Excess of which ion in drinking water causes 'blue baby syndrome'?

Question

Accepted Answer

Option 2, $NO_{3}^{-}$ (nitrate ion), is the correct answer because an excess of nitrate ions in drinking water is known to cause "blue baby syndrome," also known as methemoglobinemia. This condition occurs when nitrate is converted to nitrite in the body, which then reacts with hemoglobin to form methemoglobin. Methemoglobin is unable to effectively release oxygen to body tissues, leading to oxygen deficiency and resulting in the characteristic blue tint to the skin, particularly in infants.

Here's why the other options are incorrect:

1. $F^{-}$ (fluoride ion): While fluoride is important for dental health in small amounts, excessive fluoride can lead to dental and skeletal fluorosis, not blue baby syndrome. Fluoride does not interfere with oxygen transport in the blood.

3. $SO_{4}^{2-}$ (sulfate ion): High levels of sulfate in drinking water can cause a laxative effect and dehydration, especially in infants, but it does not cause blue baby syndrome. Sulfate does not affect hemoglobin or oxygen transport.

4. $Cl^{-}$ (chloride ion): Chloride is a common component of salts and is generally not harmful at typical concentrations found in drinking water. It does not cause blue baby syndrome and does not interfere with the blood's ability to carry oxygen.

In summary, the presence of excess nitrate ions in drinking water is specifically linked to blue baby syndrome due to their conversion to nitrites, which impair the oxygen-carrying capacity of hemoglobin. This makes Option 2 the correct choice.