AFSL

Hydrogen sulphide (H₂S) gas is prepared in the laboratory by treating iron sulphide with dil. H₂SO₄.

Intermittent supply of H₂S gas by Kipp’s apparatus :

(Working principle of Kipp’s apparatus )

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{Intermittent = Stopping and starting at irregular intervals}

H₂S gas is frequently required for salt analysis in the laboratory. The special apparatus which is used for the preparation of H₂S is called Kipp’s apparatus.

Kipp’s apparatus consist of three bulbs A, B and C. The bulbs are interconnected with each other. The upper bulb C contains a long stem which reaches upto bulb A. When dil. H₂SO₄ is poured from bulb C, liquid level rises in bulb A and reaches upto bulb B just to cover iron sulphide. Then the reaction between iron sulphide and H₂SO₄ takes place to form H₂S gas. As H₂S gas is formed, the pressure inside bulb B increases and forces the dil. H₂SO₄ up into the upper bulb C. Then the contact between FeS and dil. H₂SO₄ breaks and the formation of gas is stopped (ceased). When the tap is opened H₂S comes out and the pressure decreases. As a result liquid level again rises at bulb B and H₂S gas start to evolve again. In this way H₂S gas is prepared intermittently by Kipp’s apparatus.

Physical Properties of H₂S :

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1. It is colourless gas with rotten egg smell.
2. It is soluble in water.
3. It is poisonous gas.{In its low concentration it causes nausea, dizziness and headache. At high concentration it is fatal (i.e. deadly poisonous). Conc. of H₂S in air 1 part in 1000 parts of air may be fatal.}
4. It is heavier than air{with vapour density 17}

Chemical Properties of H₂S :

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 1. Acidic character of H₂S :

It acts as a weak diprotic (dibasic) acid. It ionizes in two steps to give two protons.

• H₂S reacts with base to form two series of salts. Eg.

• H₂S reacts with basic oxides to give salt.

2. H₂S as reducing agent :-

Sulphur in H₂S bears -2 oxidation state which in minimum oxidation state of sulphur. In chemical reaction, H₂S tends to get oxidized . So H₂S acts as reducing agent.

• It reduces halogens into halogen acids.

• It reduces sulphur dioxide to sulphur.

• It reduces ferric salt to ferrous salt.

• It reduces conc. HNO₃ to NO_2.

• It reduces conc. H₂SO₄ to SO₂.

• It reduces acidified potassium permanganate solution and pink colour of KMnO₄ is discharged.

• It reduces acidified K₂Cr₂O₇ solution and orange color of K₂Cr₂O₇ changes to green.

3. H₂S as analytical reagent in salt analysis ( Precipitation of metal sulphide) :

H₂S gas is used as analytical agent to detect group II basic radicals in acidic medium i.e. HCl and group IIIB basic radicals in in alkaline medium i.e. NH₄Cl and NH₄OH.