Potassium and its compounds

Potassiumis a soft silvery metallic element belonging to group 1 (formerly IA) of the periodic table; a.n. is 19; r.a.m. is 39.098; r.d. is 0.86; m.p. is 63.7°C; b.p. is 774°C. The element occurs in seawater and in a number of minerals, such as sylvite (KC1), carnallite (KCl.MgCl₂.6H₂O), and kainite (MgSO₄.KC1₂.6H₂O). It is obtained by electrolysis. The metal has few uses but potassium salts are used for a wide range of applications. Potassium is an essential element for living organisms. The potassium ion, K⁺, is the most abundant cation in plant tissues, being absorbed through the roots and being used in such processes as protein synthesis. In animals the passage of potassium and sodium ions across the nerve-cell membrane is responsible for the changes of electrical potential that accompany the transmission of impulses. Chemically, it is highly reactive, resembling sodium in its behaviour and compounds. It also forms an orange-coloured superoxide, KO₂, which contains the O₂^- ion. Potassium was discovered by Sir Humphry Davy in 1807.

Potassium bromide is a white or colourless crystalline solid, KBr, slightly hygroscopic and soluble in water and very slightly soluble in ethanol. Its shape is cubic; r.d. is 2.75; m.p. is 734°C; b.p. is 1435°C. Potassium bromide may be prepared by the action of bromine on hot potassium hydroxide solution or by the action of iron (III) bromide or hydrogen bromide on potassium carbonate solution. It is used widely in the photographic industry and is also used as a sedative. Because of its range of transparency to infrared radiation, KBr is used both as a matrix for solid samples and as a prism material in infrared spectroscopy.

Potassium carbonate (pearl ash; potash) is a translucent (granular) or white (powder) deliquescent solid known in the anhydrous and hydrated forms. K₂CO₃ (monoclinic; r.d. is 2.4; m.p. is 891°C) decomposes without boiling. 2K₂CO₃.3H₂O (monoclinic; r.d. is 2.04) dehydrates to K₂CO₃.H₂O above 100°C and to K₂CO₃ above 130^o C. It is prepared by the Engel-Precht process in which potassium chloride and magnesium oxide react with carbon dioxide to give the compound Engel's salt, MgCO₃.KHCo₃.4H₂O. It decomposes in solution to give the hydrogencarbonate, which can then be calcined to K₂CO₃. Potassium carbonate is soluble in water (insoluble in alcohol) with significant hydrolysis to produce basic solutions. Industrial uses include glasses and glazes, the manufacture of soft soaps. It is used also in laboratory as a drying agent.

Potassium chlorate is a colourless crystalline compound, KCIO₃, which is soluble in water and moderately soluble in ethanol; monoclinic; r.d. is 2.32; m.p. is 356°C. It decomposes above 400°C giving off oxygen. The industrial route to potassium chlorate involves the fractional crystallization of a solution of potassium chloride and sodium chlorate but it may also be prepared by electrolysis of hot concentrated solutions of potassium chloride. It is a powerful oxidizing agent finding applications in weedkillers and disinfectants. Due to its ability to produce oxygen it is used in explosives, pyrotechnics and matches.

Potassium chloride is a white crystalline solid, KC1, which is soluble in water and very slightly soluble in ethanol; its shape is cubic; r.d. is 1.98; m.p. is 772°C. It sublimes at 1500°C. Potassium chloride occurs naturally as the mineral sylvite (KC1) and as carnallite (KCl.MgCl₂.6H₂O). It is produced industrially by fractional crystallization of these deposits or of solutions from lake brines. It has the interesting property of being more soluble than sodium chloride in hot water but less soluble in cold. It is used as a fertilizer, in photography and as a source of other potassium salts such as the chlorate and the hydroxide. It has low toxicity.

Potassium chromate is a bright yellow crystalline solid, K₂CrO₄, soluble in water and insoluble in alcohol. Its shape is rhombic; r.d. is 2.73; m.p. is 968.3°C. It decomposes without boiling. It is produced industrially by roasting powdered chromite ore with potassium hydroxide and limestone and by leaching the resulting cinder with hot potassium sulphate solution. Potassium chromate is used in leather finishing as a textile mordant and in enamels and pigments. In the laboratory it is used as an analytical reagent and as an indicator. Like other chromium (III) compounds it is toxic when ingested or inhaled.

Potassium chromium sulphate (chrome alum) is a violet or ruby-red crystalline solid, K₂SO₄.Cr₂(SO₄)₃.24H₂O, that is soluble in water and insoluble in ethanol. Its shape is cubic or octahedral; r.d. is 1.826; m.p. is 89°C. It loses 10H₂O at 100°C, 12H₂O at 400°C. Six water molecules surround each of the chromium (III) ions and the remaining ones are hydrogen bonded to the sulphate ions. Like all alums, the compound may be prepared by mixing equimolar quantities of the constituent sulphates.

Potassium dichromate (potassium bichromate) is an orange-red crystalline solid, K₂Cr₂07, soluble in water and insoluble in alcohol. It is monoclinic or triclinic; r.d. is 2.68. Monoclinic changes to triclinic at 241.6°C; m.p. is 396°C. It decomposes above 500°C. It is prepared by acidification of crude potassium chromate solution (the addition of a base to solutions of potassium dichromate reverses this process). The compound is used industrially as an oxidizing agent in the chemical industry and in dyestuffs manufacture, in electroplating, pyrotechnics, glass manufacture, glues, tanning, photography and lithography and in ceramic products. Laboratory uses include application as an analytical reagent and as an oxidizing agent. Potassium dichromate is toxic and considered a fire risk on account of its oxidizing properties.

Potassium hydrogencarbonate (potassium bicarbonate) is a white crystalline solid, KHCO₃, soluble in water and insoluble in ethanol; r.d. 2.17. It decomposes at about 120°C. It occurs naturally as calcinite and is prepared by passing carbon dioxide into saturated potassium carbonate solution. It is used in baking, soft-drinks manufacture and in CO₂ fire extinguishers. Because of its buffering capacity it is added to some detergents and also used as a laboratory reagent.

Potassium hydrogentartrate (cream of tartar) is a white crystalline acid salt, HOOC(CHOH)₂COOK. It is obtained from deposits on wine vats (argol) and used in baking powders.

Potassium hydroxide (caustic potash; lye) is a white deliquescent solid, KOH, often sold as pellets, flakes, or sticks, soluble in water and in ethanol and very slightly soluble in ether. Its shape is rhombic; r.d. is 2.044; m.p. is 360.4°C; b.p. is 1320°C. It is prepared industrially by the electrolysis of concentrated potassium chloride solution but it can also be made by heating potassium carbonate or sulphate with slaked lime, Ca(OH)₂. It closely resembles sodium hydroxide but is more soluble and is therefore preferred as an absorber for carbon dioxide and sulphur dioxide. It is also used in the manufacture of soft soap, other potassium salts and in Ni-Fe and alkaline storage cells. Potassium hydroxide is extremely corrosive to body tissues and especially damaging to the eyes.

Potassium iodate is a white crystalline solid, KIO₃, soluble in water and insoluble in ethanol. It is monoclinic; r.d. is 3.9: m.p. is 560°C. It may be prepared by the reaction of iodine with hot concentrated potassium hydroxide or by careful electrolysis of potassium iodide solution. It is an oxidizing agent and is used as an analytical reagent. Some potassium iodate is used as a food additive.

Potassium iodide is a white crystalline solid, KI, with a strong bitter taste, soluble in water, ethanol, and acetone. Its shape is cubic; r.d. is 3.13; m.p. is 681°C; b.p. is 1330°C. It may be prepared by the reaction of iodine with hot potassium hydroxide solution followed by separation from the iodate (which is also formed) by fractional crystallization. In solution it has the interesting property of dissolving iodine to form the triiodide ion I₃^-, which is brown. Potassium iodide is widely used as an analytical reagent, in photography and also as an additive to table salt to prevent goitre and other disorders due to iodine deficiency.

Potassium manganate (VII) (potassium permanganate) is a compound, KMnO₄, forming purple crystals with a metallic sheen, soluble in water (intense purple solution), acetone and methanol but it is decomposed by ethanol; r.d. is 2.70. Decomposition is slightly above 100°C and is complete at 240°C. The compound is prepared by fusing manganese (IV) oxide with potassium hydroxide to form the manganate and electrolysing the manganate solution using iron electrodes at about 60°C. An alternative route employs production of sodium manganate by a similar fusion process, oxidation with chlorine and sulphuric acid, then treatment with potassium chloride to crystallize the required product.

Potassium manganate (VII) is widely used as an oxidizing agent and as a disinfectant in a variety of applications and as an analytical reagent.

Potassium monoxide is a grey crystalline solid, K₂O. Its shape is cubic; r.d. is 2.32. Decomposition occurs at 350°C. It may be prepared by the oxidation of potassium metal with potassium nitrate. It reacts with ethanol to form potassium ethoxide (KOC₂H₅) and with liquid ammonia to form potassium hydroxide and potassamide (KNH₂).

Potassium nitrate (saltpetre) is a colourless rhombohedral or trigonal solid, KNO₃, soluble in water, insoluble in alcohol; r.d. is 2.109. Transition to trigonal forms at 129°C; m.p. is 334°C. It decomposes at 400°C. It occurs naturally as nitre and may be prepared by the reaction of sodium nitrate with potassium chloride followed by fractional crystallization. It is a powerful oxidizing agent (releases oxygen on heating) and is used in gunpowder and fertilizers.

Potassium nitrite is a white or slightly yellow deliquescent solid, KNO₂, soluble in water and insoluble in ethanol; r.d. is 1.91; m.p. is 440°C. It may explode at 600°C. Potassium nitrite is prepared by the reduction of potassium nitrate. It reacts with cold dilute mineral acids to give nitrous acid and is also able to behave as a reducing agent (if oxidized to the nitrate) or as an oxidizing agent (if reduced to nitrogen). It is used in organic synthesis because of its part in diazotization and in detecting the presence of the amino groups in organic compounds.

Potassium sulphate is a white crystalline powder, K₂SO₄, soluble in water and insoluble in ethanol. Its shape is rhombic or hexagonal; r.d. is 2.66; m.p. is 1069°C. It occurs naturally as schonite (Strassfurt deposits) and in lake brines from which it is separated by fractional crystallization. It has also been produced by the Hargreaves process which involves the oxidation of potassium chloride with sulphuric acid. In the laboratory it may be obtained by the reaction of either potassium hydroxide or potassium carbonate with sulphuric acid. Potassium sulphate is used in cements, in glass manufacture, as a food additive and as a fertilizer (source of K⁺) for chloride-sensitive plants, such as tobacco and citrus.

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