Tungsten thiocyanates, complex

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]. ... 

Tungsten (and molybdenum) have been sorbed on Dowex 1 (Cl ) resin from sea-water acidified to pH -1 a 0.5 M solution of NaOH and NaCl has been used as eluent. The sorption of the thiocyanate complex on an anion-exchange column has been used for separation of W from rare earth elements [33]. 

Tungsten, molybdenum, and vanadium interfere in the determination of niobium. In contrast to the corresponding tungsten complex, the niobium-thiocyanate complex is decomposed by oxalic acid. Fe(ni), U, Ti, and Ta do not interfere if they are present in no greater than hundred-fold amounts relative to niobium. Phosphate and fluoride interfere, but the latter can be masked with aluminium ions [37]. 

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. 

Molybdenum gives a similar reaction. If, however, a thiocyanate is added, the red complex ion [Mo(SCN)6]3 is formed, and upon the addition of concentrated hydrochloric acid the red colour disappears and the blue colour due to tungsten remains. 

Thiocyanates of tungsten have not been prejsared. Complex salts containing pyridine and of composition (CgIi5NH)2.W0(SCN)5.a H20, (C5H5N)2.W(0H).3(SCN)3,3 and 2(C5l-l5N.HCNS).W02(SCN)4 have been isolated. 

In strongly acidic medium (8-9 M HCl) containing SnCh, thiocyanate does not give a colour reaction with Mo the absorbance should be measured after about 20 min. The colour of the tungsten complex is stable for 1 -2 hr. 

The sample solution, containing not more than 150 pg of W should be slightly alkaline or neutral. If it contains tartrate, it may also be acidic. Add 2.5 ml of thiocyanate solution, 5 ml of SnCh solution, and enough cone. HCl to bring its concentration in the solution to 8-9 M.. The total volume of the solution should be -25 ml. After 15 min extract the tungsten complex with two portions of isoamyl alcohol. Dilute the extract with the solvent in a 25-ml standard flask, and measure the absorbance at 403 nm, using the solvent as reference. 

Spec A = 665 nm of the titanium-methylene blue-ascorbic acid redox reaction Spec A= 420 nm of the yellow Tiron derivative (pH = 5.2 to 5.6) Spectrophotometric determination of tungsten with thiocyanate after both tungsten(VI) and molybdenum(VI) are extracted into chloroform as benzoin a-oxime complexes... 

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