Precipitation Hydrogen

Commercially, phosphinic acid and its salts are manufactured by treatment of white phosphoms with a boiling slurry of lime. The desired product, calcium phosphinite [7789-79-9], remains ia solution andiasoluble calcium phosphite [21056-98-4] is precipitated. Hydrogen and phosphine are also formed, the latter containing sufficient diphosphine to make it spontaneously flammable. The details of this compHcated reaction, however, are imperfectly understood. Under some conditions, equal amounts of phosphoms appear as phosphine and phosphite, and the volume of the hydrogen Hberated is nearly proportional to the hypophosphite that forms. 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acryhc acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubihty in water and retain this property after functionahzation with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by coohng the solution to 0 °C [92]. Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

Sulphides. Acquaint yourself with the solubility products of zinc and cadmium sulphides (see Appendix 1, Table 12). Prepare the sulphides of these metals from solutions of zinc and cadmium nitrates. What should be used as the precipitant—hydrogen sulphide or ammonium sulphide Note the colour of the precipitates and test their reaction with 1 N solutions of sulphuric and hydrochloric acids. 

When boiled with concentrated acetic acid, rhodium cyanide is precipitated, hydrogen cyanide escaping. Since the corresponding iridicyanide is not affected by similar treatment, this affords a useful method of separating rhodium from iridium. 

Ferric iron in solution exists in several forms. The more important forms ass Fe(SO)f. FeSOd+. FE OH), and FeOH2+. The sulfate complexes are greatly favored over the hydroxyl complexes, To illustrate the imponance of iron complex formation, a typical leach solution will be considered in equilibrium with precipitated hydrogen jarosile, having an approximate ftee-sulfate activity of 0.02. The equilibrium is represented by... 

Characters and Tests.—Colourless, generally opaque, cubic crystals, readily soluble in water, less so in spirit of wine. It commonly has a feebly alkaline reaction. Its solution mixed with mucilage of starch gives a blue colour on the addition of a minute quantity of solution of chlorine (to set free iodine), thus proving that the salt is an iodide. Its aqueous solution mixed with solution of tartaric acid or hydrogen-sodium tartrate, furnishes a crystalline precipitate (hydrogen-potassium tartrate), indicating the presence of potassium. 

A1(0H)3 will precipitate. Hydrogen micro-bubbles from the reduction of H ions will push the formed aluminium hydroxide to the opening of the pit where it forms a deposit of white pustules. 

CoAsS, are also used as sources. The ore is roasted and Co is precipitated as the hydroxide and then reduced to Co with carbon (hep below 417 - C, cep to m.p.). The metal is silvery white and readily polished. It dissolves in dilute acids and is slowly oxidized in air. Adsorbs hydrogen strongly. The main use of cobalt is in alloys. Cobalt compounds are used in paints and varnishes, catalysts. Cobalt is an essential element in the diet. World production 1976 32 000 tonnes metal. 

By analogy, ammonium salts should behave as acids in liquid ammonia, since they produce the cation NH4 (the solvo-cation ), and soluble inorganic amides (for example KNHj, ionic) should act as bases. This idea is borne out by experiment ammonium salts in liquid ammonia react with certain metals and hydrogen is given off. The neutralisation of an ionic amide solution by a solution of an ammonium salt in liquid ammonia can be carried out and followed by an indicator or by the change in the potential of an electrode, just like the reaction of sodium hydroxide with hydrochloric acid in water. The only notable difference is that the salt formed in liquid ammonia is usually insoluble and therefore precipitates. 

NH4. HCO3. CU and Na and the least soluble salt sodium hydrogen carbonate, is precipitated when ionic concentrations increase, and is removed by vacuum filtration. 

In presence of hydrochloric acid, tin(II) in aqueous solution (1) is precipitated by hydrogen sulphide as brown SnS, and (2) will reduce mercury(II) chloride first to mercury(I) chloride (white precipitate) and then to metallic mercury. 

Tin(IV) in aqueous acid gives a yellow precipitate with hydrogen sulphide, and no reaction with mercury(II) chloride. 

When hydrogen sulphide is bubbled into an acidic solution of an antimony or a bismuth salt an orange precipitate, SbjSs, or a brown precipitate, BijS, is obtained. Bismuth(III) sulphide, unlike antimony(IIl) sulphide, is insoluble in lithium hydroxide. 

Hence the orange colour of a dichromate is converted to the green colour of the hydrated chromium(III) ion, Cr ", and sulphur is precipitated when hydrogen sulphide is passed through an acid solution.)... 

Since most metallic sulphides are insoluble, many are precipitated when hydrogen sulphide is passed through solutions containing ions of the metals. Some are precipitated in acid, and others in alkaline... 

These are practically insoluble in water, are not hydrolysed and so may be prepared by addition of a sufficient concentration of sulphide ion to exceed the solubility product of the particular sulphide. Some sulphides, for example those of lead(II), copper(II) and silver(I), have low solubility products and are precipitated by the small concentration of sulphide ions produced by passing hydrogen sulphide through an acid solution of the metal salts others for example those of zincfll), iron(II), nickel(II) and cobalt(II) are only precipitated when sulphide ions are available in reasonable concentrations, as they are when hydrogen sulphide is passed into an alkaline solution. 

For a cobalt(ll) salt, the precipitation of the blue->pitik cobalt(II) hydroxide by alkali, or precipitation of black cobalt(II) sulphide by hydrogen sulphide provide useful tests the hydroxide is soluble in excess alkali and is oxidised by air to the brown CoO(OH) . 

Addition of an alkali metal hydroxide solution to an aqueous solution of a nickel(II) salt precipitates a finely-divided green powder. nickel(II) hydroxide NilOHfj on heating this gives the black oxide. NiO. which is also obtained by heating nickel(II) carbonate or the hydrated nitrate. Black nickel(II) sulphide, NiS, is obtained by passing hydrogen sulphide into a solution of a nickel(II) salt. 

CopperilT) sulphide, CuS, is obtained as a black precipitate when hydrogen sulphide is passed into a solution of a copper(II) salt. 

Copper(II) ions in aqueous solution are readily obtained from any copper-containing material. The reactions with (a) alkali (p. 430), (b) concentrated ammonia (p 413) and (c) hydrogen sulphide (p. 413) provide satisfactory tests for aqueous copper(II) ions. A further test is to add a hexacyanoferrate(II) (usually as the potassium salt) when a chocolate-brown precipitate of copper(II) hexacyanoferrate(II) is obtained ... 

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