NRES 201 Lecture 25 Outline of Last Lecture I. What is a colloid?II. Particle size and surface areaIII. Importance of the clay fractionIV. Clay mineralsV. Organic matterOutline of Current Lecture VI. Cation ExchangeVII. Cation exchange capacity (CEC)VIII. Determination of CECCurrent LectureCation Exchange- A property of all soil colloids, including -o Clay mineralso Hydrous oxideso Organic matterSignificance of Cation Exchange- The process holds nutrients in a form that iso Potentially usable by plantso Protected against leaching- Regulates soil pH and provides buffer capacity - Maintains a stable ionic environment- Determines plant composition- Distinguishes soil from solution culturesMechanisms - Nature of cation retentiono Cations are not fixed to colloidal surfaceso Thermal energies create a hemisphere of motiono The greater this motion, the more exchangeable an ion becomes. - The exchange processo A cation from the bulk solution must penetrate an exchangeable cation's hemisphere of motion These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Exchange occurs when the former is closer than the latter to the colloidal surfacePrinciples of Cation Exchange- Reversibility o Adsorbed cations are displaceable- Charge equivalenceo Exchange occurs on a charge - for - charge basis- Speedo Cation exchange is rapido Limited by ion movement to and from exchange sites- Mass Actiono Adsorption is favored by a higher ion concentration - Anion Effectso Cation exchange is shifted by an anion that causes a difference in: Dissociation Solubility Volatility- Cation Selectivity o Cations differ in their affinity for exchange sites depending mainly on: Ionic charge- A higher valence increases retention- Usually the most important factor Ionic radius- Smaller ions are held more tightly Extent of hydration- Greater hydration increases ionic radius and reduces the tightness of bindingCation Exchange Capacity- Defined aso The sum total of cations that can be adsorbed per unit weight of material at a specific pHEquivalent Weight- Defined aso The weight of a substance that will contribute or reach with 1 gram of
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