Hach DR/700 - Page 566

61-10
COBALT, continued
SAMPLING AND STORAGE
Collect samples in acid-washed plastic bottles. Adjust the sample pH to 2
or less with nitric acid (about 5 mL per liter). Preserved samples can be
stored up to six months at room temperature. Just before analysis, adjust
the sample pH between 3 and 8 with 5.0 N Sodium Hydroxide Standard
Solution. Do not exceed pH 8 as this may cause some loss of cobalt as a
precipitate. Correct test results for volume additions; see Sampling and
Storage, Volume Additions, (Section I) for more information.
ACCURACY CHECK
Standard Solution Method
Prepare 1.0-mg/L cobalt standard solution by diluting 10.0 mL of a
10-mg/L working stock solution to 100 mL in a volumetric flask. The
working stock solution should be prepared daily by diluting 10.00 mL of
cobalt Standard Solution, 1000 mg/L as Co, to 1000 mL with
demineralized water. This is a 10-mg/L cobalt standard solution.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two
representative lots of reagent. Testing 1.0 mg/L Co concentration
solutions, the standard deviation was ±0.004 mg/L Co.
Testing zero concentration samples, the limit of detection was
0.007 mg/L Co. The limit of detection was calculated as three times the
standard deviation when testing zero concentration samples (adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
The following may interfere in concentrations exceeding those listed
below.
AL
3+
32 mg/L
Ca
2+
1000 mg/L as (CaCO
3
)
Cd
2+
20 mg/L
C1
-
8000 mg/L
Cr
3+
20 mg/L
Cr
6+
40 mg/L
Cu
2+
15 mg/L
F
-
20 mg/L
Fe
2+
interferes directly and must not be present.