APPENDIX 2

app2edta.doc 09/09/98

Preparation of Standard 0.01 M EDTA Solutiona

(Directions for 1.000 L. Solution. Modify accordingly for othervolumes).

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

Procedure

Dry about 4 g. of the purified dihydrate Na2H2Y·2H2O (F.Wt. 372.24) at

80oC. for 1 h. to remove superficial moisture (Note 1). Cool to roomtemperature in a desiccator. Weigh (to the nearest milligram) about3.8 g. into a 1-L volumetric flask (Note 2). Use a powder funnel toensure quantitative transfer; rinse the funnel well with water beforeremoving it from the flask. Add 600 to 800 mL of water (Note 3) andswirl periodically. Dissolution may take 15 min. or longer. When all thesolid has dissolved, dilute to the mark with water and mix well (Note4). In calculating the molarity of the solution, correct the weight of the

salt for the 0.3% moisture it ordinarily retains after drying at 80oC.

Notes

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

(2) The solution can be prepared from theanhydrous disodium salt, if desired. The weighttaken should be about 3.6 g.

(3) Water used in the preparation of standardEDTA solutions must be totally free of polyvalentcations. (Use deionized water).

(4) As an alternative, an EDTA solution that isapproximately 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. AnIntroduction, Saunders,

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