Thallium

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)

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]

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)

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]

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 Tl_I(s) + I₂(g)

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)

Tl(I) is not precipitated by sulfate ions.

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)

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)

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

2 Tl(s) + 2 H₂O(l) 2 TlOH(aq) + H₂(g)

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]

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