Preparation of Standard EDTA solution.

APPENDIX 2

app2edta.doc 09/09/98

Preparation of Standard 0.01 M EDTA Solution^a

(Directions for 1.000 L. Solution. Modify accordingly for other volumes).

NOTE: Do not store EDTA solutions in glass-stoppered flasks. The solution tends to "freeze" stoppers.

Procedure

Dry about 4 g. of the purified dihydrate Na₂H₂Y·2H₂O (F.Wt. 372.24) at 80^oC. for 1 h. to remove superficial moisture (Note 1). Cool to room temperature in a desiccator. Weigh (to the nearest milligram) about 3.8 g. into a 1-L volumetric flask (Note 2). Use a powder funnel to ensure quantitative transfer; rinse the funnel well with water before removing it from the flask. Add 600 to 800 mL of water (Note 3) and swirl periodically. Dissolution may take 15 min. or longer. When all the solid has dissolved, dilute to the mark with water and mix well (Note 4). In calculating the molarity of the solution, correct the weight of the salt for the 0.3% moisture it ordinarily retains after drying at 80^oC.

Notes

(1) W.J. Blaedel, H.T. Knight, Anal. Chem., 1954, 26, 741.

(2) The solution can be prepared from the anhydrous disodium salt, if desired. The weight taken should be about 3.6 g.

(3) Water used in the preparation of standard EDTA solutions must be totally free of polyvalent cations. (Use deionized water).

(4) As an alternative, an EDTA solution that is approximately 0.01 M can be prepared and standardized by direct titration against a Mg²⁺ solution of known concentration (see Text, p. 581).

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(a) Skoog, West, Holler, Analytical Chemistry. An Introduction, Saunders,

NY, 6th ed., 1994, p. 580.