Method for determining the ammonium salt content in dilute nitric acid

At 5 mg/L, in a neutral solution, neutral formaldehyde reacts with ammonium salts to form hexamethylenetetramine; one mole of ammonium produces one mole of acid. 4NH4NO3 + 6HCHO → (CH2)6N4 + 6H2O + 4HNO3. The acid produced in this reaction can be determined by titration with an alkaline solution, using phenolphthalein as an indicator. (2) Reagents and solutions a. Standard H2SO4 solution: C(H2SO4) = 0.1000 mol/L b. Standard NaOH solution: C(NaOH) = 0.1000 mol/L c. 20% sodium hydroxide solution d. Phenolphthalein indicator: 10 g/L (with ethanol as the solvent) e. 5% neutral formalin solution: 1.8 L of industrial 36% formalin is added to 8.2 L of distilled water. Then, 25.00 mL of this solution is taken, and its acidity is determined using a sodium hydroxide standard solution with phenolphthalein as an indicator; based on this acidity value, the amount of sodium hydroxide solution required to neutralize the residual formaldehyde is calculated. The neutralized formaldehyde solution should be thoroughly shaken. (3) Analysis steps a. Add to a 500 mL Erlenmeyer flask: 100 mL of distilled water ； 25.00 mL of sample solution ； Phenol phenolphthalein indicator, 6 drops. b. Add 20% NaOH solution drop by drop for neutralization, and adjust with 0.1 mol/L H2SO4 or 0.1 mol/L NaOH until the solution just turns pink. c. Add about 100 mL of 5% neutral formaldehyde. d. Titrate with the sodium hydroxide standard solution C (NaOH) = 0.1000 mol/L until the solution turns pink. (4) Calculation: The ammonium salt content in dilute nitric acid is given by: 1000 × 25.00 / 0.018 = 1000 × NH4+/CNaOH × VNaOH, in mg/L. Where: VNaOH is the volume of the standard sodium hydroxide solution consumed, in mL ；