Oxford UniversityBIOCHEMIST 223

An experiment has been conducted to determine the soil Cation Exchange Capacity (CEC) In the experiment, 10 g of soil was used, the volume of solution containing NH4⁺ was 100 mL, and the concentration of NH4⁺ was 14 μgmL^-1.) 

a) Calculate the soil CEC

b)What is the purpose of using alcohol in the determination of soil CEC? 

c)In the third leaching step for the determination of CEC, K₂SO4 solution was used. Why? 

Answer:

a)

Given, NH4⁺concentration = 14 mcgmL^-1 = 0.014 mgmL^-1

Volume of solution = 100 mL = 0.1 L

Weight of soil sample = 10 g = 0.01 kg

                  = 0.014 mgNH4⁺ / L  X  0.1 L / Sample X  0.01 kg / kg soil

                  =  0.000014mgNH4⁺ / kg soil

                  = 14^-6mgNH4+/kg soil

Now convert it to cmol/g, for NH4⁺,the atomic weight is approximately 18 g/mol, so we calculate the cmol of exchangeable NH4+in 1 g soil

               = 14^-6mg NH4⁺ / g soil  X  1g/1000mg  X 1 mol NH4⁺/18 g  X 100 cmol/mol

               = 0.78 cmolNH4+/kg soil

Therefore CEC = 12 cmol/kg

b) In determination of soil CEC alcohol is added to remove excess saturating solution that is excess free ammonium ions are rinsed from the soil by using alcohol

c) Firstly we are leaching the soil with ammonium salt solution, ammonium ions in the ammonium salt solution replaces the other cations like Ca²⁺, Mg²⁺, Al³⁺, H⁺ etc.  Ultimately all the exchange sites in the soil will be occupied by the ammonium cation. All the soil have been saturated with ammonium so to remove excess of the ammonium solution, the soil is again leached to K₂SO_{4 .} That is to remove this adsorbed ammonium cation we further leach the soil with potassium salt solution. This potassium basically replaces this ammonium ions and thus all the exchange side is further occupied by potassium. Thus the concentration of ammonium can be determined that will give the CEC of the solution.