Chemical Properties of Bismuth

Bismuth is not readily attacked by air at ordinary temperatures, but on heating in air it is converted to trioxide. When heated in air to its boiling point it burns with a faint bluish-white flame, forming bismuth trioxide, which condenses as a yellow deposit of "flowers of bismuth" (flores bismuti) upon a cold surface placed in the flame. It reacts slowly at ordinary temperatures with water from which carbon dioxide has been expelled, becoming coated with an hydrated oxide; and at red heat there is some evidence to indicate that it decomposes steam slowly.

Spark spectrum of dilute solutions of Bismuth trichloride

1 per cent. Solution.	0.1 per cent. Solution.	0.01 per cent. Solution.
3792.9	. . .	. . .
3695.5	. . .	. . .
3595.7	. . .	. . .
3510.8	. . .	. . .
3430.9	. . .	. . .
3396.7	. . .	. . .
3067.7	3067.7	3067.7
3023.8	3023.8
2992.2	2992.2
2989.0	. . .	. . .
2938.3	. . .	. . .
2897.9	2897.9	. . .
2854.8	2854.8	. . .
2846.1	2846.1	. . .
2414.8	. . .	. . .

Bismuth is not attacked by hydrochloric acid in the absence of air, but in the presence of air it is slowly dissolved. No hydrogen is evolved. It is readily dissolved by cold nitric acid or aqua regia and by hot concentrated sulphuric acid; it is possible that the solvent action of nitric acid is due to the presence of nitrous acid. The nitrous acid is presumed to act catalytically, and the reaction may possibly be represented by the equation

2Bi + 6HNO₃ = Bi(NO₂)₃ + Bi(NO₃)₃ + 3H₂O

In the presence of nitric acid, any bismuth nitrite formed is at once converted to bismuth nitrate with the production of oxides of nitrogen. Bismuth thus resembles silver, mercury and copper in this reaction. With nitric acid of density 1.2, and at a temperature of 65° C., bismuth reacts with instantaneous evolution of nitrogen tetroxide (even in an atmosphere of hydrogen), and evolution of this gas continues until all the metal is dissolved. Neither ammonia nor hydroxylamine is produced by the action of nitric acid on bismuth under any conditions. Bismuth does not react with phosphoric acid, either in dilute or concentrated solution. It is oxidised slowly by chloric acid and the product is only partially soluble in water.

Metallic bismuth does not react with hydrogen even when heated. A black powder, described as pyrophoric bismuth, can be prepared by the reduction of bismuth compounds with hydrogen. From solutions of its compounds, bismuth may be displaced by hydrogen under pressure. From solutions of bismuth trichloride up to normal concentration, and with pressures between 15 and 250 atmospheres and temperatures between 150° and 200° C., this displacement may be represented by the expression



From this it is calculated that from a normal solution of bismuth trichloride at 20° C. and with hydrogen at 100 atmospheres pressure, one per cent, of bismuth would be precipitated in thirty-seven years. There are reasons for believing that the reaction is ionic. Precipitation of bismuth from acetic acid solutions takes place more rapidly than from solutions in hydrochloric acid. Hydrogen under a pressure of 60 atmospheres will also displace bismuth from the triphenyl derivative dissolved in xylene, according to the equation

2(C₆H₅)₃Bi + 3H₂ = 6C₆H₆ + 2Bi

At 225° C. this reaction is complete.

Bismuth does not react readily with the halogens when the latter are perfectly dry; the presence of moisture, however, greatly accelerates reaction. The metal is attacked only superficially by fluorine even at red heat; it reacts with chlorine, bromine and iodine to form in each case an impure halide, which may possibly be a mixture of the di-halide with the tri-halide.

Bismuth is oxidised by ozone, the product being a mixture of oxides. The metal combines directly with sulphur, selenium and tellurium when melted with those elements. It combines with sulphur when a mixture of the two elements is submitted to pressure. It does not react with dry sulphur dioxide even on heating, but when heated with sulphurous acid under pressure bismuth trisulphide is formed.

Bismuth does not combine directly with nitrogen, and with phosphorus only with difficulty. With arsenic and antimony it forms alloys; it is doubtful if intermetallic compounds are formed with either. It is oxidised to trioxide by the action of nitric oxide at 200° C. It is very slowly attacked by ammonium nitrate.

Bismuth will dissolve to a slight extent in solutions of alkalis, and evidence has been obtained of the formation of alkali bismuthites.

The position of bismuth in the electromotive series is a little doubtful; in the series as usually given its position is anomalous, since it lies between antimony and arsenic, thus:

Cs . . . Zn, Cd, Fe, Co, Ni, Sn, Pb, H, Sb, Bi, As, Cu, Hg, Ag, Pd, Pt, Au . . . F

If the elements are arranged in the order of the heats of formation of the chlorides the following series is obtained:

⁺Hg, Tl, Pb, Bi, Sn, Sb, As, P, Te, Se, S^-

which is in fair agreement with the order of the electrode potentials. All the elements in this series are electronegative to elements which are truly electropositive, such as the alkali metals, and are electropositive to those which are truly electronegative, such as the halogen elements. They may be regarded as amphoteric, a view that is supported by the behaviour of their compounds with alkali metals. Many of these compounds are metallic in character, and are decomposed by water, probably hydrolytically, with the formation of a hydride of the more electronegative element. (Evidence for this has been obtained in the case of sodium stannide.) These compounds with alkali metals are, however, soluble without decomposition in liquid ammonia, and the solutions behave as electrolytic conductors. Thus in a solution of sodium bismuthide in liquid ammonia it is probable that the anion is composed of a group of bismuth atoms, sodium forming the cation, since these solutions behave similarly to those of ordinary salts, and show no metallic properties. The electromotive series for these amphoteric elements corresponds with the series given above derived from the heats of formation of their chlorides, although the position of bismuth and phosphorus is doubtful owing to the sluggish action of their alkali compounds in solution in liquid ammonia. From the decomposition voltages of metallic bromides dissolved in aluminium bromide the electromotive series is found to be

⁺Al, Zn, Cd, Hg, Sb, Bi^-

In the electrolysis of fused alloys of copper, tin and bismuth at 1000° C. copper migrates to the cathode and tin and bismuth to the anode.

Bismuth can be precipitated from solution completely by tin, zinc, cadmium, iron, manganese and magnesium, but only partially by lead and copper.

The electrode potential of bismuth has been determined with respect to various cells. The normal potential between bismuth and a normal solution of a bismuth salt in the cell

Hg | KCl | N Bismuth salt | Bi

is, for bismuth sulphate -0.490 volt, for bismuth chloride -0.315 volt and for bismuth nitrate -0.500 volt. By calculation from the hydrogen-bismuth cell, using hydrochloric acid, the specific potential is -0.1635 volt at 15° C., -0.1599 volt at 25° C. and -0.1563 volt at 35° C. In molar solutions of bismuth perchlorate the bismuth may be present either as BiO⁺ or as BiOH⁺⁺. The electrode potential has been calculated from e.m.f. measurements upon such solutions using a bismuth electrode. Assuming that all the bismuth is present as BiO⁺ the potential is -0.314 volt; assuming that it is all present as BiOH⁺⁺ the potential is -0.298 volt.

Smooth crystalline cathode deposits of bismuth can be obtained by electrolysis of a solution of bismuth perchlorate. The recommended solution contains 4 grams of bismuth trioxide and 10.4 grams of perchloric acid in 100 c.c. of solution, with, as addition reagents, 0.03 per cent, of glue and 0.08 per cent, of cresol. In the absence of addition reagents, the decomposition voltage of bismuth perchlorate is 1.62 volts; for deposition a current density of 3.1 amperes per square decimetre is recommended. At present there appears to be very little practical demand for electro-deposited bismuth, but it is possible that the process may be applicable to the manufacture of certain components for electrical and magnetic apparatus. The process does not appear to be suitable for the refining of bismuth.

In a normal solution of bismuth silicofluoride the electrode potential, compared with the hydrogen electrode, is -0.295 volt. It is reported that a Japanese company employs a solution of this nature for the refining of bismuth.

The overvoltage for bismuth in 2N H₂SO₄ at 25° C. is 0.388±0.004 volt.

The anodic corrosion of bismuth in nitrate solutions has been studied; under suitable conditions bismuthyl nitrate is formed. In alkaline solutions bismuth dissolves anodically to form alkali bismuthite. When the concentration of the solution exceeds one gram of bismuth per litre the anode becomes passive as a result of the formation of a coating of oxides of bismuth. Bismuth also dissolves at the cathode in alkaline solution.

Sols of bismuth have been prepared in a variety of ways. In the earlier methods, organic salts of bismuth were reduced in very dilute solution, as for the preparation of bismuth monoxide. The electric pulverisation method has also been employed. More recently, investigators have employed chiefly reduction methods, formaldehyde, sodium bisulphite and sodium thiosulphate being used as reducing agents. The sols produced are frequently very unstable, but the stability is greatly increased by the addition of protective colloids.

There is evidence that many of these sols are contaminated by oxide. Evidence of sol formation has been observed during electrolysis of distilled water between bismuth electrodes using a low voltage.

In accordance with its position in the Periodic Table, bismuth shows more metallic properties than any other element of this sub-group; it resembles the other members in showing variable valency, but compounds other than tervalent are, in general, either unstable or of doubtful existence. Five oxides have been described, viz. bismuth monoxide Bi₂O₂, trioxide Bi₂O₃, tetroxide Bi₂O₄, pentoxide Bi₂O₅, and hexoxide Bi₂O₆. The lower oxides are definitely basic, although only the halides and some organic compounds of bivalent bismuth have been described, while the higher oxides show feeble acidic properties. No definite acid has been isolated, but alkali bismuthates, derived from quadri- and quinquevalent bismuth, have been prepared. These are, however, very unstable, being decomposed by water with evolution of oxygen. Salts of tervalent bismuth are readily hydrolysed by water, many intermediate products of great complexity being formed, the structure and composition of which are still to some extent uncertain; the hydrolysis is, however, incomplete, as the final product is, in each case, an oxy-salt of the type BiOX, e.g. BiOCl. A characteristic feature of tervalent compounds, particularly the halides, is the tendency to form complex compounds with halogen acids. In these complexes the bismuth atom appears to become quinquevalent and to enter a complex anion.

Bismuth and Hydrogen

From the position of bismuth in the Periodic Table it is to be expected that a stable hydride of bismuth would be formed only with difficulty. Until comparatively recently the existence of bismuth hydride was doubtful, but later investigations have revealed that a gaseous tri-hydride can exist, and that it resembles in many ways the trihydrides of antimony and arsenic. A solid hydride has also been reported. Hydrogen is not absorbed by bismuth in the electric discharge tube.

A substance described as bismuth dihydride, Bi₂H₂, has been obtained by the action of zinc and hydrochloric acid upon bismuth trichloride. It is a grey solid which decomposes when heated in vacuo or in a current of hydrogen. The true nature of this substance is, however, not fully established.

Bismuth and the Halogens

In the principal compounds of bismuth with the halogens, the bismuth is tervalent. It is possible that an unstable pentafluoride, or an oxy-trifluoride, exists at low temperatures, but there is no evidence for the existence of other quinquevalent compounds except perhaps in complexes. Bivalent compounds of bismuth with the halogens (with the exception of fluorine) have been reported, but it is doubtful if these have been obtained in a pure state, and they are not very stable. With the exception of the fluorides, the halides are all hydrolysed by water, but the hydrolysis is not complete, the final product being an oxyhalide. The halides tend to form complexes with the corresponding halogen acid; these complexes are themselves acidic, and in all cases except the fluoride, stable salts have been obtained.

Bismuth and Chlorine

Although the chief compound of bismuth and chlorine is bismuth trichloride, BiCl₃, many of the older investigators held that a lower chloride, bismuth dichloride, BiCl₂ or Bi₂Cl₄, also existed. It is stated that this compound is formed as a black substance when a slow current of chlorine is passed over powdered bismuth heated nearly to the melting point; after prolonged treatment it changes to a light amber liquid from which the trichloride can be obtained by sublimation. It is claimed that bismuth dichloride may also be obtained by heating a mixture of bismuth and mercurous chloride to between 230° and 250° C., by heating bismuth ammonium chloride in a current of hydrogen at 300° C., by the reduction of bismuth trichloride by bismuth, phosphorus, zinc, tin, mercury and certain organic compounds or by hydrogen, and by heating bismuth trichloride with phosphorus trichloride. More recently, thermal investigations of the system Bi-BiCl₃ have been undertaken, but the evidence obtained concerning the existence of the dichloride is conflicting. Herz and Guttmann found a maximum on the liquidus curve at a composition corresponding to BiCl₂, the melting point being 163° C. and the density 4.85 to 4.88 (the latter value being lower than the density of the equivalent mixture of bismuth and bismuth trichloride); while Eggink could find no evidence for the existence of BiCl₂ but suggested that both BiCl and BiCl₄ (see fig.) were formed. According to Marino and Becarelli, the so-called bismuth dichloride is really a solid solution; but although the melting point of this solution is higher than that of either bismuth or bismuth trichloride, they were unable to determine any maxima on the liquidus curve on account of sublimation. This solid solution undergoes a transformation into a β-variety, the change being accompanied by a marked evolution of heat; on fusion and cooling, these β-crystals change into a-crystals of different composition and two liquid phases separate out.3 An investigation into the free energy of fused bismuth trichloride did not afford any evidence in favour of the existence of a lower chloride.

Freezeng Point Curve of the System Bismuth-Clorine

The compound has certainly not been obtained pure, although by cooling the melt obtained by heating bismuth trichloride with bismuth, black, needle-shaped crystals have been obtained.5 The impure substance is dull black, and very hygroscopic; it melts readily. Many of the reactions ascribed to it could almost equally well be ascribed to a mixture of bismuth and its trichloride. When heated to about 300° C. in the absence of air the substance decomposes into bismuth and bismuth trichloride. Heated in air it forms a mixture of bismuth, bismuth trioxide and bismuthyl chloride. It is readily decomposed by water according to the equation

3BiCl₂ + 2H₂O = Bi + 4HCl + 2BiOCl

It combines with chlorine to form the trichloride. With a concentrated solution of potassium hydroxide the so-called black bismuth suboxide is obtained, which rapidly oxidises to the yellow trioxide. Dilute acids decompose it yielding salts of tervalent bismuth and metallic bismuth.

The substance does not combine with ammonia. It has been suggested, however, that a double compound with ammonium chloride is formed having the composition BiCl₂.NH₄Cl although the dichloride is decomposed by a concentrated solution of ammonium chloride.

Bismuth and Oxygen

Several oxides of bismuth have been reported - they include, bismuth monoxide, BiO, trioxide, Bi₂O₃, tetroxide (or dioxide), Bi₂O₄, pentoxide, Bi₂O₅, and hexoxide, Bi₂O₆. The most stable of these is the trioxide, which is mainly basic in its reactions, possessing only very feeble acidic properties. The higher oxides are unstable and very slightly acidic; alkali bismuthates, derived from the pentoxide, are however more stable, and are employed as oxidising agents.

Higher Oxides of Bismuth

Various higher oxides of bismuth have been described from time to time, although in many cases it is uncertain that the substances obtained were pure. Three of these oxides will be discussed here, namely, bismuth tetroxide, Bi₂O₄, bismuth pentoxide, Bi₂O₅, and bismuth hexoxide, Bi₂O₆.

As early as 1818 an oxide of bismuth containing more oxygen than the trioxide was prepared, but this, and many of the preparations subsequently described by other investigators, were probably mixtures of the tetroxide and the pentoxide, and may even in some cases have contained an alkali bismuthate. In most cases the higher oxide was prepared by the action of an oxidising agent upon a suspension of the trioxide in an alkaline solution. Oxidising agents that have been employed include ozone, hydrogen peroxide, potassium persulphate, alkali hypochlorite, chlorine, bromine, and potassium ferricyanide. Molten bismuth trioxide has also been oxidised by air, potassium chlorate and potassium nitrate. In addition, higher oxides have been obtained by electrolytic methods.

Many of the methods just mentioned, involving the oxidation of bismuth trioxide in the presence of alkalis, have been repeated, but in no case was a compound of uniform composition obtained; and the compound previously considered to be bismuthic acid, HBiO₃, always contained less oxygen than corresponds with this formula and was not uniform in composition, and further the product shows no sign of salt formation with a concentrated solution of potassium hydroxide. Probably the product obtained is a mixture of higher oxides which possess no acidic properties. Attempts to obtain a uniform product by electrolytic methods have also proved unsuccessful.

Bismuth and Sulphur

Bismuth and sulphur combine when heated together. Two sulphides have been described; they are, bismuth monosulphide, BiS or Bi₂S₂, and bismuth trisulphide, Bi₂S₃. No evidence has yet been obtained in favour of a sulphide higher than the trisulphide. Bismuth trisulphide is the more stable compound of the two; indeed, the view has been put forward that the monosulphide is really a mixture of the trisulphide and metal. An examination of the freezing point curve of mixtures of bismuth and sulphur fails to reveal any indication of the existence of bismuth monosulphide. A brief description of the results of investigations on this substance, however, follows.

Bismuth, Oxygen and Sulphur

Numerous compounds of bismuth with oxygen and sulphur have been described. The majority of these are basic salts of uncertain constitution. Certain naturally-occurring minerals may possibly be oxysulphides, such as karelinite and bolivite, but the evidence does not appear to be sufficient to decide their true nature. A substance of composition corresponding to Bi₂O₃S has been obtained as a greyish- black powder by the action of dry hydrogen sulphide upon bismuth pentoxide (the latter probably containing tetroxide), and by passing hydrogen sulphide through a suspension of bismuth pentoxide in boiling benzine. It is stable in air up to 120° C., but when heated above that temperature is converted into bismuth trioxide and sulphur dioxide. It dissolves in hydrochloric acid with evolution of hydrogen sulphide.

Bismuth and Selenium

Three compounds of bismuth and selenium have been reported, Bi₂Se, BiSe and Bi₂Se₃. Of these BiSe and Bi₂Se₃ are probably true compounds, the latter being the more stable. Octahedral crystals of bismuth subselenide, Bi₂Se, are stated to be formed when selenium is melted with a large excess of bismuth. The existence of this compound, however, has not been confirmed. The melting point curve of the system Bi-Se indicates the formation of the monoselenide, BiSe, which decomposes at 602° C. This compound is dimorphous, with a transition point at 422° C. On heating bismuth with excess of selenium, the triselenide, Bi₂Se₃, is formed. The mineral silaonite, found in Mexico, to which the formula Bi₈Se₃ was formerly given, is more probably a mixture of the triselenide Bi₂Se₃ with bismuth.

Bismuth and Tellurium

By the action of hydrogen telluride or sodium telluride upon a salt of bismuth, a monotelluride, BiTe, has been obtained, which is described as being unstable in air, as having reducing properties and as being soluble in acids. No evidence for the existence of this compound has, however, been obtained from thermal and microscopic examinations of the system Bi-Te, bismuth tritelluride, Bi₂Te₃, being the only definite compound of the two elements indicated. Bismuth telluride occurs in certain minerals, while a thiotelluride, Bi₂Te₂S, occurs as tetradymite. It is probable that the mineral montanite contains a bismuth tellurate, (BiO)₂TeO₄, in a hydrated form. No compound of this type appears, however, to have been obtained artificially.

Bismuth and Antimony

No binary compound of bismuth and antimony has as yet been obtained; the two metals form a continuous series of solid solutions.

Two antimonates of bismuth have been prepared by the addition of a concentrated solution of potassium meta-antimonate to a solution of bismuth ammonium citrate. The first, which is variously described as bismuth oxymeta-antimonate, (BiO)SbO₃, or bismuth orthoantimonate, BiSbO₄, is obtained as an amorphous white precipitate; while, in the presence of excess of ammonia, a basic orthoantimonate, (2BiO)₃SbO₄.H₂O, is obtained as a gelatinous precipitate. The composition of the latter does not appear to have been definitely ascertained.