Abstract
We shall report the results of an estimation of free iron oxides in soils by improved JEFFRIES's magnesium ribbon-potassium oxalate method, comparing with TRUOG's sodium sulfide-oxalic acid method. Improved magnesium ribbon reduction method is as follows; Procedure : Place 2 to 5 g of air dried fine soil (<2mm) into a 300 ml beaker, add 30 ml of 10% H_2O_2 and stir thoroughly. Then, place it on a boiling water bath to digest organic matter as soon as possible, and evaporate to dryness. Repeat it until practically all organic matter would be decomposed. Usually, these two treatments shall be enough. Add 40ml of potassium oxalate solution containing 4.15g potassium oxalate and 1Oml of oxalic acid soluon containing 0.95 g oxalic acid. Stir thoroughly until all of the dried. organic matter-free soil would be loosened from the bottom of the beaker. Then, place it on a boiling water bath, and heat it to about 85〜90℃. Stir violently with a glass rod coiled around on the end with a magnesium ribbon (1O cm length weighing about 0.2g), keeping at the same temperature for 3〜5 minutes or until clay and sand would become white or gray. Take down the beaker and take out the glass rod. Let it stand for a moment. Then, filtrate the supernatant by means of the decantation method. Add potassium oxalate solution and oxalic acid solution to the residue, heat and reduce it with the magnesium ribbon in the same manner. Let it stand for a moment again and filtrate the supernatant by means of the decantation method. Wash free from iron with 30 ml portion of acidified 0.5% NaCl solution adjusted to pH about 3.5 with N-HCl. Usually, four times washings shall be enough. The magnesium ribbon shall be washed with some portion of dilute HCl and the washing shall be put into the filtrate and made up them to a definite volume. This solution, we can reserve for the determination of free iron oxides. Transfer a definite volume of this solution to a small evaporating-dish. Oxalate contained in solution should be decomposed. The residue shall be converted to chloride or sulfate. And then, determine the ferric oxide by titration with standard 0.01 N mercurous nitrate. The procedure is as follows; A solution of HgNO_3,approximately 0.01 N, shall be prepared using 5% HNO_3. After keeping for a week, the solution would be standardized by titration against a standard solution of FeNH_4(SO_4)_2 or Fe_2(SO_4)_3. As a change in normality would be insignificant even after several months, the initial standardization would be sufficed. The iron should be in the ferric state and should be neither more than 10 mg with respeet to Fe_2O_3 nor less than 1 mg. The solution (at room temperature) is diluted to a volume of about 100 ml and added 5 ml of 40% NH_4SCN solution. Then, titrate the solution with the HgNO_3. It should be noted that the solutioh should be stirred vigorously and the HgNO_3 is added dropwise and slowly near the endpoint, in which the red color would be absent. The solution becomes turbid when the endpoint is overrun. Summary : 1) The procedure is simple, and adaptable to routine laboratory work, and suitable for the determination of many samples. 2) As the MnO_2 added to soil has been reduced and fully recovered by this method, the method seems to be suitable for determining some kinds of forms of manganese oxides in soils. 3) Free iron oxides in soils determined by this method give somewhat lower value than that of TRUOG's nascent hydrogen sulfide method, and higher value than TAMM's method. 4) 4 to 6% as Fe_2O_3 was removed by this method and 17% by TRUOG's method from the fine particles (0.2mm) of micaceous hematite containing 54.7% of iron as Fe_2O_3.
