What is the Diazo blank composition in the 3-diazo reagents setup?

• A. 1.5% HCl
• B. 0.5% sodium nitrite
• C. 0.1% sulfanilic acid
• D. 50% methanol

Answer: (A)

Diazotization needs an acidic medium to generate nitrous acid and form the diazonium intermediate, which then couples to give the colored azo dye. A diazo blank is prepared under those acidic conditions but without nitrite, so no diazonium salt forms and no color appears. That’s why the acid concentration is chosen: to provide the necessary environment while omitting the nitrite. Hence, 1.5% hydrochloric acid is the correct diazo blank composition. The other options either add nitrite (which would drive diazotization) or supply the coupling substrate or solvent, but don’t create the blank condition.