[Gerald C Koch]

The electrical engineer in me would look at FeCl₃ as source of Fe^III, but chemists might know better. The trace quantities of Fe^III you are going to add make it unlikely that the Cl^- has any effect on your developer's properties.

Ferric chloride is a poor choice for making a solution of known ferric ion concentration. It is one of those compounds of indefinite composition. It hydrolyses in moist air and loses chlorine. It is also deliquescent. So there is no exact composition. The three compounds I suggested in a previous post are all stable and have definite compositions.