Thallium

Chemical reactions


Reaction of thallium with acids


Tl(I) is oxidized to Tl(III) by aqua regis

NO3(aq) + 3 Cl(aq) + 4 H+(aq) 1.5 Cl2(aq) + NO(aq) + 2 H2O(l)
Tl+(aq) + Cl2(aq) Tl3+(aq) + 2 Cl(aq)


Reaction of thallium with air


Thallium reacts with oxygenm O2, forming a passivating thallium oxide surface. When heated, thallium oxide is formed.

4 Tl(s) + O2(g) 2 Tl2O(s) [grey]


Reaction of thallium with ammonia


Tl(I) is not precipitated by ammonia and does not form complexes either.


Tl(III) is precipitated by ammonia as Tl(III) oxide due to the formation of hydroxide

H2O(l) + NH3(aq) NH4+(aq) + OH(aq)
2 Tl3+(aq) + 6 OH(aq) Tl2O3(s) [brown] + 3 H2O(l)


Reaction of thallium with chromate


Tl(I) is precipitated in acetic acid, under neutral and alkaline conditions. The precipitate is insoluble in strong alkali

2 Tl+(aq) + CrO42−(aq) Tl2CrO4(s) [yellow]


Reaction of thallium with halogens


Thallium reacts vigorously with fluorine, F2, chlorine, Cl2, and bromine, Br2, forming the corresponding thallium(III) halides:

2 Tl(s) + 3 F2(g) 2 TlF3(s)
2 Tl(s) + 3 Cl2(g) 2 TlCl3(s)
2 Tl(s) + 3 Br2(l) 2 TlBr3(s)


Tl(I) is precipitated by chloride, bromide and iodide:

Tl+(aq) + Cl(aq) TlCl(s) [white]
Tl+(aq) + Br(aq) TlBr(s) [white]
Tl+(aq) + I(aq) TlI(s) [yellow]


Tl(I) is oxidized to Tl(III) by aqua regis

NO3(aq) + 3 Cl(aq) + 4 H+(aq) 1.5 Cl2(aq) + NO(aq) + 2 H2O(l)
Tl+(aq) + Cl2(aq) Tl3+(aq) + 2 Cl(aq)


Tl(III) is not precipitated by chloride and bromide, but forms complexes with up to 4 halogens

Tl3+(aq) + 4 Cl(aq) [TlCl4](aq)
Tl3+(aq) + 4 Br(aq) [TlBr4](aq)


Tl(III) is precipitated by iodide. The precipitate is dissolved by excess iodide. Heating Tl(III) iodide reduces Tl(III) to Tl(I)

Tl3+(aq) + 3 I(aq) TlI3(s) [black]
TlI3(s) + I(aq) [TlI4](aq)
TlI3(s) + Δ TlI3(s) + Δ TlI(s) + I2(g)


Reaction of thallium with hydroxide ions


Tl(I) is not precipitated by hydroxide, as TlOH is soluble in water.


Tl(III) is precipitated by hydroxide as Tl(III) oxide

2 Tl3+(aq) + 6 OH(aq) Tl2O3(s) [brown] + 3 H2O(l)


Reaction of thallium with sulfate


Tl(I) is not precipitated by sulfate ions.


Reaction of thallium with sulfide


Tl(I) is precipitated in acetic acid, under neutral and alkaline conditions. The precipitate is soluble in inorganic acids and insoluble in sodium sulfide solution

2 Tl+(aq) + S2−(aq) Tl2S(s) [black]


Tl(III) is not precipitated by hydrogen sulfide in inorganic acid, instead Tl(III) is reduced to Tl(I)

Tl3+(aq) + H2S(aq) Tl+(aq) + S(s) + 2 H+(aq)


Reaction of thallium with sulfoxides


Tl(III) is reduced to Tl(I) by sulfur dioxide and heat

Tl3+(aq) + SO2(g) + 2 H2O(l) Tl+(aq) + SO42−(aq) + 4 H+(aq)


Reaction of thallium with water


Thallium reacts with water, forming thallium hydroxide, TlOH, and hydrogen gas, H2

2 Tl(s) + 2 H2O(l) 2 TlOH(aq) + H2(g)


Qualitative analysis


The reactions should be done at 20 °C.

1. Dissolve the thallium salt in water.
2. Add a solution of 0.4% bismuth nitrate (Bi(NO3)3) in 20% nitric acid (HNO3). Remove precipitates if formed.
3. Add a solution of 10% sodium iodide (NaI) in a saturated sodium thiosulfate solution.

Thallium will give a red precipitate [1]. The sensitivity of the test is 10 mg/L [1]


Quantitative analysis


Method 3500-Tl C Inductively Coupled Plasma Method [5]. A portion of the sample is digested in a combination of acids. The digest is aspirated into an 8,000 K argon plasma where resulting light emission is quantified for 30 elements simultaneously.

Method limit of detection in water = 0.005 mg/L
Method limit of detection in soil = 1.00 mg/kg