Sulphide ions in alkaline solution react with solid sulphur to form polyvalent sulphide ions. The equilibrium constant for the formation of S2−2 and S2−3 from S and S2− ions are 1.7 and 5.3 respectively. Equilibrium constant for the formation of S−23 from S is:
3.11
S+S−2⇌S−22; K1−−−eq−1
2S+S−2⇌S−23; K2 −−−eq−2
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S+S−22⇌S−23; K3 −−−eq−3
eq−3=−(eq−1)+(eq−2)
K3=K2K1=5.31.7=3.11