Page 19 - Instrumental Analysis - Pharm D Clinical- 07-PA403

UV-visible Spectrophotometry

Problems

1- The absorbance of a 0.002% w/v phenol in 0.1 N sulphuric acid at
270 nm is equal to 0.880 when measured in 2 cm pathlength.
Calculate the A(1%, 1 cm) and ε for phenol (MW = 94).

A270 0.88
A(1% , 1 cm) = ──── = ────── = 220

b c 2 x 0.002

A(1%, 1 cm) x MW 220 x 94

ε = ───────────── = ─────── = 2068 L mol–1 cm–1

10 10

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2- Calculate the concentration in µg/mL of a solution of tryptophan (MW
204.2) (ε = 5432 L mol–1 cm–1) in 0.1 M HCl, giving A = 0.613 in 4 cm
cell at its λmax 277 nm.
A=εbc

0.613 = 5432 x 4 x c

c = 2.82 x 10–5 M = 2.82 x 10–5 x 204.2 x 1000 = 5.76 µg/mL

x MW x 1000

M g/L mg/L (µg/mL or ppm)

3- Unknown sample has %T = 50.0. What is its absorbance value?
%T = 50
T = 0.50
A = – log T = – log 0.5 = 0.301

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