Page 131 - Modern Analytical Chemistry

1400-CH05 9/8/99 3:59 PM Page 114

114 Modern Analytical Chemistry

S spiked

kC S
Slope =
V f
–C V
A o
x-intercept = kC V
C S y-intercept = A o
V f
V S
(a)

S spiked

Slope = k

–C V
A o
x-intercept = kC V
Figure 5.7 V f y-intercept = A o
V
Examples of calibration curves for the f
method of standard additions. In (a) the
signal is plotted versus the volume of the
V
S
added standard, and in (b) the signal is C S( )
plotted versus the concentration of the V f
added standard after dilution. (b)
where Y is S spike and X is V s . The slope of the line, therefore, is kC S /V f , and the
y-intercept is kC A V o /V f . The x-intercept is the value of X when Y is 0, or
kC V kC S
Ao
0 = + ´x-intercept)
(
V f V f
( kC V V f ) CV
Ao /
Ao
x-intercept = – =–
S /
( kC V f ) C S
Thus, the absolute value of the x-intercept is C A V o /C S .
Since both V o and C S are known, the x-intercept can be used to calculate the ana-
lyte’s concentration.

EXAMPLE .7
5
A fifth spectrophotometric method for the quantitative determination of the
concentration of Pb 2+ in blood uses a multiple-point standard addition based
on equation 5.6. The original blood sample has a volume of 1.00 mL, and the
2+
standard used for spiking the sample has a concentration of 1560 ppb Pb . All
samples were diluted to 5.00 mL before measuring the signal. A calibration
curve of S spike versus V s is described by