Which substance can interfere with the Mercuric Nitrate method?

The Mercuric Nitrate method is commonly used for determining the concentration of certain substances, particularly for measuring the presence of sulfide. Sulfides can interfere with the accuracy of this method because they can react with mercuric nitrate and lead to erroneous results by precipitating or forming complexes that affect the measurement. This interference can result in reduced sensitivity or false readings, ultimately impacting the accuracy of the test results.

In contrast, the other substances listed—nitrate, calcium, and magnesium—are less likely to cause interference in the Mercuric Nitrate method. While these substances can affect various chemical reactions, they do not react with mercuric nitrate in a way that significantly disrupts the measurement of sulfides. Nitrate typically does not interfere directly in this method, and both calcium and magnesium generally do not produce any significant issues related to the specific determination being made with mercuric nitrate. Therefore, sulfide is correctly identified as the substance that interferes with the Mercuric Nitrate method.