Niobium thiocyanates

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]. 

To the sample solution containing not more than 50 pg of Nb ( complexed with tartrate), add concentrated HCl and the SnCb and thiocyanate solutions until the solution is 3 M in HCl, 5% in SnCl2, and 12% in KSCN. After 5 min, extract the niobium thiocyanate complex with 2 portions of diethyl ether. Transfer the extracts to a 25-ml standard flask, make up to the mark with ether, and measure the absorbance at 385 nm against a reagent blank solution. 

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. 

Addition of excess of potassium thiocyanate to an alkaline solution of a niobate, followed by zinc and concentrated hydrochloric acid, produces a golden-brown colour which may be almost red in the presence of larger quantities of niobium. It is stated that ndther tantalum nor titanium gives any coloration under the same conditions.3 Addition of... 

Niobium pentachloride decomposes readily in the presence of moisture thus it is very important in this synthesis that the acetonitrile be rigorously dried. The method described by Coetzee et al.1 is suitable for the purification of this solvent. The niobium pentachloride starting material should also be freed of any oxy species. This can be effected at about 100° by sublimation of the pentachloride away from the less volatile oxychlorides using a standard vacuum sublimer with a water-cooled probe. Appearance of commercial samples is a poor guide to their purity, and this purification step should not be omitted. The potassium thiocyanate should be purified by recrystallization and then thoroughly dried. 

Some Thiocyanate Complexes op Vanadium, Niobium, and Tantalum... 

Since niobium pentachloride and tetrachloride decompose readily in the presence of oxygen or moisture, all solvents and vessels used in the reactions must be dried by accepted methods. The niobium chlorides should also be free of any oxy species. The potassium thiocyanate and 2,2 -bipyridine should be purified by recrystallization from water and ether, respectively, and then dried at a pressure of ca. 10" torr for 72 hrs. 

In an inert-atmosphere dry-box, 2.36 g (0.01 mole) of niobium tetrachloride is suspended in 25 ml of acetonitrile in a 100-ml round-bottomed flask equipped with a magnetic stirring bar. Niobium(IV) chloride is conveniently prepared by reduction of niobium(V) chloride with aluminum metal. " This mixture is stirred for ca. 1 hr and then 5.84 g (0.06 mole) of potassium thiocyanate dissolved in 25 ml of acetonitrile is added. Stirring is continued for 4 hr to allow for complete reaction. The red solution is filtered through a medium-porosity sintered-glass frit, separating the insoluble potassium chloride from the soluble K2[Nb(NCS)e]. The KCl precipitate is washed with three 5-ml portions pf... 

Iron(III) does not interfere, if it is converted into Fe(II). Niobium is effectively masked by oxalate. Fluoride and nitrate both interfere. Before its determination by the thiocyanate method, tungsten is usually separated from the majority of metals by precipitation or extraction, often with a-benzoinoxime [6,7]. 

The thiocyanate method (in various modifications) has been used in determinations of molybdenum in plant material [158], natural waters [28,41], cast iron and steel [25,56,159,160], tantalum, niobium, and tungsten [2], vanadium and its compounds [27] corundum and lithium niobate [55], ores and minerals [161,162], uranium concentrates [29,163] lithium fluoride [164], platinum chloride [42], and fertilisers [56]. A review of applications of the thiocyanate method for determining Mo has been given [165]. 

Niobium can be separated from Ta and other metals in HF or HCl media by extracting into CHCI3, CCI4, cyclohexane, 1,2-dichloroethane, or xylene the ion-associates, which are formed by Nb (and Ta) complexes with tetraphenylarsonium cation, TO A [8,9], DAM [10], or Aliquat 336 [11-13]. The thiocyanate complexes of Nb with triphenylphosphonium cation [14] or with amides [15] have also been extracted. 

Niobium V) reacts with thiocyanate in HCl solutions to form a yellow complex, which has been a basis of determining Nb. The niobium is determined spectrophotometrically either after extraction of the complex [34-36] or in an aqueous acetone medium. The sensitivities in both cases are similar, but the extraction method is less subject to interference by other metals. Diethyl ether is commonly used as the solvent, but ketones, esters, and higher alcohols are also suitable. 

Niobium is extracted in the presence of SnCl2 which reduces Fe(III) and other interfering oxidants. The concentrations of SnCl2, HCl, and thiocyanate greatly affect the intensity of the colour and the reproducibility of the results. These concentrations should not be lower than 4% SnCh, 2 M HCl, and 10% KSCN. Since some thiocyanic acid is also extracted by ether, one is recommended to use ether saturated with thiocyanic acid. A mixture of diethyl ether and CCI4 (1+1) makes a useful extractant since it is denser than water. 

The thiocyanate complex of niobium may be extracted in the presence of ethylenebis(triphenylphosphonium) cation [38], promazine [39], triphenylguanidine [40], and dibenzo-18-crown-6 ether [41]. Chloroform, trichloroethylene, toluene and benzene are used as the solvents. 

The thiocyanate method has been used for determining niobium in steels [35,118], tantalum and its compounds [6,7,119], cobalt alloys [37], uranium [120], rocks and minerals [1,121], sodium metal [122], and thin Nb-Ti films [123]. Niobium has been determined in various metals and alloys with the use of Bromopyrogallol Red [124]. 

Niobium(v) forms a 1 2 complex with salicylhydroxamic acid in aqueous HCl which has a formation constant of ca. 2.5 x 10" P mol-. " The extraction of niobiumfv) with N-phenylacetylhydroxamic acid (A) in benzene and CHCI3 has been studied in the presence of pyrocatechol (B) or thiocyanate and a Nb A B complex of stoicheiometry 1 2 2 and a Nb I NCS one of stoicheio-metry 1 2 3 have been identified. " ... 

Most LWR fuel rod cladding is made of Zircaloy (and its derivatives), which is an alloy of primarily zirconium and tin. Other alloying elements include niobium, iron, chromium, and nickel. Zircaloy was chosen because it has a very low cross section for thermal neutrons. Naturally occurring zirconium contains about l%-5% hafnium. The hafnium must be removed because it has a very high thermal neutron cross section and is often used in making control rods for reactors. The separation process used in the United States is a liquid-liquid extraction process. It is based on the difference in solubility of the metal thiocyanates in methyl isobutyl ketone. In Europe, a process known as extractive distillation is used to purify zirconium. This method employs a separation solvent that interacts differently with the zirconium and hafnium, causing their relative volatilities to change. This enables them to be separated by a normal distillation process. The separated zirconium is then alloyed with the required constituents. 

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