H2S存在解离平衡：
H2S ==可逆== H+ + HS- Ka1
HS- ==可逆== H+ + S2- Ka2
由于第2步电离程度远远小于第1步电离,
所以第2步电离可以忽略,
即 [HS-] = [H+],
Ka2 = [H+][S2-] / [HS-] = [S2-]
即 [S2-] = Ka2