it is expected that the ferric chloride, just like other inorganic coagulants, dissociates during the process of coagulation, hence, the reduced pH of the treated effluent. This dissociation also leaves footprints of chlorides. However, you stated that this occurred in one of your runs of the experiment. The trend should be the same with the residual chloride increasing with concentration of ferric chloride used.

Dear Hopewell,

When dissolved in water, iron(III) chloride undergoes hydrolysis and gives off heat in an exothermic reaction. The resulting brown, acidic, and corrosive solution is used as a flocculant in sewage treatment and drinking water production, and as an etchant for copper-based metals in printed circuit boards. As a result of this reaction chloride ions are released.

In industrial application, iron(III) chloride is used in sewage treatment and drinking water production. In this application, FeCl3 in slightly basic water reacts with the hydroxide ion to form a floc of iron(III) hydroxide, or more precisely formulated as FeO(OH)−, that can remove suspended materials. The byproduct of this hydrolysis is again chloride ion.

[Fe(H2O)6]3+ + 4 HO− → [Fe(H2O)2(HO)4]− + 4 H2O → [Fe(H2O)O(HO)2]− + 6 H2O

The hydrolysis extent of ferric chloride depends on temperature and the pH of the medium.

Reference

Water Treatment Chemicals (PDF). Akzo Nobel Base Chemicals. 2007. Retrieved 2007-10-26.

Hoping this will be helpful,

Rafik

Ferric chloride is about 65% chloride by mass so if you are dosing 10 mg-Fe/L you can expect about 19 mg/L chloride increase. In practice a bit more can be expected as ironsalts for wastewater might not be 100 % pure.