p Block Elements
12.0 Phosphorus
12.0 Phosphorus
It glows in the dark and was, therefore called phosphorus (Greek word, phos=light, and phero= 1 carry)
Occurrence:
Since phosphorus is an active element, it is not found free in nature. It is widely distributed in nature in the combined state. it occurs as phosphate in the rocks and in the soil and as phosphoproteins in all living beings. It is an essential constuients of bones, teeth, blood and nervous tissue. It is necessary for the growth of plants. Bone ash contains about $80\%$ calcium phosphate, it is present in milk, eggs, and excreta of seabirds. The principal minerals of phosphorus are
(i) Phosphorite $C{a_3}{(P{O_4})_2}$
(ii) Fluorapatite $3C{a_3}{(P{O_4})_2}.Cs{F_2}$
(iii) Chlorapatite $3C{a_3}{(P{O_4})_2}.CaC{l_2}$
Extraction:
Phosphorus is extracted either from phosphorite or bone ash by application of the following processes.
(i) Retort process or old process.
(ii) Electrothermal process or modern process.
(i) Retort process or old process.
The phosphorite mineral or bone ash is digested with concentrated sulphuric acid about 60%. Insoluble calcium sulphate and orthophosphoric acid are formed.
The syrupy liquid is separated from insoluble residue by filtration. The liquid is evaporated when it changes into metaphosphoric acid with the evolution of water.
The metaphosphoric acid is mixed with powdered coke and distilled in fireclay retorts at a bright red hot. The acid is reduced to phosphorous by carbon which comes in vaporized form. The vapours are condensed below water.
(ii) Electrothermal process or modern process.
The mixture of phosphorite, carbon, and silica is dried and then introduced into the electric furnace. The furnace is an iron tank lined inside with refractory bricks. Carbon electrodes are fitted on either side of the furnace. The furnace has two exits, one for removal of vapours in the upper part of the furnace and the other for removal of slag in the lower part of the furnace. The charge is introduced through the closed hopper arrangement.
The mixture is heated at $1400^\circ C - 1500^\circ C$ by the discharge of an alternating current between carbon electrodes. Silica combines with calcium phosphate and forms phosphorus pentaoxide which is reduced by carbon into phosphorus.
Vapours of phosphorus and carbon monoxide leave the furnace through the upper exit and are condensed under water. The liquid slag is tapped out periodically through an exit in the base.
Purification:
Phosphorus obtained is further purified by melting under acidified potassium dichromate solution when the impurities are oxidised . It is redistilled.
