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In the Vollhard method Chloride is precipitated from a solution using escess silver ions. $$\ce{Ag+_{(aq)} + Cl-_{(aq)} -> AgCl_{(s)}}$$ After precipitation the solution is filtered and ammonium thiocyanate is added to the silver supernate with a small amount of ferric ammonium sulfate as an indicator where thiocyanate reacts preferentially with the silver:

$$\ce{Ag+_{(aq)} + SCN-_{(aq)} -> AgSCN_{(aq)}}$$

Once the all of the silver has reacted with thiocyanate the $\ce{Fe^3+}$ will then react to form $\ce{Fe[SCN]^2+}$ which produces a dark red color and indicates an equivalent point. and the chloride concentration is calculated by difference.

$$\ce{Fe^3+_{(aq)} + SCN-_{(aq)} -> Fe[SCN]^2+_{(aq)}}$$

Since the silver in the precipitate, why will you have to filter $\ce{AgCl}$ before you can start back titration instead of treating the solution? Would filtering also necessary for $\ce{AgBr}$ and $\ce{AgI}$?

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  • $\begingroup$ Welcome to Chemistry.SE! Good question, but please note the edits made and try to demonstrate and understanding of the underlying principles in future questions. We are a community happy to help, but not act as instructors. Requiring users to understand the underlying principles first helps maintain quality on this site. $\endgroup$ – A.K. Sep 22 '18 at 18:36
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Yes filtering is necessary as thiocyanate can complex the solid silver chloride per:

$$\ce{AgX_{(s)} + SCN-_{(aq)} -> Ag[SCN]X-_{(aq)}\tag{X = Cl, Br, I}}$$

This additional reaction requires more thiocyanate for back titration and thus gives a lower chloride concentration than is true.

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