell, a sickening effect that a few Sciencemadness members have had personal experiences with. If you are in an area with a strong smell of hydrogen sulfide, and then that strong smell suddenly disappears for no apparent reason, immediately leave the area. At the concentration where this occurs, the gas is potentially deadly, with a toxicity not very far from the infamous hydrogen cyanide, not to mention undetectable. Safety plans should be in order during any experiment which makes use of hydrogen sulfide, including a way to quickly leave the room/area. In the event of large amounts of the gas being released inside, large amounts of steam help to insure the oxidation of hydrogen sulfide to less harmful chemicals, and open containers of base can neutralize it as well.

Frequently find time during working with this gas to get some fresh air, even if you are certain you are not breathing any in.

In case of hydrogen sulfide poisoning, the patient should immediately leave the contaminated room (or be carried from there) and given access to outside air. In case of severe poisoning, the patient should immediately receive medical attention. If no medical attention is available, the patient should be given pure oxygen to breathe. Using methylene blue, a general antidote against blood agents that mess with hemoglobin or cytochrome oxidase, can also be recommended: the dosage is 50-100 ml of 1% solution intravenously. Nitrites, such as an isopropyl nitrite inhalation, also work as antidotes. Do not use both methylene blue and nitrite.

Storage

Storage of hydrogen sulfide presents great difficulties, as even the slightest leak will create a foul smelling environment. Hydrogen sulfide tanks and solutions are best stored in closed bottles, in a special cabinet containing scrubbers capable of removing hydrogen sulfide.

Disposal

Since the gas has a strong unpleasant smell, it's best to neutralize it first. There are several way to neutralize hydrogen sulfide.

Burning it will release sulfur dioxide, which has a less disagreeable odor, and in contact with hydrogen sulfide will be reduced to elemental sulfur. Bubbling hydrogen sulfide will result in sodium hydrosulfide, that will slowly hydrolyze to release hydrogen sulfide back in the air.

Hydrogen peroxide will reduce hydrogen sulfide to elemental sulfur or oxidize it to sulfuric acid in an alkaline environment, that can be easily neutralized.[2]

References

  1. http://pubs.acs.org/doi/abs/10.1021/je00031a019
  2. http://antoine.frostburg.edu/chem/senese/101/redox/faq/h2o2-h2s-so2.shtml

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