No abstract

The oxidation potential of dithionite (Na~S204) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At tile same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics ocem~ at approximately pH 7.3. A buffer is needed to hold the pl~ at the optimum level because 4 moles of OH are used up in reaction with each mole of Na2S204 oxidized. Tests show that NaHCO3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 rain treatments normally given for iron oxide removal from soils and clays.A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffere~l Na2S2Oa-citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was about 17 percent as contrasted to 35-80 percent with other methods. INTRODUCTIONThe removal of" amorphous coatings and crystals of free iron oxides, particularly hematite and goethite, which act as cementing agents, is important in many types of analysis of softs and clay minerals. The removal of free iron oxides aids in dispersion of the silicate portion, which is essential for effective segregation into different particle size fractions. For x-ray diffraction studies the removal of free iron oxides greatly enhances the parallel orientation of layer silicate clays and brings out some x-ray diffraction peaks that are otherwise difficult or impossible to detect. Differential and integral thermal analysis, electron micrographs and cation exchange capacity are greatly improved after removal of free iron oxides. Use of citrate chelating agent (Aguilera and Jackson, 1953) with Na~S204 not only helps with iron extraction but removes some coatings of alumina and thereby assists in the dissolution of free silica cements that are stabilized by alumina coatings, as will be shown in the present paper.

The total unit cell planar specific surface was computed as the sum of planar sorption surface (by a glycerol gravimetric method) and the mica unit cell interplanar surface (corresponding to the K). This sum was found to be constant for a given unit cell formula weight, averaging 773 m. 2 /g. with a standard deviation of ±12.7 or about ±2%. For example, a Colorado vermiculite had 1.63% K 2 O equivalent to 16.3% mica residue with a unit cell interplanar surface of 124 m. 2 /g.; this added to the measured 631 m. 2 /g. of planar sorption surface (glycerol sorbed on expanded or cleavage planes) gives a total planar surface of 755 m. 2 /g. Similarly, for coarse clay from Fithian, Illinois, the 5.61% K 2 O is equivalent to 56.1% of illite, with unit cell interplanar surface of 426 m. 2 /g. This, added to 235 m. 2 /g. of measured planar sorption surface, gives a total of 661 m. 2 /g. which when corrected to exclude 15% unexpanded minerals (kaolinite and chlorite) gives a total planar specific surface of 775 m. 2 /g. Wyoming montmorillonite had 803 m. 2 /g. of planar specific surface, comparing well with the theoretical, 808 m.Vg. This principle of unit cell planar specific surface constancy of 2:1 layer silicates shows that the mechanism of K release from mica is by cleavage, and gives an accurate tool for analysis of the rather generally occurring interstratified mixtures of expanding 2:1 layer silicate minerals with micas in soils and sediments.

In a new method for specific surface determination, montmorillonite is made to sorb exactly two interlayers of glycerol (a true monolayer on each planar surface) at 35° C. in a vacuum of < 1 mm. of Hg. Vermiculite sorbs only one interlayer under these conditions. Both montmorillonite and vermiculite sorb only a mono‐interlayer of glycerol at 110° C. in a previous method. The two analytical methods thus give the amount of glycerol corresponding to the second interlayer in montmorillonite, and this makes possible quantitative analytical determination of montmorillonite and of vermiculite. To illustrate, Wyoming montmorillonite sorbs 21.1% glycerol at 110° C., and 41.6% at 35° C. in vacuum, which gives an analysis of 98% montmorillonite, whereas Colorado vermiculite sorbs the same amount of glycerol under both these conditions, equivalent to only a monointerlayer. The presence of fine amorphous material in montmorillonitic fine clays is revealed by a higher specific surface than obtained with 100% montmorillonite. Dissolution of the allophane from Black Cotton soil (India) resulted in a specific surface analysis of 94% montmorillonite (5 to 10% kaolinite is present). Ladybrook fine clay fraction (Queensland, Australia) showed 90% montmorillonite and 10% of mono‐interlayer expanding mineral.