Page 221 - Modern Analytical Chemistry

1400-CH07 9/8/99 4:03 PM Page 204

204 Modern Analytical Chemistry

which simplifies to
C + K A,I ´ C I - ( C A o )
A
E =
( C Ao )
( C Ao ) ´
C A K A,I C I
= - +
( C Ao ) ( C Ao ) ( C Ao )
K A,I ´ C I
=( R A - )1 + 7.16
( C Ao )
A more useful equation for the relative error is obtained by solving equation 7.13
for C I and substituting back into equation 7.16
é K A,I ´() ù
C I o
R - ) 1 + ´ 7.17
E = ( A ê R I ú
)
ë C ( Ao û
The first term of equation 7.17 accounts for the incomplete recovery of analyte, and
the second term accounts for the failure to remove all the interferent.
7
EXAMPLE .11
Following the separation outlined in Example 7.10, an analysis is to be carried
out for the concentration of Cu in an industrial plating bath. The concentration
ratio of Cu to Zn in the plating bath is 7:1. Analysis of standard solutions
containing only Cu or Zn give the following standardization equations

S Cu = 1250 ´(ppm Cu)

S Zn = 2310 ´(ppm Zn)
(a) What error is expected if no attempt is made to remove Zn before analyzing
for Cu? (b) What is the error if the separation is carried out? (c) What is the
maximum acceptable recovery for Zn if Cu is completely recovered and the
error due to the separation must be no greater than 0.10%?

SOLUTION
(a) If the analysis is carried out without a separation, then R Cu and R Zn are
equal to 1.000, and equation 7.17 simplifies to

K Cu,Zn ´(ppm Zn) o
E =
(ppm Cu) o
From equation 7.11 the selectivity coefficient is
k Zn 2310
.
K Cu,Zn = = =185
k Cu 1250
Although we do not know the actual concentrations of Zn or Cu in the sample,
we do know that the concentration ratio (ppm Zn) o /(ppm Cu) o is 1/7. Thus
( . )( )
1
5
18
4
0
E = = .264, or 26 . %
7
()
(b) To calculate the error, we substitute the recoveries calculated in Example
7.10 into equation 7.17