The spot test procedure for determining the SSA of soils is described as follows (Santamarina et al., 2002): Methylene blue solution was prepared by mixing 1 g dry powder of MB with 200 mL of deionized water. Ten grams of oven-dried soil was mixed with 30 mL deionized water. Then, the MB solution was added into this soil

suspension with 0.5 mL increments. After each 0.5 mL addition of MB, soil suspension was mixed by magnetic stirrer for 1 min; then, a small drop was removed from the solution and placed onto Fisher brand filter paper. If the unadsorbed MB forms a permanent blue halo around the soil aggregate spot onto the filter paper, it means that MB has replaced cations in the double layer and coated the entire surface.

Surface area is determined from the MB amount that required reaching the end- point from the following equation:

SSA =(1/319.87)*(1/200)*(0.5N)*Av*AMB*(1/10)

where N is the number of MB increments added to the soil suspension solution, Av is Avogadro's number (6.02*1023/mol), and AMB is the area covered by one MB molecule (typically assumed to be 130 Å2). For practical purposes, it is sufficient to dry the dye at 105 °C to constant weight in order to attain anhydrous state of MB.

I have tried several times but did not obtain the halo.