What acid is added to the sample bottle prior to sampling for iron and manganese?

The correct choice for the acid that is typically added to the sample bottle prior to sampling for iron and manganese is nitric acid (NO3). This is due to nitric acid's effectiveness in preserving the sample and preventing the precipitation of metals. When sampling for trace metals like iron and manganese, it is crucial to avoid any alterations in the sample's chemistry that might occur if it were to be contaminated or otherwise changed during transport or storage.

Nitric acid serves to maintain the sample's stability by controlling the pH, thereby ensuring that dissolved metals remain in solution and reducing the likelihood of their precipitation. Additionally, nitric acid helps to avoid influences from other substances that could interfere with the analysis, ultimately leading to a more accurate determination of metal concentrations.

The other acids listed are less suitable for this purpose. For instance, hydrochloric acid may alter the speciation of metals, and acetic acid is not typically used for preserving metal samples due to its weaker acidity and potential interference with the analysis. Sulfuric acid, while strong, can also lead to the formation of sulfates that may complicate the analysis of certain metals. Hence, nitric acid is specifically favored in protocols for sampling processes pertaining to iron and manganese in water samples.