Tungsten colorimetric

For a number of years, phenolic substances were dosed by colorimetric techniques, based on redox reactions usually known as Folin Ciocalteau methods, even if a number of adjustments were developed to fit different matrix characteristics. The Folin Cioalteau reagent is a mixture of phosphomolybdic and phosphotingstic acids, with molybdenum in the 6+ oxidation state and, when the reaction takes place, it is reduced to form a complex called molybdenum blue and tungsten blue. In this complex, the mean oxidation state is between 5 and 6 and the formed complex is blue so it can be read spectrophotometrically at 750 nm. 

Heath, Ghem. Trade J., 1920, 66, 629 Travers, Oompt. rend., 1918, 166, 416. Fora colorimetric method of estimating small quantities of molybdenum in tungsten, see King, Ind. Eng. Chem., 1923, 15, 350. 

Fig. 25. Reflectometer for colorimetric measurements. 1 Photodetector, 2 mirror, 3 slide in measurement position, 4 white reference slide, 5 lens, 6 aperture, 7 filter, 8 infra-red filter, 9 lamp casing, 10 tungsten halogen lamp, 11 filter disk.

The scandium recovery process is ouUined schematically in Figure 5. The tungsten ore tailings are dissolved in the hydrazin sulfuric reagent, the pH is adjusted to 2.0, and the resulting solution is checked for the presence of ferric ion colorimetrically using the thiocyanate spot test. Any ferric ion content is reduced to the ferrous state by the addition of elemental iron as per the following equation ... 

Generally, phenols reduce solutions of phosphomolybdic or phospho-tungstic acid, giving rise to the formation of lower oxides of molybdenum or tungsten. This reaction was formerly used to detect the presence of phenols as well as for the colorimetric determination of phenols (40 — 43). However, it is not specific for phenols, and all reducing organic or inorganic compounds react as well. 

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