Phosphine Evolution During the Quantitative Determination of Phosphorus in Cast Iron
All standard methods for the quantitative determination of phosphorus in ores and metallurgical materials are based upon a preliminary conversion to orthophosphate anion PO4-3; nitric acid is generally employed as the reagent to ensure the necessary oxidizing attack on the sample. The analysis is usually continued by employing appropriate reagents to precipitate either magnesium ammonium phosphate, MgNH4PO4*6H2O or ammonium phosphomolybdate, (NH4)3PO4*12MoO3*H2O. When magnesium ammonium phosphate is precipitated, the determination is a gravimetric one involving final ignition to magnesium pyrophosphate, Mg2P2O7. If ammonium phosphomolybdate is precipitated, the analysis may be completed gravimetrically, titrimetrically, or colorimetrically. Of the numerous procedures proposed for the subsequent treatment of the phosphomolybdate, the following have received considerable attention: the precipitate may be filtered, dried, and weighed as (NH4)3PO4*12MoO3; it may be ignited and weighed as P2O5*24MoO3; it may be dissolved in standard alkali, and the excess base titrated with standard acid; it may be reduced with acid and zinc to yield Mo+3, which is then titrated with standard permanganate; it may be dissolved in ammonia, then treated with magnesia mixture to yield the magnesium ammonium phosphate precipitate; it may be reduced to an intensely colored substance called "molybdenum blue," and the solution used for a colorimetric measurement. (Despite the fact that this heteropoly blue is stoichiometrically such a poorly defined compound that the nature of the material has not been unequivocally established, it is found to yield comparatively rapid and reproducible results for the quantities of phosphorus encountered in metal analyses, and is more sensitive than the previously favored "molybdenum yellow" product.) At the present time, the preferred methods are the gravimetric (magnesium ammonium phosphate ignited to magnesium pyrophosphate) - which is considered to be the most accurate (and most tedious) method, the titrimetric (ammonium phosphomolybdate + alkali + acid), and the photometric (molybdenum blue).
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