Thiosulfate - Reactions

Reactions

Thiosulfates are stable only in neutral or alkaline solutions, but not in acidic solutions, due to decomposition to sulfite and sulfur, the sulfite being dehydrated to sulfur dioxide:

S2O32− (aq) + 2 H+ (aq) → SO2 (g) + S (s) + H2O

This reaction may be used to generate an aqueous suspension of sulfur and demonstrate the Rayleigh scattering of light in physics. If white light is shone from below, blue light is seen from sideways and orange from above, due to the same mechanisms that color the sky at mid-day and dusk.

Thiosulfates react with halogens differently, which can be attributed to the decrease of oxidizing power down the halogen group:

2 S2O32− (aq) + I2 (aq) → S4O62− (aq) + 2 I− (aq)
S2O32− (aq) + 4 Br2 (aq) + 5 H2O(l) → 2 SO42− (aq) + 8 Br− (aq) + 10 H+ (aq)
S2O32− (aq) + 4 Cl2 (aq) + 5 H2O (l) → 2 SO42− (aq) + 8 Cl− (aq) + 10 H+ (aq)

In acidic conditions, thiosulfate causes rapid corrosion of metals; steel and stainless steel are particularly sensitive to pitting corrosion induced by thiosulfate. Addition of molybdenum to stainless steel is needed to improve its resistance to pitting (AISI 316L hMo). In alkaline aqueous conditions and medium temperature (60°C), carbon steel and stainless steel (AISI 304L, 316L) are not attacked, even at high concentration of base (30%w KOH), Thiosulfate (10%w) and in presence of Fluoride ion (5%w KF).

The natural occurrence of the thiosulfate group is practically restricted to a very rare mineral sidpietersite, Pb4(S2O3)O2(OH)2, as the presence of this anion in the mineral bazhenovite was recently disputed.

Read more about this topic:  Thiosulfate

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