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Flame Photometric Determination of Exchangeable Calcium and Magnesium in Soils¹

ABSTRACT

The Beckman Model DU spectrophotometer, with No. 9200 flame attachment, and No. 4300 photomultiplier assembly was used to determine exchangeable Ca and Mg in soils. The study involved an investigation of the instrumental adjustments that would give the highest metal-to-background emission ratio, including effect of flame composition. The effects of Al, Fe, Mn, and PO₄ upon Ca and Mg flame emissions were investigated. Thirty-two soils were analyzed chemically, and by 2 flame photometric procedures for Ca and Mg, with special emphasis on the effects of method of extraction and preparation of test solution. Two procedures for photometric determinations of exchangeable Ca and Mg are proposed:

1. A "standard procedure" which calls for the elimination of the acetate ion and the removal of R₂O₃ and PO₄, and a resultant test solution of 0.2 N NH₄Cl;

2. A simpler "optional procedure" which consists of the direct flame photometric reading of the soil NH₄OAc extract.

In the photometric determination of Ca and Mg in NH₄OAc solution, the greatest source of error stemmed from the mutual interferences of the two cations, the Ca imparting a slight plus error to the Mg, and the Mg imparting a minus error to the Ca. No such interferences were observed when the determinations were made in the solution of 0.2 N NH₄Cl. For soils low in exchangeable Ca and Mg, it is suggested that the exchangeable Ca and Mg may be equally well determined by the simpler "optional procedure". However, for soils of high Ca and/or Mg contents or where phosphate is suspected, the use of the "standard procedure" is recommended.

¹ Received Oct. 18, 1954.

² Associate Soil Chemist and former Assistant Chemist, respectively, University of Tennessee Agr. Exp. Sta., Knoxville. The authors wish to express their appreciation to Dr. John Dean for aid in instrumental adjustment.

Received for publication October 18, 1954.