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 absorbance's using a 1.00 cm covet: What is the molar absorptivity (element of) of the indophenol product, and what is the concentration of ammonia in the lake water? Element of = [NH_3]_lake water =

Explanation / Answer

Apsorbance of Indophenol = 0.697 - 0.450

=0.247

Ammonia concentration 5.5×10^-4 mol/litre

Volume of ammonia solution =2.50 ml

No of mole of ammonia = (5.5×10^-4/1000)×2.5= 1.38 × 10^-6

Mole of indophenol ion =1.38 × 10^-6

Concentration of indophenol = (1.38×10^-6/25)×1000= 5.5× 10^-5M

Molar absorptivity = Absorbance/(path length × concentration)

Molar absorptivity = 0.247/(1 × 5.5×10^-5)

=4.5 × 10^3M^-1 cm^-1

Absorbance in sample = 0.450 -0.045 = 0.405

[NH3] = 0.405/(1× 4.5 × 10^3) = 9×10^-5M

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