Blue tungstates

Mild and reversible reduction of 1 12 and 2 18 heteropoly-molybdates and -tungstates produces characteristic and very intense blue colours ( heteropoly blues ) which find application in the quantitative determinations of Si, Ge, P and As, and commercially as dyes and pigments. The reductions are most commonly of 2 electron equivalents but may be of 1 and up to 6 electron equivalents. Many of the reduced anions can be isolated as solid salts in which the unreduced structure remains essentially unchanged and... 

Wolframat, n. tungstate, wolfraraate. Wolfram-blau, n. wolfram blue, mineral blue... 

When testing for a tannin reserve or tannin index, the presence of natural color aids detection. Testing for tannin is simple and relatively accurate using either a permanganate reduction test (development of permanent purple color) or lignin/tannic acid polymer test (tungstate blue color). 

The most important mineral example is natural scheelite. ScheeUte emits a bright blue emission in a broad band centered at 425 nm (Fig. 4.9) with a decay time of several ps. Calcium tungstate CaW04 has long been known as a practical phosphor, and has been carefully studied. The intrinsic blue luminescence center is the complex ion in which the central W metal ion is... 

The Falin-Ciocalteu reagent (FCR) is a complex formed in a reaction between sodium tungstate and sodium molybdenate in hydrochloric add and phosphoric acid, which turns yellow after lithium sulphate is added. The reagent reads in an alkaline environment with reducing compounds. Such a reaction gives a blue chromophore which is observed by colorimetry. The Folin-Ciocalteu method is highly sensitive - both to phenolic and non-phenolic compounds, e.g. proteins, vitamin C, vitamin Bj, folic acid, Cu(I). The method is applied most frequently to determine the total content of phenolic compounds [34,35]. If that is the case, a sample for determination should be prepared in a proper manner to minimise the effect of non-phenolic... 

Green-fluorescing Zn2Si04 Mn2+ and magnesium tungstate that fluoresces blue under short-wavelength UV are mixed with the sorbents (silica gel, aluminum oxide) of TLC plates in proportions of ca. 3 wt% to improve the contrast in the detection of UV-absorbing compounds. 

Caution. Use only plastic or ceramic spatulas when handling these wet products, inasmuch as metal (even stainless-steel) spatulas will cause reduction of the tungstates to a blue, reduced form. 

In the Biuret reaction, a purple colour develops when the protein is treated with alkaline copper sulphate. This reaction is dependent on peptide bonds and not on the side chains of individual amino-acids present. In the Folin-Ciocalteu reaction, the protein is treated with tungstate and molybdate under alkaline conditions and the formation of a complex such phenylalanine and tyrosine gives rise to a blue colour. Lowry developed one of the most widely used protein assays in which a combination of the above reactions is involved07, l8). 

A. r>itr>o80-tert-oetane To a 1-L, three-necked flask equipped with an addition funnel, a mechanical stirrer, and a thermometer are added 120 ml of methanol, 51.7 g of tert-octylamine (0.4 mol) and 90 mL of water containing 1.2 g (0. 0028 mol) of the tetrasodium salt of ethylenediaminetetraacetic acid and 2.52 g (0. 0076 mol) of sodium tungstate dihydrate. The solution is cooled to 15°C in an ice bath and hydrogen peroxide (361 mL of a 16% solution, 1.7 mol) (Notes 1 and 2) is added over 5 hr. The blue reaction mixture is stirred for an additional 16 hr and the product is extracted with petroleum ether (3 x 50 mL). Unreacted amine is removed by washing twice with 2 N hydrochloric acid. After the blue organic layer is washed with brine, it is dried over MgS04. Petroleum ether is removed by distillation at atmospheric pressure. Continued distillation of the product affords 29.7 g of nitroso-tert-octane... 

Mixed heteropolymolybdates and tungstates are present in the 6-heteropoly series. The synthesis of the ammonium salts of the 6-heteropoly anions with nickel as the central atom of the general formula (NH4)4[NiMo6 nWn024H6] has been reported187). All compounds thus obtained are light blue. The preparation of the free acids has also been described187). 

Salts containing pentavalent tungsten may be obtained by the reduction of alkali tungstate in concentrated hydrochloric acid. Salts of types M(I)2(WOC1s) (green), M(I)(WOQ4) (brown-yellow), andM(I)(WOCl4 H20) (blue) have been isolated. Thiocyanato andbromo salts are also known. 

Zinc and hydrochloric acid If a solution of a tungstate is treated with hydrochloric acid and then a little zinc added, a blue colouration or precipitate is produced. The product is called tungsten blue and has a composition near to the formula W205. 

Iron(II) sulphate solution reddish-brown colour. Upon adding dilute mineral acid, the colour changes to blue the colour becomes paler and more green upon warming but returns to blue on cooling (difference from tungstate). 

Mixed oxide-hydroxide materials, called blue oxides, are obtained by reduction of acidified solutions of molybdates or tungstates (or suspensions of M03) with Sn11, S02, and so on. Because they are noncrystalline, the structures are not known and the cause of the color is uncertain, but speculation abounds.1... 

These are dark blue species obtained by reduction of both isopoly- and heteropoly molybdates and tungstates, and thus the class is broader than the name implies. It appears that in general, these reductions are reversible and major structural features are retained. In the case of the [M6Oi9]2 ions, reduction by one electron gives a 3-ion in which the added electron is weakly trapped at one metal atom but is thermally mobile. 

A delicate reaction for ferrous iron consists in adding a solution of sodium phospho-tungstate, acidified with hydrochloric acid, to the solution suspected of containing a ferrous salt. The whole is rendered alkaline with caustic soda, when a blue coloration is produced if a ferrous salt is present.8 This reaction is more delicate than that with potassium ferricyanide. 

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