Method 61b with final determination by autoanalysis

The acid digest from Method 6.1b consists of a solution of 0.05 g sample in 50 ml of 50% v/v H2SO4. There are therefore 50,000 pg sample in 50 ml, or 1000 pg ml-1. If the fertilizer is a super triplephosphate with approximately 20% P, this would give a solution of approximately 200 pg P ml-1. The autoanalysis method is the one described in Chapter 7 for total phosphorus in plant materials, which requires an optimum sample concentration of from 20 to 100 pg P ml-1, therefore 5 ml of the super triplephosphate sample solution should be diluted with 50% v/v H2SO4 to 20 ml in a volumetric flask to give a solution of approximately 50 pg P ml-1. The minimum phosphorus content in the fertilizer to give an adequate peak height is 2 % P, or 4.6% PO. Single superphosphate has approximately 8% P, so the resulting 80 pg P ml-1 solution should not need further dilution. As can be seen from the above discussion, the number of pg P ml-1 read from the standard curve is divided by 10 to give the % P in the sample of fertilizer.

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