Question

Example 4. The organic functional groups, particularly carboxyl and phenolic, are major contributors to cation exchange properties of soils and sediments. A fraction of soil organic matter was found to have a molar mass of 5000 g mol-1. The organic matter contained 85 carboxyl groups mol-1 (pKa = 4.8) and 16 phenolic groups mol-1 (pKa = 6.0). At a pH of 7.0, what is the CEC of the organic matter per 1 kg of soil? Answer: 2000 cmol(+)/kg OM or 2000 meq/g OM

Step 1. Using pK values find concentration of deprotonated (negatively charged) groups at pH = 7 for carboxylic groups.

Step 2. Using pK values find concentration of deprotonated groups at pH = 7 for phenolic groups

.Step 3. Find the total concentration of deprotonated groups in charge (-)/mol units

Step 4. Knowing molar mass of OM matter find the number of moles of organic groups per kg of organic matter

Step 5. Find CEC of OM