Molybdenum thiocyanates

Thiocyanates of Molybdenum.—Thiocyanates containing molybdenum as a cation are not known, but compounds similar to the complex cyanides, in which the metal figures as the central atom of a complex anion, have been prepared. 

To determine W admixtures in molybdenum, the molybdenum thiocyanate complex is first extracted in the presence of thioglycolic acid, then the tungsten thiocyanate complex is extracted after addition of TiCla as reducing agent 128]. ... 

One of the pioneering works on FI liquid-liquid extraction preconcentration was on the determination of molybdenum in plant ash solutions [39]. An aqueous/organic phase ratio of 10 was used to extract the molybdenum thiocyanate complex with a sample throughput of 30 h but the sensitivity achieved in this early attempt appears to be somewhat insufficient for samples with low molybdenum contents. A more recent attempt by the same group produced a more sensitive method for molybdenum. 

Arsonium salts have found considerable use in analytical chemistry. One such use involves the extraction of a metal complex in aqueous solution with tetraphenyiarsonium chloride in an organic solvent. Titanium(IV) thiocyanate [35787-79-2] (157) and copper(II) thiocyanate [15192-76-4] (158) in hydrochloric acid solution have been extracted using tetraphenyiarsonium chloride in chloroform solution in this manner, and the Ti(IV) and Cu(II) thiocyanates deterrnined spectrophotometricaHy. Cobalt, palladium, tungsten, niobium, and molybdenum have been deterrnined in a similar manner. In addition to their use for the deterrnination of metals, anions such as perchlorate and perrhenate have been deterrnined as arsonium salts. Tetraphenyiarsonium permanganate is the only known insoluble salt of this anion. 

Discussion. Molybdenum(VI) in acid solution when treated with tin(II) chloride [best in the presence of a little iron(II) ion] is converted largely into molybdenum(V) this forms a complex with thiocyanate ion, probably largely Mo(SCN)5, which is red in colour. The latter may be extracted with solvents possessing donor oxygen atoms (3-methylbutanol is preferred). The colour depends upon the acid concentration (optimum concentration 1M) and the concentration of the thiocyanate ion (1 per cent, but colour intensity is constant in the range 2-10 per cent) it is little influenced by excess of tin(II) chloride. The molybdenum complex has maximum absorption at 465 nm. 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

Discussion. Molybdenum may be determined colorimetrically by the thiocyanate-tin(II) chloride method (for details, see Section 6.14) or by the dithiol method described here. 

Dithiol is a less selective reagent than thiocyanate for molybdenum. Tungsten interferes most seriously but does not do so in the presence of tartaric acid or citric acid (see Section 17.34). Tin does not interfere if the absorbance is read at 680 nm. Strong oxidants oxidise the reagent iron(III) salts should be reduced with potassium iodide solution and the liberated iodine removed with thiosulphate. 

Kawabuchi and Kuroda have concentrated molybdenum by anion exchange from seawater containing acid and thiocyanate [497] or hydrogen peroxide [497,498], and determined it spectrophotometrically. Korkisch et al. [499] concentrated molybdenum from natural waters on Dowex 1-X8 in the presence of thiocyanate and ascorbic acid. 

In a method described by Kiriyama and Kuroda [500], molybdenum is sorbed strongly on Amberlite CG 400 (Cl form) at pH 3 from seawater containing ascorbic acid, and is easily eluted with 6 M nitric acid. Molybdenum in the effluent can be determined spectrophotometrically with potassium thiocyanate and stannous chloride. The combined method allows selective and sensitive determination of traces of molybdenum in seawater. The precision of the method is 2% at a molybdenum level of 10 xg/l. To evaluate the feasibility of this method, Kiriyama and Kuroda [500] spiked a known amount of molybdenum and analysed it by this procedure. The recoveries for 4 to 8 xg molybdenum added to 500 or 1000 ml samples were between 90 and 100%. 

An adsorbing colloid formation method has been used to separate molybdenum from seawater prior to its spectrophotometric determination by the thiocyanate procedure [517]. 

Theory Molybdenum (VI) is mostly converted to molybdenum (V) when an acidic solution of the former is treated with tin (II) chloride preferably in the presence a little Fe2+ ion. The resulting molybdenum (V) form a red complex with thiocyanate ion as follows ... 

The chromatographic separation of technetium from molybdenum is based on the different extent to which molybdate and pertechnetate are adsorbed from alkaline and acid solutions. The distribution coefficient of molybdate between the anion exchanger Dowex 1-X8 and 3 M NaOH is 12, while it is 10 for pertechnetate under the same conditions. Molybdate is also adsorbed to a much lesser extent from hydrochloric acid solutions than pertechnetate. Thus, molybdemun can be eluted by hydroxide or HCl solutions while nitric acid, perchlorate or thiocyanate are used for the elution of technetium . 

The non-aqueous medium favors the formation of the red-violet complex. The yellow complex is poorly soluble in a wide variety of organic solvents. Molybdenum, uranium, and iron interfere with the determination since these elements form colored thiocyanate complexes. Disadvantages of this method are the long time required and the formation of the yellow thiocyanate complex which cannot be excluded. 

Magnesium nitrate, Tin(ll) fluoride, 4693 Manganese(lV) oxide, Calcium hydride, 4705 Molybdenum(VI) oxide, Graphite, 4717 Nitric acid, Formaldehyde, 4436 Nitric acid, Formic acid, 4436 Nitric acid, Formic acid. Urea, 4436 Nitric acid, Metal thiocyanate, 4436 Oxalic acid, Urea, 0725 Ozone, Acetylene, 4846... 

If a drop of 0-02N aqueous sodium sulphide is added to a drop of a solution containing arsenate or phosphate on a filter paper and a drop of aqueous ammonium molybdate acidified with sulphuric acid also added, a blue colour develops 2 ferro- and ferri-cyanides and thiocyanate should be absent. The formation of molybdenum blue is used in the colorimetric estimation of arsenic (see p. 321). 

Won, Ck. H. Sensitive Spectrophotometric Determination of Submicrogram Amounts of Orthophosphate in Natural Water by a Modified Molybdenum(V)-thiocyanate Method. J. Chem. Soc. Japan, Pure Chem. Sect. 85, 859 (1964). 

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