Leaching tests solute transfer

Leaching is controlled by the rate at which liquid flows past individual particles. For situations and materials where the liquid flows around the particles in the solid matrix, the rate of flow is driven by the hydraulic head of the liquid above the solid material and the efficiency of solute transfer is controlled by the contact between the surface area of the particles and the liquid. Where the rate of flow is very fast and the time over which a state of solute transfer equilibrium is established is slow, leaching may not reach true chemical equilibrium hence not all the leachable components will be leached as quickly as in the situation where the rate of flow is slower. Leaching tests are all based on the assumption that equilibrium or semi-equilibrium is reached in the timescale of the test. 

Transfer 3.3 g of sample, accurately weighed, into a porcelain dish or casserole, heat on a hot plate until completely charred, then heat in a muffle furnace at 480° for 8 h or overnight, and cool. Cautiously add 5 mL of nitric acid, evaporate to dryness on a hot plate, then heat again in the muffle furnace at 480° for exactly 15 min, and cool. Extract the ash with two 10-mL portions of water, filtering each extract into a separator. Leach any insoluble material on the filter with 6 mL of Ammonium Citrate Solution, 2 mL of Hydroxylamine Hydrochloride Solution, and 5 mL of water (see Lead Limit Test, Appendix IIIB, for preparation of these solutions), adding the filtered washings to the separator. Continue as directed under Procedure in the Lead Limit Test, Appendix IIIB, beginning with Add 2 drops of phenol red TS to the separator.. .. ... 

Decompose the test portion by sintering with sodium peroxide and leaching with water and HC1. Transfer the solution to a distillation flask and evaporate a portion of the solution. Reduce arsenic to arsenic(III) by treatment with potassium bromide and hydrazine sulphate. Adjust the acidity and distill AsC13, collecting the distillate in water. Reduce an aliquot portion of the solution with potassium iodide and ascorbic acid, and treat with sodium borohydride to reduce arsenic ions to the volatile hydride, arsine. Rapidly sweep the arsine into a hydrogen—nitrogen (or argon)-entrained... 

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