When testing for anions why is dilute nitric acid added first (Chloride, Bromide and Iodide tests)
The correct answer and explanation is:
Dilute nitric acid is added first when testing for anions such as chloride, bromide, and iodide to remove impurities and prevent unwanted reactions. Specifically, it helps to eliminate any carbonate (CO3²⁻) or hydroxide (OH⁻) ions that may be present in the sample. These ions can interfere with the test by producing precipitates or reactions that could be mistaken for the actual halide ion test results.
For example, carbonate ions react with silver nitrate to form a white precipitate of silver carbonate, and hydroxide ions can form silver oxide precipitates. These would confuse the identification of chloride, bromide, or iodide ions. The addition of dilute nitric acid ensures that carbonate ions are converted into carbon dioxide gas, which escapes from the solution, and hydroxide ions are neutralized to water. This cleaning step guarantees that when silver nitrate is added later, any precipitate formed is due to the halide ions only.
Moreover, nitric acid is chosen because it is a strong acid that provides hydrogen ions to react with unwanted ions but does not introduce interfering ions itself. Unlike hydrochloric acid, which contains chloride ions, nitric acid does not add any additional halide ions that could interfere with the halide test. This specificity is crucial for accurate identification.
Once the sample is treated with dilute nitric acid, silver nitrate is added. Silver ions react with chloride, bromide, or iodide ions to form characteristic precipitates: white for chloride, cream for bromide, and yellow for iodide. These colors help to distinguish between the different halide ions.
In summary, dilute nitric acid is used first to remove carbonate and hydroxide ions that would otherwise cause misleading precipitates during the silver nitrate test. Its use ensures the test’s accuracy by avoiding contamination from other ions and preventing false positives.