Hach DR/700 Procedures Manual page 566

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.
3+
AL
2+
Ca
2+
Cd
-
C1
3+
Cr
6+
Cr
2+
Cu
-
F
2+
Fe
32 mg/L
1000 mg/L as (CaCO
20 mg/L
8000 mg/L
20 mg/L
40 mg/L
15 mg/L
20 mg/L
interferes directly and must not be present.
61-10
)
3