Question

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 × 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:

Sample: Absorbance: (625nm)

A 0.479

B 0.726

C 0.045

What is the molar absorptivity (ε) of the indophenol product, and what is the concentration of ammonia in the lake water?

Answer 1

Sample A = 10 mL lake water + 5 mL Phenol+ 2 mL Sodium Hypoclorite Sample

SampleB = 10 mL lake water + 5 mL Phenol+ 2 mL Sodium Hypoclorite + 2.5 mL 5.5 x 10^-4 M NH3 Sample + sandard Soltion

Sample C = 10 mL distilled water + 5 mL Phenol+ 2 mL Sodium Hypoclorite Blank

       OD

Sample A         Sample                                          0. 479

SampleB          Sample + Standard Solution            0. 726

Sample C        Blank                                             0.045

To get the OD of standard solution alone = OD_A-OD_B = 0.726 - 0.479 = 0.247

Given standard NH₃ solution concentration is 5.5 x 10-4 M and we have taken 2.5 mL and diluted to 25 mL.

Therefore, the net concentration of [NH₃] from standard solution = 2.5 mL x 5.5x10^-4 M/ 25 mL = 5.5 x 10^-5 M

From Beers lamberts law;

OD = e.c.l

0.247 = e x 5.5 x 10^-5 M x 1 cm

e = 4.5 x 10³ M^-1 cm^-1

Ammonia concentration in the lake water :

OD_sample = OD_A - OD_C = 0.478 - 0.045 = 0.433

Again from Beers law;

OD_sample = e.c.l

0.433= 4.5x10³ M^-1cm^-1 x C x 1 cm

[NH₃] = C = 9.62x 10^-5 M

Therefore, Answer : Molar absorptivity (ε) of the indophenol = 4.5 x 10³ M^-1cm^-1

The concentration of ammonia in the lake water [NH₃]_{lake water} = 9.62 x 10^-5 M