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Phosphate Assay

In an acid medium, molybdate is transformed into a heteropolyacid, which subsequently reacts with phosphate to produce a complex. Molybdate exists as a dimer, which polymerizes and reacts with phosphate to form duodecamolybdophosphoric acid (12 MPA).

12MoO4 + H3PO4 + 24H   ---> H3PMo12O40 + 12H2O

Upon reduction, the acid forms a blue product, referred to as "heteropoly blue"

This colourimetric method is based on the method of Chen, P.S., Toribora, T.Y., Warner, H. (1956; Microdetermination of phosphorus. Anal Chem. 28:1756-1760). The ashed sample is mixed with an acid solution of ammonium molybdate which forms phosphomolybdic acid by combining with the phosphate present. The acid is reduced by the the addition of ascorbic acid to produce a blue colour whose intensity is proportional to the amount of phosphate present.

Calcium Assay

In this method, a cresolphthalein dye is used to form a complex with Ca2+, which is violet coloured at alkaline pH. This colourimetric method is based on the principles described in Schwarzenbach, G. (1955; The complexones and their analytical application. Analyst 80, 713 - 729). The ashed sample is mixed with an alkaline solution of cresolphthalein complexone (CPC), a.k.a. Phthalein Purple, buffered with sodium borate which forms a o-CPC complex by combining with the calcium ions present. The colour intensity of the complex formed is directly proportional to the calcium concentration.

Ca2+  o-CPC      --- alkaline pH --->    calcium-o-CPC complex

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