Hach DR/850, Procedures Manual, Page: 188 / 460

188
Summary of Method
The colorimetric method for measuring hardness supplements the
conventional titrimetric method because it can measure very low
levels of calcium and magnesium. Also some interfering metals
(those listed above) in the titrimetric method are inconsequential
in the colorimetric method when diluting the sample to bring it
within the range of this test.
The indicator dye, calmagite, forms a purplish-blue color in a
strongly alkaline solution and changes to red when it reacts with
free calcium or magnesium. Calcium is chelated with EGTA to
destroy any red color due to calcium and then the sample is
chelated with EDTA to destroy the red color due to both calcium
and magnesium. Measuring the red color in the different stages of
chelation gives results as the calcium and magnesium hardness
concentrations.
REQUIRED REAGENTS
Cat. No.
Hardness Reagent Set (100 Tests) ................................................................................ 23199-00
Includes: (1) 22417-32, (1) 22418-32, (1) 22419-26, (1) 22297-26
Quantity Required
Description Per Test Unit Cat. No.
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.......... 22419-26
EGTA Solution ......................................................... 1 drop....................50 mL.......... 22297-26
REQUIRED APPARATUS
Cylinder, 100-mL mixing ............................................. 1........................... each............ 1896-42
Dropper, measuring, 0.5 and 1.0 mL ............................ 2....................... 20/pkg.......... 21247-20
Sample Cell, 10-20-25 mL, w/cap ................................ 3......................... 6/pkg.......... 24019-06
OPTIONAL REAGENTS
Calcium Standard Solution, 2.0 mg/L as CaCO
3
...................................946 mL.......... 20581-16
Nitric Acid, ACS....................................................................................500 mL.............. 152-49
Nitric Acid Solution, 1:1........................................................................500 mL............ 2540-49
Sodium Hydroxide Standard Solution 5.0 N ...............................100 mL MDB............ 2450-32
OPTIONAL APPARATUS
pH Meter, sens
ion
™ 1 , portable, with electrode .......................................... each.......... 51700-10
Thermometer, –20 to 110 °C....................................................................... each.......... 26357-02
HARDNESS,
continued