Hach DR/700 Procedures Manual page 395

HARDNESS,
INTERFERENCES
For the most accurate calcium test result, the test should be rerun on a
diluted sample if the calcium is over 1.0 and the magnesium is over
0.25 mg/L as CaCO
respective concentrations.
The following cause a detectable error in test results.
3+
Cr
2+
Cu
EDTA, chelated
2+
Fe
3+
Fe
2+
Mn
2+
Zn
SUMMARY OF METHOD
The colorimetric method for measuring hardness supplements the more
conventional titrimetric method through its ability to measure very low
levels of calcium and magnesium. Also some interfering metals (those
listed above) in the titrimetric method will be rendered inconsequential
when diluting the sample to bring it within the range of this test. The
indicator dye used is calmagite which forms a purplish-blue color in a
strongly alkaline solution and changes to red when contacting free
calcium or magnesium. Calcium and magnesium determinations are
made by chelating calcium with EGTA to destroy any red color due to
calcium and then chelating the calcium and magnesium with EDTA to
destroy the red color due to both calcium and magnesium. By
measuring the red color in the different states, calcium and magnesium
concentrations are determined.
continued
. No retesting is needed if either is below those
3
52-69
0.25 mg/L
0.75 mg/L
0.2 mg/L as CaCO
1.4 mg/L
2.0 mg/L
0.20 mg/L
0.050 mg/L
3