187
between 3 and 8 with 5.0 N Sodium Hydroxide Standard Solution
just before analysis. Correct the test results for volume additions;
see Correction for Volume Additions in Section 1 for more
information.
Accuracy Check
Using a 2.00 mg/L (as CaCO
3
) standard solution as sample,
perform the hardness procedure described above. The results
should be 2.00 mg/L calcium (as CaCO
3
).
Method Performance
Precision
In a single laboratory using a standard solution of 2.00 mg/L Mg
as CaCO
3
and 1.88 mg/L Ca as CaCO
3
with the instrument, a
single operator obtained a standard deviation of ± 0.09 mg/L Mg
as CaCO
3
and ± 0.08 mg/L Ca as CaCO
3
.
Estimated Detection Limit
The estimated detection limit for program 30 is 0.13 mg/L
magnesium hardness and 0.08 mg/L calcium hardness. For more
information on the estimated detection limit, see Section 1.
Interferences
For the most accurate hardness test result, the test should be rerun
on a diluted sample if the calcium is over 1.0 or the magnesium is
over
0.25 mg/L as CaCO
3
. No retesting is needed if either is below
those respective concentrations.
The following cause a detectable error in test results.
Traces of EDTA or EGTA remaining in sample cells from
previous tests will give erroneous results. Rinse cells thoroughly
before use.
Interfering Substance Level at Which Substance Interferes
Cr
3+
0.25 mg/L
Cu
2+
0.75 mg/L
EDTA, chelated 0.2 mg/L as CaCO
3
Fe
2+
1.4 mg/L
Fe
3+
2.0 mg/L
Mn
2+
0.20 mg/L
Zn
2+
0.050 mg/L
HARDNESS, continued
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