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Chemical Properties of Thallium

Thallium forms two series of salts : thallous salts (TlX), derived from the powerfully basic oxide thallous oxide (Tl2O); and thallic salts (TlX3), derived from the weakly basic oxide thallic oxide (Tl2O3). All the salts of thallium are poisonous, producing symptoms like those of lead poisoning.

Thallous compounds and the compounds of the alkali metals have many points of similarity. The salts derived from colourless acids are generally colourless, and those which are soluble in water crystallise readily, usually as anhydrous salts. They are volatile at a red heat. Many soluble thallous salts are isomorphous with the corresponding salts of ammonium, potassium, or rubidium, a fact that was early recognised. Thallous hydroxide is a strong base. It is readily soluble in water, forming a solution which readily absorbs carbon dioxide, is soapy to the touch and alkaline in reaction.

Both in appearance and in solubility, a number of sparingly soluble thallous salts (chloride, bromide, iodide, sulphide, chromate, etc.) closely resemble the corresponding salts of lead.

In aqueous solution the thallous salts are ionised to approximately the same extent as the corresponding salts of the alkali metals. The following values for the percentage dissociation at 18° are given by A. A. Noyes and Falk: -


At 18° the ionic mobility of the thallous ion is 65.55, and, adopting Kohlrausch's value for its temperature-coefficient, the ionic mobility at 25° is 75.48. The corresponding value for potassium is 64.7 at 18°. The transport number for the thallous ion in dilute solution is 0.479 at 25°. The equivalent conductivities at 25° of a number of thallous salts are tabulated below: -


Numerous investigations have been carried out dealing with the influence of other salts on the solubilities of thallous salts. The results may be briefly summarised as follows. For uni-univalent thallous salts the changes of solubility caused by other salts are in qualitative agreement with the deductions from the ionic theory and the law of chemical equilibrium, and quantitative agreement, though never exact, is approached most closely with the least soluble thallous salts. For unibivalent thallous salts, the solubilities are affected by salts having a common univalent ion (i.e. the thallous ion) in a similar manner; but the influences exerted by other salts, having bivalent ions in common with the thallous salts, are not even in approximate agreement with theoretical deductions unless it be assumed that the unibivalent salts undergo dissociation in two stages, e.g. Tl2SO4Tl + TlSO4'; TlSO4' ⇔ Tl + SO4'', and that even in dilute solution the concentrations of the intermediate ions are considerable.

Thallous hydroxide is considerably less stable than an alkali hydroxide as is indicated by its relatively small heat of formation. It is easily dehydrated (even at 100°) to thallous oxide, which has a very small heat of hydration. Further, thallium is considerably less electropositive than the alkali metals. According to the best experimental results available (1916), the following sequence of metals in the electromotive series is correct and indicates the position of thallium : -

.... Zn, Cd, Fe, Tl, Co, Sn, Ni, Pb ....

Thallium has therefore a decided tendency to assume the ionic state. The electrode potential of thallium at 25° is -0.6170 volt (normal calomel electrode being zero). For very dilute solutions, the potential of the thallium electrode varies with the concentration of thallous ion in the liquid bathing it in strict accordance with the Nernst formula; a slight deviation is noticeable when the concentration of thallium ion reaches 0.1 normal. The accuracy with which the Nernst formula is capable of expressing the results is strong evidence against the hypothesis of the existence of subvalent thallium ion Tl2, which has been supposed by Denham to exist.

Thallic compounds

Thallic compounds resemble the compounds of aluminium to a certain extent. The thallic salts are in general readily soluble in water, and crystallise with considerable amounts of water of crystallisation. They are derived from a very weak base, thallic hydroxide, which is practically insoluble in water, and which in case of dehydration resembles auric hydroxide. The thallic salts are therefore considerably hydrolysed by water, and solutions of these salts are only stable in the presence of an excess of acid. A solution of thallic sulphate, for example, containing a slight excess of sulphuric acid, gives a brown precipitate of thallic hydroxide when diluted or warmed. Thallic salts are decidedly unstable, anhydrous thallic chloride, for instance, losing chlorine at temperatures below 100°, and they exhibit a great tendency to form complex salts.

Thallous salts may be oxidised to thallic salts and vice versa, the former conversion being not so readily accomplished as the latter. The reduction of thallic to thallous salts may be readily and quantitatively effected by the ordinary reducing agents, and it therefore happens that the addition of ammonium sulphide to a solution of a thallous salt leads to the precipitation of thallous sulphide and sulphur. Further, potassium iodide gives a precipitate of thallous iodide and iodine when added to a thallic salt. Thallic salts are quantitatively reduced to thallous salts by thallium itself. Thallous salts may be oxidised by potassium permanganate, the reaction when effected in the presence of hydrochloric acid and under certain conditions being sufficiently exact to be used for the estimation of thallium. Practically complete oxidation of thallous salts can also be brought about by chlorine or bromine.

Thallous and thallic salts exhibit to a high degree the curious property of combining with each other to form what may be termed intermediate salts, such, for example, as the chlorides of the composition Tl2Cl3(TlCl3.3TlCl) and

TlCl2(TlCl3.TlCl). Since these salts usually resemble the thallous salts in being sparingly soluble in water, it often happens that such intermediate salts are produced during the oxidation of thallous, or the reduction of thallic salts. The modus operandi of these processes, however, is not definitely known.

Thermochemistry of Thallium Compounds

A number of heats of formation, etc., are given in the following table: -

CompoundHeat of Formation CalsHeat of Solution Cals.
Tl2S (cryst.)17.7...
Tl2Te (cryst.)12.2...

[Tl2O] + H2O = 2[TlOH] + 3.25 Cals,
TlOHaq. + = + 13.74 Cals,
TlOHaq. + = + 13.70 Cals,
TlOHaq. + +16.4 Cals,
TlOHaq. + ½ = ½ + 15.6 Cals.

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