When the soda extract containing thiosulfate ion is treated with excess of ${\mathrm{AgNO}}_3$ solution followed by boiling then:

The ion of Disulfur Trioxide treated with silver ion in the following way,${\mathrm{S}}_2{\mathrm{O}}_3^{2-}+2{\mathrm{Ag}}^{+}⟶\underset{ \left(\mathrm{white}\right)}{{\mathrm{Ag}}_2{\mathrm{S}}_2{\mathrm{O}}_3\downarrow }$

At first , no ppt. occur because:

$2{\mathrm{S}}_2{\mathrm{O}}_3^{2-}+{\mathrm{Ag}}^{+}⟶{\left[\mathrm{Ag}{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2\right]}^{3-}$

${\mathrm{Ag}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ turns dark on standing:

${\mathrm{Ag}}_2{\mathrm{S}}_2{\mathrm{O}}_3\downarrow +{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{Ag}}_2\mathrm{S}\downarrow +2{\mathrm{H}}^{+}+{\mathrm{SO}}_4^{2-}$

So. when the soda extract containing thiosulfate ion is treated with excess of ${\mathrm{AgNO}}_3$ solution followed by boiling then black precipitate is formed.