in analytical chemistry. The following example describes the determination of phosphorus. A sample containing the phosphate is mixed with an acid solution of MoVI, for example ammonium molybdate, to produce , which has an α-Keggin structure. This anion is then reduced by, for example, ascorbic acid or SnCl2, to form the blue coloured β-keggin ion, . The amount of the blue coloured ion produced is proportional to the amount of phosphate present and the absorption can be measured using a colorimeter to determine the amount of phosphorus. Examples of procedures are: the analysis of phosphate in sea water. standard methods for determining phosphorus and silicon content of metals and metal ores. (e.g. BSI and ISO standards) the determination of germanium and arsenic The comparison of the measured absorption against readings taken for analyses of standard solutions means that a detailed understanding of the structure of the blue complex was unnecessary. This colorimetric method is ineffective when comparable amounts of arsenate are present in solution with phosphate. This is due to the strong chemical likeness of arsenate and phosphate. The resultant molybdenum blue for arsenate, using the same procedure, does produce a slightly different spectral signature, however. Recently, paper-based devices have become very attractive to use colorimetric determination for making inexpensive, disposable and convenient analytical devices for the determination of reactive phosphate in the field. By using an inexpensive and portable infrared Lightbox system, one can create uniform and repeatable lighting environments to take advantage of the peak absorbance of the molybdenum blue reaction in order to improve limit of detection of paper-based devices. This system may act as a substitute for expensive, lab-equipment spectrometers. Colorimetric determination of glucose The Folin–Wu and the Somogyi–Nelson methods are both based on the same principles. In the first step, glucose (or a reducing sugar) is oxidised using a solution of Cu(II) ion, which is reduced to Cu(I) by the process. In the second step, the Cu(I) ions are then oxidised back to Cu(II) using a colourless hetero-polymolybdate complex, which is, in the process, reduced to give the characteristic blue colour. Finally the absorption of the hetero-poly molybdenum blue is measured using a colorimeter and compared to standards prepared from reacting sugar solutions of known concentration, to determine the amount of reducing-sugar present. The Folin–Wu method uses a reagent that contains sodium tungstate. The exact nature of the blue complex in this procedure is not