Studies on Pre-Treatment of Soils for Mechanical Analysis by Elutration Method (Part 2)

Abstract

(A) The effectiveness of 5% oxalic acid for dispersing aggregates of "Onji" as a pre-treatment of mechanical analysis by elutriation method was studied. It was found that the higher the temperature of the solution, the more the effective was to disperse aggregates of "Onji" colloid, but it dissolved rather more minerals of fresh sand. According to the result obtained, the most effective method of pretreatment was as follows : 5 g of sample is shaken in the shaking bottle, with 500 ml of 5% oxalic acid solution, previously heated at about 50℃ (more higher degrees preferably in winter), for 10' by shaking machine. Sample containing humus is necessary to be treated with about 50 ml of 15% H_2O_2 on the water bath before adding oxalic acid to decompose its humus. Solubility of fresh minerals by 5% oxalic acid solution at about 50℃ was found almost neglisible. (B) TRUOG and his associates suggested the complete removal of the oxides of iron and alumina by the use of an oxalic acid-sodium sulfide treatment. It has undoubtedly many advantages for special studies, but it should be used with some modifications in ordinary mechanical analysis. Simplification of TRUOg's method was studied, and satisfactory result was obtained as follows : Place 5 or 10 g of soil in a liter conical beaker, add 10 ml of 15% H_2O_2,and decompose the humus on a water bath. Of soils with no humus, H_2O_2 treatment for about 30' is preferable to break up the combined granules. When decomposition of humus is nearly complete, add some drops of NH_4OH, boil for 10' to drive off the excess of H_2O_2. Prepare a liter seperating fnnnel for removal of the most of clay. Set two lines, showing the following relation, on this funel. One is of showing about 200 ml from the bottom and the other is of 4 cm higher than the above one. Transfer the suspension to this funnel, add 30 ml of 10% Na_2SiO_3・9H_2O solution, dilute with H_2O to the upper line, shake the funnel for a minute, stand for 10', siphone off the suspension to the lower line, fill up with H_2O to upper line, and repeat the same procedure** two or four times according to the clay content. * Adding Na_2SiO_3 is not necessary for allitic soils. ** This is schemed to speed up the filteration procedure which follows. Transfer the content of the funnel to 500 ml of ERLENMEYER flask, dilute with H_2O to abont 300 ml, warm the suspension to 80-90℃, add 10 g of NH_4Cl mixture of 2.1 g citric acid (C_6H_8O_7・H_2O) and 14.7 g Na_2HPO_4・12H_2O, 15ml of 20% Na_2S・9H_2O solution, and shake the flask for 2' vigorously. At this procedure, pH of suspension is kept 7〜6,H_2S is driven off, and the color of the suspension turn black. Add finely powdered oxalic acid crystals, stir vigorously to dissolve FeS and then add drop by drop (1 : 1) NH_4OH to turn the suspension again black and to form FeS by the H_2S in the solution, stir for 1 or 2', and add again oxalic acid crystals to dissolve FeS. Transfer the suspension on BUCHNER funnel baving about 8 cm of inner diameter. Filter the suspension by suction with water pump. In order to remove sulfer which contaminates the soil, treat the residue on the funnel twice with 95% ethyl alcohol to remove the moisture at first, and then three times with CS_2 solution (1 volume of CS_2 is added to two volumes of 95% ethyl alcohol). Three or four times with 95% ethyl alcohol to remove CS_2,two times with hot 5% oxalic acid solution, and finally three or four times with conc. NaCl solution. Transfer the most part of sands to a flask, washing down the still adhered soil particles to the flask with hot water. The pre-treatment of the sample is thus completed, and is now ready for mechanical analysis by the elutriation method of using modified KOPECKY elutriator proposed by Soeiety of Agr. Japan (1924).