The photometric determination of phosphorus in fertilizers using the phosphovanado‐molybdate complex

Abstract

amounts up to 1000 p.p.m. of the following ions : ammonium, calcium, ferric iron, potassium, sodium, citrate, silicate, sulphate and many others. MethodThe following is a description of the method as used in the Research Laboratory of Fisons Ltd.The details given apply to the Spekker photoelectric absorptiometer, but the method can easily be applied to other makes of absorptiometer.At first separate solutions of nitric acid, vanadate and molybdate were used in that order, but it was found later, in accordance with a statement by Barton,, that a composite solution containing all three served equally well. ( I ) Composite reagent : for each litre, there are required 140 ml. conc. HNO,, 1.0 g. ammonium vanadate, NH,VO,, and 20 g. Analar ammonium molybdate, (NH,) ,Mo,O,, .4H ,O. The ammonium molybdate is dissolved in about 400 ml. distilled water at about 50" c., and then the solution is cooled. The ammonium vanadate is dissolved in about 300 ml. boiling distilled water, cooled and the nitric acid added gradually with stirring. The ammonium molybdate solution is then added gradually to the other solution with stirring. The mixed solution is finally diluted to I 1. with distilled water. ( 2 ) Standard phosphate solution: 3.834 g. dried Analar potassium dihydrogen phosphate, KH,PO,, per 1. For actual use, this solution is diluted ten times so that I ml. of the diluted solution is equivalent t o 0.2 mg. P,O,. Reagents(3) Nitric acid : I vol. acid of sp. gr. 1.42 to 2 vols. water. (4) Ammonium hydroxide: I vol. solution of sp. gr. 0.88 (5) Hydrochloric acid : I vol. acid of sp. gr. 1.18 t o I vol.to I vol. water.water. This rapid method makes use of the stable yellow colour complex developed when an excess of a molybdate solution is added to an acidified solution of a vanadate and an orthophosphate; all three reagents required can be used in a composite solution. Details for the determination of water-soluble and total phosphate are given using the Spekker photoelectric absorptiometer. Results agree within 2.5% of those obtained by the standard gravimetric method, those on nine fertilizers being given. This paper gives an account of a rapid method of determination of P,O,, suitable for use in the works laboratory.The development of the photoelectric absorptiometer has made it possible to compare colours much more accurately than can be done visually. Advantage is taken of this in the method for the determination of phosphorus in soils described by Tinsley and Pizer.l This method makes use of the reaction in which a reducing agent, e.g. stannous chloride, acts upon a solution containing a mixture of an orthophosphate and a molybdate, yielding the so-called ' molybdenum blue ' colour.Colorimetric methods are particularly suitable for analysis of extracts from soils, since the maximum sensitivity is found in very dilute solutions. In the case of fertilizers, however, the concentrated solutions obtained can readily be diluted accurately to the required range of P,O, content. A method was developed several years ago in the l...

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Root Morphological Plasticity for Heterogeneous Phosphorus Supply inZea maysL.

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