RAPID DETERMINATION OF EXCHANGEABLE CATIONS USING AN ACCELERATED SOLVENT EXTRACTION PROCEDURE

Cation exchange capacity (CEC) and exchangeable bases are important properties of soils and sediments. They relate information on a soils ability to sustain plant growth, retain nutrients, buffer acid deposition or sequester toxic heavy metals. Cation exchange occurs due to the negative charges carried by soil particles, in particular clay minerals, sesquioxides and organic matter. These negative charges are cancelled out by the absorption of cations from solution.

Several standard methods exist for the determination of cation exchange capacity (Sumner and Miller, 1996). Among these are two methods that estimate the CEC by summation of exchangeable bases (Ca, Mg, Na, K) and exchangeable Al. Both methods involve the extraction of exchangeable bases and Al using neutral salt solutions. The standard method chosen for comparison is the more rapid of the two standard methods. The selected standard method involves 3 sequential extractions of a sample with a 0.2 M NH4Cl solution. The soil-solution slurry is shaken for 15 minutes, and the solution is separated from the solid by centrifugation. The method is labor intensive and requires approximately 2.5 hours to complete a batch of eight samples. Facing the need to determine exchangeable bases on hundreds of samples, a rapid method of extraction was desired. Therefore, a new method was developed using a Dionex ASE 100 accelerated solvent extraction system. This method entails the extraction of exchangeable cations using the same 0.2 M NH4Cl solution used in existing standard methods. The reaction occurs in a stainless steel chamber at 21 C and 1034 kPa. Reaction time and the number of sequential extractions were varied to produce results that are comparable to the standard method. The new methods were tested on a Cazenovia series soil A horizon, and compared with results using the standard method. Results of the standard test determine a total CEC for the soil of 1.95 cmol kg-1. Exchangeable bases, Ca, Mg, Na and K contribute 58.5%, 16.3%, 19.9% and 3.3% of the total CEC, respectively. Exchangeable Al accounts for the remaining 2.1% of the exchange capacity. Comparable results may be achieved by a single 10-minute extraction using the ASE 100. Replicate analyses of both the standard test and the new method agree within 5%.