$\frac{12 H _{2} O + S _{8} \longrightarrow 4 S _{2} O _{3}^{2-}+24 H ^{+}+16 e ^{\circ}}{2 S _{8}+12 H _{2} O \longrightarrow 8 S ^{2-}+4 S _{2} O _{3}^{2-}+24 H }$

for balancing in basic medium add equal number of $OH ^{\ominus}$ that of $H ^{+}$

$2 S _{8}+12 H _{2} O +24 OH ^{\ominus} \longrightarrow 8 S ^{2-}+4 S _{2} O _{8}^{2-}$

$+24 H _{2} O$

$2 S _{8}+24 OH ^{\ominus} \rightarrow 8 S ^{2-}+4 S _{2} O _{8}^{2-}+12 H _{2} O$

$S _{8}+12 OH ^{\ominus} \rightarrow 4 S ^{2-}+2 S _{2} O _{8}^{2-}+6 H _{2} O$

$a =12$