$100 \mathrm{ml}$ of $0.0018\%(\mathrm{w}/\mathrm{v})$ solution of ${\mathrm{Cl}}^{-}$ion was the minimum concentration of ${\mathrm{Cl}}^{-}$ required to precipitate a negative sol in one h. The coagulating value of ${\mathrm{Cl}}^{-}$ion is