Determination of Total Iron in Soils by Disodium Dihydrogen Ethylenediamine Tetraacetate Titration KUANG LU CHENGl, ROGER H. BRAY,
AND
TOUBY KURTZ
C-nioersity of Illinois, Urbana, Ill.
beaker and dilute to about 30 ml. with water. Adjust the solution to pH 2 to 3 with sodium acetate and acetic acid. Add 5 drops of indicator (either Tiron or salicylic acid) and a few crystals of ammonium persulfate. Mix and titrate immediately with Versenate solution. If Tiron is used as the indicator, the color change a t the end point is from blue to yellow; if salicylic acid is used, the color change a t the end point is from purplish red to yellow or colorless. I n either case the end point is reached when the blue or red tint has completely disappeared.
ISODIUM dihydrogen ethylenediamine tetraacetate (Versenate) has been widely used for the determination of total hardness in water (9)and of calcium and magnesium in soil and plant material (1). The complex formation of iron with Tiron (disodium-1,2-dihydroxybenzene-3,5-disulfonate)and with Versenate has been recently studied and their constants of formation have been determined potentiometrically and photometrically (6.7 ) . This paper describes the application of these reagents to a direct method for determining total iron in soils. For determining total iron in soil and in silicate materials, gravimetric (6),colorimetric, and volumetric oxidation-reduction methods (8, 10) are usually used. Because the gravimetric procedure lacks specificity, careful and complete separation from many interfering ions is required prior to determination. Colorimetric procedures are very accurate and sensitive, but they are no simpler than the proposed volumetric procedure and have thr disadvantage of limited range. The volumetric oxidation-reduction method requires the preparation of ferrous ion solutions which can be titrated t o ferric ions by ceric sulfate. Reduction t o ferrous iron requires the use of the silver reductor introduced by Walden, Hammett, and Edmonds (9). I n the procedure given beloty, the soil is decomposed by perchloric acid digestion or by sodium carbonate fusion and the ferric iron in the resulting solution is titrated directly x ith Versenate (disodium dihydrogen ethylenediamine tetraacetate), using Tiron or salicylic acid as the indicator. This procedure is as accurate as the ceric titration and much simpler. Versenate complexes many metals, but most of these complexes are highly stable only in neutral or alkaline media. Among the elements present in more than trace amounts in soils, only the ferric ion is tightly tied up by Versenate in acid solutions of pH 2 to 3. Ions of nickel, antimony, lead, and tin, which form complexes of varying stabilities a t this pH, rarely occur in soils in amounts sufficient to interfere ( 4 ) .
40
20
1
1
3
2
4
5
PH
Figure 1. Determination of Iron in Soil
I s SOIL FILTRATE AFTER PERCHLORIC ACID DIGESTION. Pipet a suitable aliquot of the filtrate into a small beaker. The aliquot should contain about 3 to 20 mg. of iron, so 25 ml. of filtrate is satisfactory in most cases. Adjust to pH 2 to 3 with sodium acetate and acetic acid and titrate with Versenate as described in the standardization of the Versenate solution, except that the addition of ammonium persulfate is not necessary (no ferrous ions should be present in the solution after either perchloric acid digestion or sodium carbonate fusion). Calculation. If 2 grams of soil and 25 ml. of the 100 ml. of filtrate are taken, then the percentage of FelOa can be calculated from the equation
REAGENTS
Versenate Solution. Dissolve 4 grams of disodium dihydrogen ethylenediamine tetraacetate in 1 liter of water. Indicator. Dissolve 5 grams of disodium-l,2-dihydroxybenzene-3,5-disulfonate in 100 ml. of water, or dissolve 1 gram of salicylic acid in 100 ml. of ethyl alcohol. Sodium acetate, 207, aqueous solution. Glacial acetic acid. Standard Iron Solution. Dissolve 1.0000 gram of electrolytic iron in 10 ml. of 1 to 1 hydrochloric acid and dilute to about 100 ml. with water. Warm this solution on the steam bath until all the iron has been dissolved, then make up to 1 liter with water. This solution contains 1 mg. of iron per ml.
70Fe20B= T’
X ml. of Versenate used X 4 X 1.43 2000
x
100
where V is the iron titer in milligrams of iron per milliliter of the standardized Versenate solution. RESULTS AND DISCUSSION
PROCEDURE
Digestion of Soil. Weigh 2 grams of fine soil (
