Exchangeable Calcium and Magnesium in Calcareous Soils

A KCl solution buffered at pH = 8.3 by triethanol amine is used as an extractant. 

The micronutrient cations can be estimated in a single extraction with diethylene triamine pentaacetic acid (DTPA) which has excellent chelating property with the micronutrient elements. Adequate precaution must be taken against any likely contamination from the reagents and glass wares in micronutrient assay work. Only double distilled water should be used. Specific hallow cathode lamps for each elements are used on AAS and requisite standards for instrument calibration are prepared as per instructions in the operation manual. 

Note The standard atomic conditions for the respective micronutrient elements are to be followed from Instruction Manual of the Atomic Absorption Spectrophotometer. 

19 ARSENIC DETERMINATION BY CONVERSION TO THEIR HYDRIDES AND ASPIRATION INTO AN AAS 

Arsenic is ubiquitous in nature and is found in detectable concentrations in all environmental matrices. The occurrence of As in the continental crust of Earth is usually given as 1.5 to 2.0 mg/1. The distribution of arsenic in nature is extremely variable, showing little correlation with geological formation, climate, or soil. Numerous minerals, rocks, sediments and soils contain arsenic partly as constituent of sulfide minerals or complex sulfides of metal cations and partly as a constituent retained by soils and/or sediments in occluded or adsorbed forms. The latter is manifested primarily by the adsorption or occlusion of As on hydrous A1 and Fe oxides, but these are not necessarily the only source. Arsenic is also adsorbed on clay colloid, is bound to organic matter and may form slightly water soluble compounds with Al, Fe, Ca and Mg in the soil matrix. Some of the more common minerals in soils are arsenopyrite (FeAsS), Orpiment (AsgSg) etc. 

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