Hardness
© Hach Company, 2003. All rights reserved. Printed in the U.S.A. 3/04 1ed
Page 4 of 4 Hardness_None_Other_CLC_Eng_Ody.fm
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Summary of Method
The colorimetric method for measuring hardness supplements the conventional
titrimetric method because the colorimetric method can measure very low levels
of calcium and magnesium. Also, some metals (those listed the table above) that
interfere in the titrimetric method may be inconsequential when diluting the
sample to bring it within the range of this test. The indicator dye is calmagite,
which forms a purplish-blue color in a strongly alkaline solution and changes to
red when it reacts with 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. Test results are measured at 522 nm.
Required Reagents
Quantity Required
Description per test Unit Cat. No.
Hardness Reagent Set (100 Tests) ..................................................................................................................23199-00
Includes:
Alkali Solution for Calcium and Magnesium Test........................1 mL ................ 100 mL MDB.....22417-32
Calcium and Magnesium Indicator Solution.................................1 mL ................ 100 mL MDB.....22418-32
EDTA Solution, 1 M ...........................................................................1 drop.............. 50 mL SCDB .....22419-26
EGTA Solution....................................................................................1 drop.............. 50 mL SCDB .....22297-26
Required Apparatus
Cylinder, 100-mL, graduated mixing.....................................................1 ....................... each......................1896-42
Dropper, measuring, 0.5 and 1.0 mL......................................................2 ....................... 20/pkg...............21247-20
Sample Cells, 10-20-25 mL, w/cap.........................................................3....................... 6/pkg.................24019-06
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