To estimate the chloride ion present in tap water, volumetric analysis has been commonly used in laboratory. Which of the following solutions can be used for the above mentioned titration ? a) BaCl2…

The estimation of chloride ions ($\mathrm{Cl}{}^{-}$) in tap water is typically performed using argentometric titration (Mohr’s method or Volhard’s method). Among the given options, $\mathrm{AgNO}{}_3$ is the correct choice because:

• Chloride ions react with silver ions ($\mathrm{Ag}{}^{+}$) to form a precipitate of silver chloride ($\mathrm{AgCl}$): $$\mathrm{AgNO}{}_3+\mathrm{Cl}{}^{-}\to \mathrm{AgCl}(\mathrm{s})+\mathrm{NO}{}_3{}^{-}$$
• The endpoint is detected by the formation of a reddish-brown precipitate of silver chromate ($\mathrm{Ag}{}_2\mathrm{CrO}{}_4$) when using $\mathrm{K}{}_2\mathrm{Cr}{}_2\mathrm{O}{}_7$ as an indicator (in Mohr’s method).
• $\mathrm{BaCl}{}_2$ and $\mathrm{NaOH}$ are not suitable for this titration as they do not react with chloride ions to form a detectable precipitate or undergo a redox reaction.

Thus, $\mathrm{AgNO}{}_3$ is the appropriate titrant for estimating chloride ions in tap water.