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:

Explanation / Answer

Absorbance = ebc

where,

e = molar absorptivity

c = concentration of NH3

b = path length = constant

Absorbance after substrating blank,

A = 0.398 - 0.045 = 0.353

B = 0.645 - 0.045 = 0.600

let x be the concentration of NH3 in lake water then,

For sample A,

A = 0.353 = e(x)

e = 0.353/x

feed in sample B equation,

For sample B,

A = 0.600 = e (x+5.5 x 10^-4)

0.600 = 0.353/x(x+5.5 x 10^-4)

x = 2.23 x 10^-3 is the concentration of NH3 in lake water

Feed this value,

A = 0.600 = e(2.23 x 10^-3 + 5.5 x 10^-4)

e = 215.83 M-1.cm-1 is the molar absorptivity of NH3

  

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