(1 bookmark) Treatment of ammonia with phenol in the presence of hypochlorite yi

Question

(1 bookmark) Treatment of ammonia with phenol in the presence of hypochlorite yields indophenol, a blue product absorbing light at 625 nm, which can be used for the spectrophotometric determination of ammonia. To determine the ammonia concentration in a sample of lake water, you mix 10.0 mL of lake water with 5 mL of phenol solution and 2 mL of sodium hypochlorite solution and dilute to 25.0 mL in a volumetric flask (sample A). To a second 10.0 mL solution of lake water you add 5 mL of phenol, 2 mL of sodium hypochlorite, and 2.50 mL of a 5.50 104 M ammonia solution and dilute to 25.0 mL (sample B). As a reagent blank, you mix 10.0 mL of distilled water with 5 mL of phenol, 2 mL of sodium hypochlorite and dilute to 25.0 mL (sample C). You measure the following absorbances using a 1.00 cm cuvet:

Explanation / Answer

Concentration of NH3 in sample B,

= 5.5 x 10^-4 M x 2.5 ml/25 ml

= 5.5 x 10^-5 M

Subtract blank absorbance from sample A and sample B

Sample A, absorbance = 0.381 - 0.045 = 0.336

Sample B, absorbance = 0.628 - 0.045 = 0.583

Let Cx be the concentration of NH3 in the lake water

then,

Cx/(Cx + 5.5 x 10^-5) = 0.336/0.583

0.583Cx = 0.336Cx + 1.85 x 10^-5

Cx = 1.85 x 10^-5/0.247 = 7.50 x 10^-5 M

concentration of NH3 in 10 ml lake water sample = 7.50 x 10^-5 x 25/10 = 1.87 x 10^-4 M

Related Questions

Navigate

• Browse (All)
• Browse #
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.