text: The determination of phosphorus, arsenic, silicon and germanium are examples of the use of heteropoly-molybdenum blue in analytical chemistry. The following example describes the determination of phosphorus. A sample containing the phosphate is mixed with an acid solution of Mo, for example ammonium molybdate, to produce , which has an α-Keggin structure. This anion is then reduced by, for example, ascorbic acid or SnCl, 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.