Working conditions for the determination of total phosphorus in compost using the vanado-molybdophosphoric acid colorimetric method.

The objective of this work is to determine what volume of digested and diluted sample should be used in the vanado-molybdophosphoric acid colorimetric method to measure the total phosphorus content of a compost sample, as well as to define when the pH of the aliquot taken should be neutralized in order to not affect the color development. A sample of compost produced from animal manure was subjected to dry incineration, which involved an acid digestion of 0.25 g of ash and a subsequent dilution to 50 ml with distilled water; this last mixture is called “digested and diluted sample”. A volume of 3, 5, 7 or 9 ml of digested and diluted sample was employed for the colorimetric method; in each case, the pH of the aliquot was adjusted or not, consequently, the experimental design included eight treatments. If the probable total phosphorus content per dry mass (%P) of compost is between 0.150 – 0.676, 0.270 – 1.220, 0.450 – 2.030 or 1.350 – 6.080, use a volume of 9, 5, 3 or 1 ml of the digested and diluted sample, respectively; the pH of this aliquot must be adjusted when the volume to be utilized is equal to or greater than 7 ml.