Egorov N.B.

Department of Applied Chemistry of Rare, Scattered and Radioactive Elements Institute of Physics and Technology Tomsk Polytechnic University, Russian Federation

 

THE SYNTHESIS OF SODIUM TETRATHIOSULFATOPLUMBATE HEXAHYDRATE

 

The interaction between equivalent amounts of Pb(NO3)2 or Pb(CH3COO)2 with Na2S2O3 in aqueous solutions yields poorly soluble compounds of PbS2O3 [1] and Pb3(S2O3)2(CH3COO)2 [2]. These compounds are dissolved in an excess of Na2S2O3 to form lead thiosulfate complex ions [Pb(S2O3)2]2-, [Pb(S2O3)3]4-, and [Pb(S2O3)4]6-.

The purpose of this work was to recover water-soluble lead thiosulfate complexes in the solid phase and to characterize them.

The following reagents were used: Na2S2O3×5H2O (Aldrich), Pb(NO3)2 (Aldrich), and PbS2O3, prepared by reacting Pb(NO3)2 with Na2S2O3 solutions [1].

Lead thiosulfate complexes were synthesized using two methods. In method 1, precisely weighed portions of Na2S2O3×5H2O and Pb(NO3)2 were dissolved in a minimum volume of water and mixed by adding Pb(NO3)2 solution to a Na2S2O3 solution; the relevant reaction was:

Pb(NO3)2+xNa2S2O3→Na(2x-2)[Pb(S2O3)x]+2NaNO3,                                    (1)

where x = 2 - 4.

Reaction 1 proceeded in two stages: at first, poorly soluble PbS2O3 was formed in the solution (the solubility of the product was 4×10-7), and then it was dissolved in an excess of Na2S2O3.

In method 2, a PbS2O3 powder was dissolved in a saturated solution of Na2S2O3; the corresponding reaction was:

PbS2O3+yNa2S2O3→Na2y[Pb(S2O3)(y + 1)],                                                   (2)

where y = 1 - 3.

The PbS2O3 formed in solution and mixed according to method 1 and the PbS2O3 powder used in method 2 could only be completely dissolved when the molar ratio of Pb2+:S2O32- was equal to 1:4 or higher. At other ratios, insoluble PbS2O3 remained in solution.

Thus, the lead thiosulfate complexes were recovered to the solid phase from solutions with a molar ratio of Pb2+:S2O32-= 1:4.

On heating and storage the lead thiosulfate complex solutions decomposed to form PbS and did not allow their isolation by evaporation of the solvent.

The lead thiosulfate complexes were isolated from solution using alcohol. When ethanol was used, the lead thiosulfate complexes were isolated as an oily liquid, which was separated and treated with absolute ethanol (without additional treatment, the liquid decomposed over time to form PbS). The oily liquid gradually transformed into a powder, which was filtered, washed, and dried in air.

A weak point of method 1 was the necessity to reprecipitate the product due to NaNO3 contamination, which promoted decomposition of the lead thiosulfate complexes. In this case, the product yield decreased.

The product obtained was a non-hygroscopic white powder. The yield of the lead thiosulfate complexes in method 2 was 75%.

The complexes were analyzed for lead [3] and sulfur [4]. The amount of water was determined from thermal analysis data.

The anal. calc. (%) for Na6[Pb(S2O3)4]×6H2O: Pb, 22.98; S, 28.45; H2O, 11.98.

Found (%): Pb, 23.1; 23.02; S, 28.40; 28.35; H2O, 11.81.

Thus, according to the data from the elemental and thermal analyses, the compound synthesized was formulated as Na6[Pb(S2O3)4]×6H2O (sodium tetrathiosulfatoplumbate hexahydrate).

X-Ray powder diffraction patterns were measured on a Shimadzu XRD 6000 diffractometer (CuKa radiation, l = 0.154056 nm).

The X-ray phase study showed that the compounds synthesized by methods 1 and 2 were alike, being one individual substance that possessed a characteristic X-ray pattern that contained no lines from the original components (Figure 1).

Fig

Figure 1. The XRD pattern of the Na6[Pb(S2O3)4]×6H2O

The compound synthesized was highly soluble in water. At concentrations of 0,5 M and higher, Na6[Pb(S2O3)4]×6H2O dissolved with the formation of a minor amount of PbS, and the solution turned brown. On storage, the highly concentrated solutions decomposed over time to form PbS.

At a solution concentration of 0,1 M, pearly disk-shaped crystals precipitated from the solutions after 8 - 10 h. The IR spectrum of the compound isolated from the solution was identical to that of PbS2O3. This indicates that the complex undergoes hydrolysis and decomposes in an aqueous solution.

It should be noted that the PbS2O3 prepared by a reaction between solutions of Pb(NO3)2 and Na2S2O3 had no pronounced crystal structure, unlike the PbS2O3 formed by the hydrolysis of Na6[Pb(S2O3)4]×6H2O.

 

References:

1. Freedman A.N., Straughan B.P. // Spectrochim. Acta. 1971. A 27. P. 1455–1465.

2. Norlund C. A.; Harell R. G. // Acta Chem. Scand. 1990. 44. P. 1077–1079.

3. Schwarzenbach G.; Flaschka H. Die komplexometrische Titration. Ferdinand Enke: Stuttgart, 1965.

4. Busev A.I.; Simonova L.N. The Analytical Chemistry of Sulfur. Nauka: Moscow, 1975.