1 (85)

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85Chapter 7 Collecting and Preparing Samples
15. In anoxic sediments with relatively high concentrations of sulide, 
S2–, the speciation of Cu2+ is controlled by the formation of sta-
ble copper-sulide phases, even at the very acidic pH levels obtained 
when using a strong acid, such as HNO3, as a preservative. Adding 
H2O2 before adding HNO3 oxidizes S2– to SO4
2- , which minimizes 
this problem.
16. (a) If the recovery for the interferent, RI, is 1, then equation 7.19 for 
the error reduces to
.E R 1 0 0630A= - =
 and the apparent recovery for the analyte, RA, is 1.063 or 106.3%.
 (b) If the recovery for the analyte, RA, is 1, then equation 7.19 for the 
error reduces to
( )
( ) ( . ) ( )
.E
C
K C
R R5
0 816 1
0 0630
,
A
A I I
I I
o
o
# #= = =
 and the apparent recovery for the interferent, RI, is 0.386 or 38.6%.
17. (a) he recoveries for copper and for iron are
.
.
. .R 278 3
275 9
0 9914 0 991mg
mg
Co .= =
.
.
. .R
184 9
3 6
0 01947 0 019
mg
mg
Fe .= =
 (b) he separation factor, SFe,Co, in which iron is the interferent and 
cobalt is the analyte, is
.
. . .S R
R
0 9914
0 01947 0 0196 0 020Fe,Co
Co
Fe
c= = =
 (c) he selectivity of the method for the analyte, Co, relative to the 
interferent, Fe, is
.
. .K
k
k
0 786
0 699 0 889Co,Fe
Co
Fe
= = =
 (d) If we make no attempt to separate the analyte and the interferent, 
then RCo and RFe have values of 1; thus, the expected error in the 
analysis for Co is
( )
( )
( )
( ) .
( . ) ( )
.
E R
C
K C
R1
1 1 10 2
0 889 1
1 0 0872
Co o
Co,Fe Fe o
FeCo #
#
= - + =
- + =
 or an error of +8.72%.
 (e) If we complete the separation, then the expected error in the anal-
ysis for Co is