For detecting the nitrates in sewage, the colour may be developed by adding

ANS:C - Phenol-di-sulphuric acid and pottassium hydroxide

1. Phenol-di-sulphuric Acid (Sulfuric Acid and Phenol Mixture):
   • Phenol-di-sulphuric acid, also known as the sulfuric acid and phenol mixture, is a reagent commonly used in the phenol method for detecting nitrates.
   • In this method, the sewage sample suspected of containing nitrates is treated with the sulfuric acid and phenol mixture.
   • Sulfuric acid serves to acidify the sample, creating an acidic environment conducive to the reaction between nitrate ions and phenol.
2. Potassium Hydroxide (KOH):
   • Potassium hydroxide, commonly abbreviated as KOH, is a strong base.
   • In the phenol method for detecting nitrates, potassium hydroxide is used to neutralize the acidic solution obtained after the addition of sulfuric acid and phenol.
   • The neutralization reaction helps stabilize the color development process and ensures accurate detection of nitrates.
3. Color Development:
   • When the sulfuric acid and phenol mixture react with nitrate ions in the sewage sample, a colored compound called nitrophenol is formed.
   • The intensity of the color developed corresponds to the concentration of nitrates present in the sample.
   • Potassium hydroxide is added to stabilize the color and enhance the sensitivity of the test.
4. Observation of Color Change:
   • The formation of a pink to reddish color indicates a positive result, suggesting the presence of nitrates in the sewage sample.
   • The depth of color developed is proportional to the concentration of nitrates present.