Environmental Engineering FOURTH EDITION

98 ENVIRONMENTAL ENGINEERING

Light Filter Sample Photocell Ammeter

source

Figure 5-8. Elements of a filter photometer.

where

Po = radiant power of incident light,

P = radiant power of light after it passes through the sample,

e = absorbtivity (Lmol-' cm-l),

b = path length (cm),

c = concentration of absorbing substance (moles L-l), and

A = absorbance (no units).

A photometer, as shown in Fig. 5-8, measures the difference between the intensity of
light passing through the sample (P in Eq. (5.23)) and the intensity of light passing
through clear distilled water or a reference sample (Po) and reports absorbance (A)
as well as percent transmission (%T):

mically, in colorimetric analyses, a standard dilution series is used to estimate the

concentration of an unknown sample, as illustrated in Example 5.4.

EXAMPLE 5.4. Several known concentrations of ammonia and an unknown sample were

analyzed using the phenate method, and the color was measured with a photometer.

Find the ammonia concentration of the unknown sample.

Standards Absorbance

0 pg/L of ammonia 0.050

5 pg/L of ammonia 0.085

50 bg/L of ammonia 0.402

100 pg/L of ammonia 0.747

350 pglL of ammonia 2.450

Unknown sample 1.082

10 wglL of ammonia 0.121