LaMotte AM-41 Hydroponics Test Kit - Manual (Page 12)

THE CHEM IS TRY OF THE TEST RE AC TIONS
The following information pertains to the nature of the chemical tests used
in this kit. Although the information is not totally complete, it does
provide the teacher or student with the basic chemistry of the reagents and
a description of the reactions as they are thought to occur.
CALCIUM-MAGNESIUM
*Calcium & Magnesium Buffer Ammonia Buffer *5126
*TEA Reagent Triethanolamine *3921
*CM Indicator Eriochrome/Triethanolamine *6522
Standard EDTA Reagent Disodium Salt EDTA 5254
*Sodium Hydroxide Reagent Sodium Hydroxide *4259
Calcium Hardness Indicator Tablets Calcon 5250
Calcium-Magnesium Inhibitor
Reagent CDTA 3922
*Inhibitor Solution Sodium Sulfide *9258
Ethylenediaminetetraacetic acid (EDTA) and its disodium salts form
chelate complexes with metal cations. Calcium and magnesium complexes
are colorless and the calcium-EDTA complex is more stable than the
magnesium-EDTA complex.
The Eriochrome dye also forms complexes with calcium and magnesium;
the magnesium-dye complex is more stable than the calcium-dye complex.
In the pH range from 8.5 to 11.5 an aqueous solution of the dye is blue,
while it’s calcium and magnesium complexes are red. The Calcium &
Magnesium Buffer is added to bring the sample within this pH range.
CDTA is used to complex or mask interfering metals.
If EDTA titrant is added the calcium, and then the magnesium is extracted
from their dye complexes. After sufficient EDTA is added to complex all of
the magnesium as well as the calcium, the solution will turn blue.
CAL CIUM
At a pH of about 12, Calcon indicator forms a red complex with calcium.
A sodium hydroxide solution is required to bring the pH within the desired
range. The sodium sulfide and TEA are used to complex or mask
interfering metals. EDTA combines with calcium before it combines with
magnesium and is capable of extracting calcium from the Calcon complex
and thus, restores the blue color of Calcon. Magnesium does not change
the color of Calcon at this pH.
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