For ferric hydroxide sol the coagulating power of phosphate ion is more than chloride ion.

Because ferric hydroxide sol is positively charged, it is coagulated by anions; by the Hardy-Schulze rule the greater the charge on the coagulating ion the greater its power, and $\mathrm{PO_4^{3-}}$ has a higher charge than $\mathrm{Cl^-}$.