Removal of Heavy Metal and Halide Contamination from Macronutrient Salts

In micronutrient deficiency experiments with plants the salts used to supply macronutrients must obviously be free of significant contamination. Hewitt (4), in reviewing the testing and purification of nutrient reagents, clearly pointed out that there is no guarantee that even the purest salts commercially available are sufficiently free of the known micronutrients, and that routine use of an appropriate purification procedure is amply justified by many examples of improvement gained in deficiency technique. Routine purification is further justified if it can be done more easily than analytical testing of reagents in search of the occasional highly pure batch. While recrystallization has been extensively used as a general purification procedure, Hughes (5) and Iewitt (4) have pointed out that more effective processes for specific groups of micronutrients are needed. This view is supported by data on the effectiveness of recrystallization which we have included in this report. We have applied a number of procedures with the aim of removing specific micronutrients. For example, micronutrient metals have been removed by adsorption on calcium phosphate (1), by coprecipitating with CuS and FeS (2, 4, 11) and by extracting with 8-quinolinol (4, 9), while chloride has been removed by adding AgNO3 as well as by recrystallizing (2). Some of these processes have not been highly effective, and all are somewhat time-consuming. Accordingly we have developed two new proceduresevaporative treatment with HNO3 to remove halides, and coprecipitation with Mg(OH)2 to remove heavy metal cations. Both are exceedingly simple as well as effective.