Sodium bismuthate

Sodium bismuthate is an inorganic compound, and a strong oxidiser with chemical formula NaBiO₃. It is somewhat hygroscopic, but not soluble in cold water, which can be convenient since the reagent can be easily removed after the reaction. It is one of the few water insoluble sodium salts. Commercial samples may be a mixture of bismuth(V) oxide, sodium carbonate and sodium peroxide.

A related compound with the approximate formula Na₃BiO₄ also exists.

Structure

Sodium bismuthate adopts an ilmenite structure, consisting of octahedral bismuth(V) centers and sodium cations. The average Bi–O distance is 2.116 Å. The ilmenite structure is related to the corundum structure ( Al₂O₃) with a layer structure formed by close packed oxygen atoms with the two different cations alternating in octahedral sites.

Synthesis

Bismuth oxidizes to the +V oxidation state only with difficulty in the absence of alkali. Synthesis is performed by making a suspension of bismuth trioxide in a boiling sodium hydroxide solution. It is then oxidized by addition of bromine to form sodium bismuthate.

:Bi₂O₃ + 6 NaOH + 2 Br₂ → 2 NaBiO₃ + 4 NaBr + 3 H₂O

Another synthesis of NaBiO₃ involves oxidizing a mixture of sodium oxide and bismuth(III) oxide with air (as the source of O₂):
:Na₂O + Bi₂O₃ + O₂ → 2 NaBiO₃
The procedure is analogous to the oxidation of manganese dioxide in alkali to give sodium manganate.

Reactions

Storage conditions with moisture and high temperatures are detrimental to sodium bismuthate, as it oxidizes water, decomposing into sodium hydroxide and bismuth(III) oxide:
:2 NaBiO₃ + H₂O → 2 NaOH + Bi₂O₃ + O₂
It is decomposed faster by acids. In HCl, NaBiO₃ also reacts to form chlorine gas.

NaBiO₃ may be used to detect manganese qualitatively and quantitatively. As a strong oxidizer, it converts almost any manganese compound to permanganate, which is easily assayed spectrophotometrically. To do this, some NaBiO₃ and the sample are reacted in a hot solution of sulfuric acid or nitric acid. Permanganate has a violet color and maximum absorbance at 510 nm. The reaction is:
:2 Mn²⁺ + 5 NaBiO₃ + 14 H⁺ → 2 + 5 Bi³⁺ + 5 Na⁺ + 7 H₂O

Sodium bismuthate can perform oxidative 1,2-cleavage on glycols, ketols and alpha hydroxy acids with no further oxidation of the (possible) aldehyde products:Suzuki, p. 373
:R₂C(OH)–C(OH)–R₂ → R₂C=O + O=CR₂
:R₂C(OH)–C(O)–R → R₂C=O + RCOOH
:R₂C(OH)–COOH → R₂C=O + CO₂

These cleavages can be done in the presence of acetic or phosphoric acid at room temperature. Alcohols like methanol or ethanol can be used as the reaction media, as they are oxidized slowly with sodium bismuthate. Lead tetraacetate performs similar reactions, but anhydrous conditions, as required in the use of lead tetraacetate, are not necessary for sodium bismuthate.

NaBiO₃ can be used for lab-scale plutonium separation (see bismuth phosphate process).

Safety

NaBiO₃ is a mild mechanical irritant. Upon ingestion it is moderately toxic with symptoms akin to lead poisoning: abdominal pain and vomiting. Large doses cause diarrhea and death. Continued absorption of NaBiO₃ into body causes permanent kidney damage. These effects are due to the toxicity of bismuth. Oral absolute lethal dose (LD₁₀₀) of NaBiO₃ is 720 mg/kg for rats, and 510 mg/kg for rabbits.

References

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Citations

{{Bismuth compounds

Sodium compounds
Bismuth compounds
Oxygen compounds