Treatment of ammonia with phenol in the presence of hypochlorite yields indophen

Question

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 times 10^-4 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: What is the molar absorptivity (epsilon) of the indophenol product, and what is the concentration of ammonia in the lake water?

Explanation / Answer

Absorbance of indophenol product = absorbance of B - blank = 0.729-0.045 = 0.684

Molarity of indophenol in the diluted solution = 2.5 mL * 5.5*10^-4/25mL = 5.5*10^-5 M

molar absorptivity of indophenol = 0.684/ 5.5*10^-5 M = 12436.36 M^-1cm^-1

[NH3] in sample A = absorbance A-blank/ 12436.36 M^-1cm^-1 = 3.51*10^-5 M

[NH3] in lake water = 25 mL * 3.51*10^-5 M/10mL = 8.78*10^-5 M

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