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. OCI indophenol anion 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 x 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: Absorbance (625 nm) 0.551 0.798 0.045 Sample What is the molar absorptivity (e) of the indophenol product, and what is the concentration of ammonia in the lake water? Number Number M-1cm-1 [NH =11D = 3 Jlake water

Explanation / Answer

absorbance due to ammonia added = B -A

                                                         = 0.798 - 0.551

                                                         = 0.247

the concentration of NH3 added sample is = 5.50 x 10^-4 x 2.5 / 25

                                                                     = 5.50 x 10^-5 M

E = A / b C

E   = 0.247 / 1 x 5.50 x 10^-5

E = 4.49 x 10^3 M-1 cm-1

[NH3]lake water =   A / E b

                              =[ (0.551 - 0.045 ) / 4.49 x 10^3 x 1 ) ] x 25 / 10

                               = 2.82 x 10^-5 M

[NH3]lake water =2.82 x 10^-4 M

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