I am attempting to prepare an ammonium chloride/ammonium hydroxide buffer solution ($\ce{pH}=10 \pm 0.1$) for titrating water hardness with calgamite and EDTA.

In the 17th Edition of the Standard Methods for the Examination of Water and Wastewater, the reagent preparation section for titrating water hardness (Section 2340 C.) states:

  1. Mix the $\ce{NH4Cl+NH4OH}$
    1. Add $\pu{1.25 g}$ magnesium salt of EDTA (commercially available).
  2. Dilute with distilled water.

After researching, I have assumed it to mean Magnesium Disodium EDTA (hydrate), CAS# 14402-88-1.

In reading the source material and searching online for synonyms of "magnesium salt of EDTA," I can't seem to find an exact hit on the term. Searches included PubChem, ChemSpider, Sigma-Aldrich, and Cole-Parmer, among others. Each seemed to point me toward "Magnesium Disodium EDTA Hydrate." I was hoping that with the amount of knowledge on this site, someone might recognize the term "salt of" and be able to help me out and clarify the term, or confirm that I am using the correct EDTA salt.

  • 4
    $\begingroup$ EDTA is an abbreviation commonly used for the ligand - ethylenediaminetetraacetate. The parent molecule is the corresponding ethylenediaminetetraacetic acid. So, an Mg salt of an acid. $\endgroup$ Mar 29, 2018 at 16:21

1 Answer 1


The authors have some strange kind of humor... it's a riddle.

Below you might see

2) If the magnesium salt of EDTA is unavailable, dissolve 1.179 g disodium salt of ethylenediaminetetraacetic acid dihydrate (analytical reagent grade) and 780 mg magnesium sulfate (MgSO4⋅7H2O) or 644 mg magnesium chloride (MgCl2⋅6H2O) in 50 mL distilled water. Add this solution to 16.9 g NH4Cl and 143 mL conc NH4OH with mixing and dilute to 250 mL with distilled water.

From the molar mass of $\ce{MgSO4 * 7 H2O}$ ($\pu{246.47 g/mol}$) you can calculate the number of moles of magnesium the "magnesium salt of EDTA" should contain. With the help of the data sheet of your purchased magnesium salt of EDTA you can calculate the required quantity in grams.

You also could calculate possible stoichiometries of the imaginary "magnesium salt of EDTA" if you like. But be aware that not all commercially available magnesium salts of EDTA have a well defined stoichiometry.


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