In qualitative analysis, the precipitation of the iron group (III) requires careful control of conditions to ensure selective precipitation of the desired ions. Ammonium chloride is added before ammonium hydroxide to achieve this control effectively.

Let's analyze the options to understand why ammonium chloride is used:

Option A: decrease concentration of $$\mathrm{OH}^{-}$$ ions

This option is correct. Adding ammonium chloride ($$\mathrm{NH}_4\mathrm{Cl}$$) introduces a common ion effect, where the concentration of $$\mathrm{OH}^{-}$$ ions is reduced due to the formation of $$\mathrm{NH}_4\mathrm{OH}$$. This reaction can be represented as:

$$\mathrm{NH}_4^+ + \mathrm{OH}^- \leftrightarrow \mathrm{NH}_4\mathrm{OH}$$

By reducing the concentration of $$\mathrm{OH}^{-}$$ ions, it prevents the premature precipitation of hydroxides of metals that might otherwise not selectively precipitate in the desired group.

Option B: increase concentration of $$\mathrm{Cl}^{-}$$ ions

While it is true that ammonium chloride increases the concentration of $$\mathrm{Cl}^{-}$$ ions, this is not the primary reason for its addition in this context. The focus is more on regulating $$\mathrm{OH}^{-}$$ ion concentration.

Option C: prevent interference by phosphate ions

This is not relevant in the context of adding ammonium chloride, as the main idea behind adding ammonium chloride is to control the $$\mathrm{OH}^{-}$$ ion concentration rather than dealing directly with phosphate ions.

Option D: increase concentration of $$\mathrm{NH}_4^+$$ ions

While ammonium chloride does increase the concentration of $$\mathrm{NH}_4^+$$ ions, which in turn helps in maintaining the $$\mathrm{OH}^{-}$$ concentration at a lower level, this is again a secondary consideration to the primary goal of controlling the $$\mathrm{OH}^{-}$$ ion concentration to ensure selective precipitation.

Therefore, the correct answer is:

Option A: decrease concentration of $$\mathrm{OH}^{-}$$ ions