Introduction to Sulphur

Sulphur

Occurrence of Sulphur

The abundance of sulphur in earth's crust is about 0.6%. Free sulphur occurs in large natural deposits. In Mexico and Sicily, it is being mined and used directly without further purification. Some useful sources of sulphur are sulphide ores such as copper pyrites (CuFeS₂), iron pyrites (FeS₂) also known as Fool's gold, galena (PbS), zinc blende (ZnS), cinnabar (HgS) etc.

Sulphur is an important element because of the industrial importance of sulphuric acid and agricultural importance of sulphate fertiliser. Sulphur is also important in living systems.

Position of Sulphur in The Periodic Table

Sulphur is the second member of oxygen family which belongs to group VIA in the periodic table. The family members in order of increasing atomic number are Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te) and Polonium (Po).

The electronic configuration of sulphur is 1s2 2s2 2p6 3s2 3p4 and general valence shell electronic configuration of this family is ns2 np4. The members of the family contain 6 valence electronics and so all of them are placed in group VIA. Sulphur similar to other members is the p-block element. It is a solid non-metallic element. It belongs to the second period between phosphorous and chlorine in the periodic table.

Allotropy of Sulphur

The element can exist in different allotropic forms and all of them are in the solid state. Different allotropes of sulphur arise due to the difference in crystal structures, one form differs from its another form in density, outward crystal appearance, colour, thermal stability and intrinsic energy. The followings are the allotropic modification of sulphur.

a. Crystalline form

i) Rhombic or Octahedral or α-sulphur

ii) Monoclinic or Prismatic or β-sulphur

b. Noncrystalline or amorphous form

i) Plastic or γ-sulphur

ii) Milk of sulphur or δ-sulphur

iii) Colloidal sulphur

1. Rhombic or Octahedral sulphur (α-Sulphur)

Preparation

Rhombic sulphur is the most stable among all varieties. The ordinary roll sulphur or flowers of sulphur belong to this variety. It can be obtained by the slow evaporation of a solution of roll sulphur in carbon disulphide (CS₂) when the crystals obtained are octahedral i.e. eight sided. CS₂ is a volatile liquid and as such, it soon evaporates.

Properties

1. It is transparent and pale yellow in colour.
2. It is soluble in carbon disulphide, benzene, chloroform etc. But it is insoluble in water.
3. It is non-conductor of heat and electricity.
4. Its melting point is 113°C and boiling point is 445°C and specific gravity is 2.05.
5. As the name signifies, it crystallises in the octahedral rhombic form.
6. When slowly heated to 95.5°C, it gets converted to monoclinic sulphur.

2.Monoclinic or Prismatic (β-sulphur)

It is a needle-shaped crystalline allotrope of sulphur having melting point 119°C. It is insoluble in water but soluble in organic solvents like carbon disulphide (CS₂). It changes into Rhombic form at 96°C.

3. Plastic ( γ-sulphur)

It is an amorphous form of sulphur having the rubber-like mass and gradually changes into rhombic form. It is insoluble in both water and carbon disulphide (CS₂) and has no sharp melting point. It is also called as the supercooled sulphur liquid in which sulphur atoms are randomly arranged.

4. Colloidal sulphur

It is another variety of the amorphous form of sulphur which is prepared by passing H₂S through the oxidising agent like HNO₃, KMnO₄ etc. It can also be prepared by treating the solution of sodium thayoSulphatewith dil. HCl to get white turbid mass. On heating, it changes into rhombic form.

$$H_2S+2HNO_3\longrightarrow{2NO_2+2H_2O+S↓}$$

$$KMnO_4+3H_2SO_4+5H_2S\longrightarrow{K_2SO_4+2MnSO_4+8H_2O+5S}$$

5. Milk of Sulphur

It is another amorphous form of sulphur which can be obtained by boiling milk of lime with sulphur followed by decomposing with HCl.

$$3Ca(OH)_2+12S\longrightarrow{3CaS_5+CaS_2O_3+3H_2O}$$

$$2CaS_5+CaS_2O_3+6HCl\longrightarrow{3CaCl_2+3H_2O+12S↓}$$

It is insoluble in water but soluble in CS₂ and used as a medicine.

Manufacture of Oil of Vitrol (H₂SO₄) by Contact process

Principle

(1) Production of SO₂

Sulphur dioxide gas is produced by burning sulphur with air or from iron-pyrites.

$$S+O_2\xrightarrow\Delta{SO_2}$$

$$4FeS_2+11O_2\longrightarrow{2Fe_2SO_3+8SO_2}$$

(2) Catalytic Oxidation of SO₂

Hence formed SO₂ gas is catalytically oxidised into sulphur trioxide gas in the presence of air and finely divided V₂O₅ as the catalyst. This is the key step in the manufacture of H₂SO₄ because as much as SO₃ gas is formed the high amount of H₂SO₄ is obtained.

$$2SO_2+O_2\rightleftharpoons{2SO_3+Heat}$$

(3) Formation of Oleum / absorption of SO₃

Hence, formed SO3 gas is absorbed in the presence of conc. sulphuric acid to get fuming acid called oleum.

$$SO_3+H_2SO_4\longrightarrow{H_2S_2O_7}$$

(4) Dilution of oleum

The oleum is diluted with required amount of water in order to obtain desired concentration of sulphuric acid.

$$H_2S_2O_7+H_2O\longrightarrow{2H_2SO_4}$$

Condition of maximum yield

This reaction $$2SO_2+O_2\rightleftharpoons{2SO_3+45\;cal}$$ is reversible exothermic and proceeds with a decrease in volume. In such case reactions, Le-chatelier's principle may be applied to get the maximum yield of sulphur trioxide.

i) Low temperature

The low temperature will be favourable for the forward reaction because of exothermic in nature. The yield has been found to be maximum at about 450-500°C experimentally.

ii) High pressure

High pressure favours forward reaction. But the reaction can't be carried out at very high pressure. A pressure of 2-3 atm is sufficient to make the steady flow of gases.

ii) Excess of oxygen

A slight excess of oxygen in the reacting mixture of (SO₂ + O₂) helps to carry the reaction.

iv) Use of catalyst [V₂O₅]

At very low temperature the velocity of the reaction is very slow. In order to accelerate the formation of SO₃, the use of the catalyst is extremely necessary.

v) Purity of gases

As the catalyst is open to poisoning the mixture of SO2 and air or oxygen should be pure and dry and free from all types of impurities particularly dust.

Working Process

It consists of the following units.

i) Pyrite Burners

Iron pyrites are burnt in the presence of air to get sulphur dioxide.

$$S+O_2\longrightarrow{SO_2}$$

$$4FeS_2+11O_2\longrightarrow{2Fe_2O_3+8SO_2}$$

Traces of SO₃ are formed

$$2S+3O_2\longrightarrow{2SO_2}$$

ii) Purification unit

a) Dust removers

Here steam is sprayed from the top of the tower to settle down the dust particles.

b) Scrubber

The gas from the cooler is passed into scrubbers. Here water is sprayed from the top of the tower to remove water soluble impurities.

c) Drier

Here, conc H₂SO₄ is sprayed from the top of the tower, which removes moisture from the gases.

d) Arsenic purifier

The chamber contains gelatinous ppt of ferric hydroxide (Fe(OH)₃) precipitate kept in shelve which removes the arsenic impurities from the gases.

e) Testing box

The impurities in gases mixture are detected by using tidal effect.If the gaseous mixture is found to contain any impurities then it is recycled again by passing through purification unit.

iii)Contact chamber

In this converter, cylindrical iron pipes are fitted in which catalyst (V₂O₅) is packed. The pure gases SO₂ and O₂ combines or reacts on the surface of the catalyst to produce SO₃.

$$2SO_2+O_2\rightleftharpoons{2SO_3+\Delta{H}}$$

iv) Oleum formation chamber

This is a steel line chamber in which quartz crystal are fitted internally. The SO₃ gas obtained from contact chamber is speared by conc. H₂SO₄ by which oleum is formed.

$$SO_3+conc.H_2SO_4\longrightarrow{H_2SO_7\;(Oleum)}$$

Thus, formed oleum is diluted with required amount of H₂O to get conc. H₂SO₄ about 96% strength.

Physical properties of Sulphuric acid

1. Colourless, syrupy and corrosive nature.
2. It has the high boiling point and viscous due to the formation of intermolecular hydrogen bonding.
3. It reacts vigorously with water which is exothermic in nature that is why water should not be added directly in conc. H₂SO₄ for dilution. Hence conc. H₂SO₄ is diluted by adding acid to the water slowly with constant stirring.

Chemical properties

1. Decomposition

It is decomposed into SO₂ and O₂ on strong heating.

$$2H_2SO_4\xrightarrow\Delta{2SO_2↑+2H_2O+O_2↑}$$

2. Acidic nature

It is strong dibasic acid and ionises into two steps to give two steps to give two series of salt.

3. Action on metal

Those electropositive metals which lies above hydrogen in electro chemical series such as Zn, Mg etc reacts with sulphuric acid to evolve hydrogen gas.

$$Zn+H_2SO_4\longrightarrow{ZnSO_4+H_2}$$

$$Mg+H_2SO_4\longrightarrow{MgSO_4+H_2}$$

Those metal which lies below hydrogen in electro chemical seriesreact with conc H₂SO₄ to give SO₂ gas.

$$Cu+conc.2H_2SO_4\longrightarrow{CuSO_4+SO_2↑+2H_2O}$$

4.Oxidizing nature

Concentrated sulphuric and produces nascent oxygen and hence acts as a strong oxidizing agent.

$$H_2SO_4\longrightarrow{H_2O+SO_2+[O]}$$

i)Carbon is oxidized into CO₂

$$H_2SO_4\longrightarrow{H_2O+SO_2+[O]]\times2}$$

$$\underline{C+2[O]\longrightarrow{CO_2}}$$

$$C+2H_2SO_4\longrightarrow{2H_2O+2SO_2+CO_2}$$

ii) Sulphur is oxidized into SO₂

$$H_2SO_4\longrightarrow{H_2O+SO_2+[O]]\times3}$$

$$\underline{S+3[O]\longrightarrow{SO_3}}$$

$$S+3H_2SO_4\longrightarrow{3H_2O+3SO_2+SO_3}$$

iii) Phosphorous is oxidized into phosphoric acid

$$H_2SO_4\longrightarrow{H_2O+SO_2+[O]]\times5}$$

$$2P+5[O]\longrightarrow{P_2O_5}$$

$$\underline{P_2O_5+3H_2O\longrightarrow{2H_3PO_4}}$$

$$2P+5H_2SO_4\longrightarrow{2H_2O+2H_3PO_4+5SO_2}$$

iv) HI and HBr are oxidized into iodine and bromine respectively

$$H_2SO_4\longrightarrow{H_2O+SO_2+[O]}$$

$$\underline{2HBr+H_2SO_4\longrightarrow{2H_2O+SO_2+Br_2}}$$

$$2HBr+H_2SO_4\longrightarrow{2H_2O+SO_2+Br_2}$$

$$H_2SO_4\longrightarrow{H_2O+SO_2+[O]}$$

$$\underline{2HI+[O]\longrightarrow{H_2O+I_2}}$$

$$2HI+H_2SO_4\longrightarrow{2H_2O+SO_2+I_2}$$

v) Dehydrating action/nature

Sulphuric acid is used as a dehydrating agent. It has great affinity with water to give hydrorate which is exothermic process.

a) It removes water from sugar:

Conc. H2SO4 remains water molecule from sugar with appearing black mass of carbon which is called charring of sugar and this property is called charring action.

$$C_{12}H_{22}O_{11}\xrightarrow{condenses}{12C+11H_2O}$$

b) Crystal of oxylic and formic acid are dehydrated

$$COOH—COOH\; 2H_2O+conc.H_2SO_4\longrightarrow{CO+CO_2↑+H_2SO_4.3H_2O}$$

$$HCOOH\xrightarrow{conc.H_2SO_4}{CO↑+H_2SO_4.H_2O}$$

vi) Precepitation reaction

It acts as a precipating agent and hence precipates certain metallic ions from their salt solution with characterstict colour.

$$BaCl_2+H_2SO_4\longrightarrow{BaSO_4↓+2HCl}$$

$$Pb(NO_3)_2+H_2SO_4\longrightarrow{PbSO_4↓+2HNO_3}$$

vii) Formation of volatile acid

$$2NaCl+H_2SO_4\longrightarrow{Na_2SO_4+2HCl}$$

$$Ca_3(PO_4)_2+3H_2SO_4\longrightarrow{3CaSO_4+2H_3PO_4}$$

Laboratory Preparation of H₂S gas

Principle

In the laboratory H₂S gas is prepared by the action of non oxidising acid with iron sulphide.

$$FeS+H_2SO_4\longrightarrow{FeSO_4+H_2S↑}$$

$$Fe+dil.2HCl\longrightarrow{feCl_2+H_2S↑}$$

Hence formed gas is collected in gas jar by upward displacement of air.

Nitric acid and conc. H₂SO₄ are not used in the preparation of H₂S gas because they oxidise H₂S gas to free sulphur.

Intermittent supply of H₂S by Kipp's Apparatus

For analytical purpose, H₂S gas is needed in small quantity in the regular interval in small quantity. For this purpose, Kipp's apparatus is used. It contains bulbs 'A', 'B' and'C' in which bulb 'A' has a long stem that connects bulb 'B' and 'C' as shown in the figure.

Working process

The middle bulbs contain iron sulphide pieces. Dil H₂SO₄ is poured from the top of bulb A. When acid just covers the FeS pieces after filling the bulb C reactions starts to give H₂S gas and comes out when the tap is open.

$$FeS+H_2SO_4\longrightarrow{FeSO_4+H_2S↑}$$

When the tap is closed, the gaseous pressure in bulb 'B' increases that pushes H₂SO₄ up, up to the bulb 'A' which breaks the contact between acid and FeS and hence the formation of gas is stopped. When the tap is open H₂S gas comes out and acids come in place of H₂S and this process repeats.

Purification of gas

H₂S gas is purified by passing heat through the suspension of MgO.

$$MgO+2H_2S\longrightarrow{Mg(HS)_2+H_2O}$$

$$Mg(HS)_2\xrightarrow\Delta{H_2S↑+MgS}$$

Test of H₂S

It has a pungent smell(rotten egg odour) and is tested by passing it through the filter paper wetted by lead acetate which turns into black.

$$H_2S+Pb(CH_3COO)_2\longrightarrow{Pbs↓+2CH_3COOH}$$

Physical Properties

1. It is colourless gas with the pungent smell of rotten egg.
2. It is heavier than air.
3. It is fairly soluble in gas.
4. It is poisonous gas.

Chemical Properties

(1) Combustibility and thermal stability

It is unstable compound and decomposes into their elements on heating.

$$H_2S\xrightarrow\Delta{H_2+S}$$

It burns with blue flame with forming SO₂ gas and water.

$$2H_2S+3O_2\longrightarrow{2SO_2+2H_2O}$$

In the case of limited supply of air it burns with oxygen to give free sulphur due to incomplete combustion.

$$2H_2S+O_2\xrightarrow{limited}{2S+2H_2O}$$

(2) Acidic nature

It is weak diprotic acid and hence ionizes into two steps and forms two series of salt.

$$H_2S\rightleftharpoons{HS^-+H^+}$$

$$HS^-\rightleftharpoons{H^++S^{--}}$$

(3) Reducing nature

It is a strong reducing agent in which oxidation number of sulphur changes from -2 to 0.

a) Halogen are reduced into their halo-acid

$$H_2S+X_2\longrightarrow{2HX+S}$$ (where X = Cl, Br, I)

b) Ferric chloride is reduced into ferrous chloride

$$2FeCl_3+H_2S\longrightarrow{2FeCl_2+2HCl+S}$$

c) Hydrogen pero-oxide is reduced to water

$$H_2O_2+H_2S\longrightarrow{2H_2O+S}$$

d) Ozone is reduced into oxygen

$$O_3+H_2S\longrightarrow{O_2+H_2O+S}$$

e) SO₂ is reduced into free sulphur in presence of moisture

$$SO_2+2H_2S\xrightarrow{moisture}{2H_2O+3S}$$

f) Conc. HNO₃ is reduced into NO₂

$$2HNO_3\longrightarrow{H_2O+2NO_2+[O]}$$

$$\underline{H_2S+[O]\longrightarrow{H_2O+S}}$$

$$2HNO_3+H_2S\longrightarrow{2H_2O+2NO_2+S}$$

g) Conc. H₂SO₄ is reduced into SO₂

$$H_2SO_4\longrightarrow{H_2O+SO_2+[O]}$$

$$\underline{H_2S+[O]\longrightarrow{H_2O+S}}$$

$$H_2SO_4+H_2S\longrightarrow{2H_2O+SO_2+S}$$

h) The pink colour of acidified KMnO₄ is changed into colourless

$$2KMnO_4+3H_2SO_4\longrightarrow{K_2SO_4+2MnSO_4+3H_2O+5[O]}$$

$$\underline{H_2S+[O]\longrightarrow{H_2O+S]\times5}}$$

$$2KMnO_4+3H_2SO_4+5H_2S\longrightarrow{K_2SO_4+2MnSO_4+8H_2O+5S}$$

i) Light yellow color of acidified K₂Cr₂O₇ is changed into green

$$K_2Cr_2O_7+4H_2SO_4\longrightarrow{K_2SO_4+Cr_2(SO_4)_3+4H_2O+3[O]}$$

$$\underline{H_2S+[O]\longrightarrow{H_2O+S]\times3}}$$

$$K_2Cr_2O_7+4H_2SO_4+3H_2S\longrightarrow{K_2SO_4+Cr_2(SO_4)_3+7H_2O+3S}$$

(4) Precipitation of metal sulphide

H₂S gas is used as an anylitical agent to precypite certain metallic ions from their salt solution with characterstic colour.

a) Group II metal ion (acidic medium)

When H₂S is passed through the scidified solution of group II metal ions, they are precipitated in the form of sulphide with characterstics colour.

$$H^{++}+H_2S\xrightarrow{H^+}{HgS↓+2H^+}$$

$$Cu^{++}+H_2S\xrightarrow{H^+}{CuS↓+2H^+}$$

$$Cd^{++}+H_2S\xrightarrow{H^+}{CdS↓+2H^+}$$

b) Group IIIB metal ion (in alkaline medium)

When H₂S is passed through group IIIB metal ions, there metallic ions are precipitated in the form of sulphide with charcterstics colour.

$$Ni^{++}+H_2S\xrightarrow{OH^-}{NiS↓+2H^+}$$

$$Zn^{++}+H_2S\xrightarrow{OH^-}{ZnS↓+2H^+}$$

$$Mn^{++}+H_2S\xrightarrow{OH^-}{MnS↓+2H^+}$$

Laboratory Preparation of SO₂ gas

Principle

In the laboratory, SO2 gas is prepared by the action of cupper turnings with concentrated sulphuric acid. Hence, formed gas is collected in the gas jar by upward displacement of air.

$$Cu+2H_2SO_4\longrightarrow{CuSO_4+H_2O+SO_2}$$

Procedure

Cupper turnings pieces are taken in a round bottom flask. Conc. H₂SO₄ is poured through thistel funnel and apparatus is made air tight as shown in figure.

After heating the flask, SO₂ gas is formed which can be dried by passing it through conc. H2SO4 in order to absorb moisture, pure and dry gas is obtained in the gas jar by upward displacement of air.

Physical Properties

1. It is a colour gas with a pungent suffocating odour.
2. It is 2.2 times heavier than air.
3. It is highly soluble in water and the solution is called sulphurous acid.
4. It can be easily liquefied to a colourless liquid at -10°C and solidified at -26°C.

Chemical Properties

1. Thermal diassociation (Action of heat)
   It is dissociated into SO₃ gas on heating.
   $$3SO_2\xrightarrow\Delta{2SO_3+S}$$
2. Acidic nature
   It is acidic oxide and reacts with water to give sulphurous acid which is weak and diprotic in nature.
   $$SO_2+H_2O\longrightarrow{H_2SO_3}$$
   $$H_2SO_3\rightleftharpoons{H^++{HSO_3}^-}$$
   $${HSO_3}^-\rightleftharpoons{H^++{SO_3}^{--}}$$
3. Combustibility
   It is neither combustible nor a good suppoter of combustion but when burning magnesium ribbon is passed to SO2 gas it burns continously.
   $$3Mg+SO_2\longrightarrow{2MgO+MgS}$$
4. Oxidation
   When SO₂ is catalytically oxidized with air, sulphur trioxide gas (SO₃) is formed which is exothermic.
   $$2SO_2+O_2\rightleftharpoons{2SO_3+heat}$$
5. Reducing nature
   It acts as a reducing agent due to the formation of nascent hydrogen in its solution.
   1) Halogens are reduced into their halo acid.
   $$SO_2+2H_2O\longrightarrow{H_2SO_4+2[H]}$$$$\underline{X_2+2[H]\longrightarrow{2HX}}$$$$SO_2+2H_2O+X_2\longrightarrow{2HX+H_2SO_4}$$ [Where X = Cl, Br, I]
   2)Pink colour of acidified KMnO₄ solution is changed into colorless
   $$2KMnO_4+3H_2SO_4\longrightarrow{K_2SO_4+2MnSO_4+3H_2O+5[O]}$$$$\underline{SO_2+H_2O+[O]\longrightarrow{H_2SO_4]\times5}}$$$$2KMnO_4+5SO_2+2H_2O\longrightarrow{K_2SO_4+2MnSO_4+2H_2SO_4}$$
   3)Light yellow colour of acidified K₂Cr₂O₇ becomes green
   $$K_2Cr_2O_7+4H_2SO_4\longrightarrow{K_2SO_4+Cr_2(SO_4)_3+4H_2O+3[O]}$$$$\underline{SO_2+H_2O+[O]\longrightarrow{H_2SO_4]\times3}}$$$$K_2Cr_2O_7+3SO_2+H_2SO_4\longrightarrow{K_2SO_4+Cr_2(SO_4)_3+H_2O}$$
   4)Ferric salt reduced in ferrous salt
   $$SO_2+2H_2O\longrightarrow{H_2SO_4+2[H]}$$$$\underline{FeCl_3+[H]\longrightarrow{FeCl_2+HCl]\times2}}$$$$2FeCl_3+SO_2+2H_2O\longrightarrow{H_2SO_4+2FeCl_2+2HCl}$$
6. Bleaching property
   SO₂ acts as a bleaching agent. The bleaching action is due to:
   1) Reduction
   SO₂ bleaches other substances in its aqueous solution due to the formation of nascent hydrogen.
   $$SO_2+2H_2O\longrightarrow{H_2SO_4+2[H]}$$
   $$Vegetable\;colouring\;matter+[H]\longrightarrow{colorless\;reduced\;product}$$
   This type of reduction is temporary and reversible in nature in which original color is restored on aerial oxidation.
   $$Vegetable\;colouring\;matter+[H]\rightleftharpoons{Colourless\;reduced\;product}$$
   2) Formation of addition product
   SO2 bleaches in some cases due to the formation of addition product.
   $$Vegetable\;coloring\;matter+S_2\longrightarrow{Colourless\;addition\;product}$$
7. Oxidizing property
   It acts as an oxidizing agent for some reducing agent.
   $$SO_2+2H_2S\longrightarrow{2H_2O+3S}$$$$2Mg+SO_2\longrightarrow{2MgO+S}$$$$4HI+SO_2\longrightarrow{2H_2O+2I_2+S}$$

Sodium Thiosulphate (Na₂S₂O₃.5H₂O)

When one of the oxygen atoms in the sulphate ion is replaced by sulphur, the resulting ion (S₂O₃^2-) is known as thiosulphate. The free acid, H₂S₂O₃ is not stable but thiosulphate salts are stable.

Manufacture

Sodium thiosulphate is manufactured by boiling a solution of sodium carbonate (Na₂CO₃) saturated with SO₂ with powdered sulphur.

$$Na_2CO_3+SO_2\xrightarrow{boil}{Na_2SO_3+CO_2}$$

$$Na_2SO_3+S\xrightarrow{100°C}{Na_2S_2O_3}$$

Crystals of sodium thiosulphate pentahydrate are obtained on cooling the solution.

Application

1. It is used as antichlor to remove residual Cl2 from the fabric after bleaching.
2. Largely used in photography as a fixing agent. It reacts with the unreduced silver bromide on the film.
3. In the laboratory, it is used in the volumetric estimation of iodine.

Bibliography

Pant, Manju, Bhoj Raj Bhattarai and Nab Raj Adhikari.Comprehensive Chemistry. 6th edition . Vol. 1 . Kathmandu: Heritage Publisher and Distributors Pvt. Ltd., 2072.