Fe III -SCN

Hypophosphite can be determined, in the presence of phosphate, from the colour it gives with ammonium molybdate in H2SO4 medium [145]. Hypophosphite may also be determined by its bleaching of the colour of the Fe(III)-SCN complex [146 ]. 

Figure 9-4. a and fi curves for the system Fe(III)-SCN". Dashed n line for Example 1 solution. 

Copper(II) has often been colorimetrically determined by its intensely blue ammonia complexes. The a diagram in Figure 9-1 shows that a similar situation occurs to that for the Fe(III)-SCN complexes, namely that a constant excess NH3 concentration will be required if the color is to be proportional to the Cu(II) concentration. Even more useful, now, is the use of EDTA in the determination of metal ions. Copper ion is often titrated in ammonia buffers. Let us examine the constants to see if quantitative complexing of Cu(II) by EDTA should occur in the presence of NH3. 

Patel and Patel (1999) proposed a FIA-spectrophotometric method that enables Ci2 i6 trimethylammonium and cetylpyridinium chloride to be determined by means of forming a complex between the cationic surfactants and Fe (III)-SCN. The effect of FIA variables on determining surfactant in nitric acid medium was studied however, possible interferences (ions present in formulation) were not encountered. This method of determining cetylpyridinium chloride in soap and shampoo was compared with an established FIA method, in which the complex was formed with an anionic dye (Orange II), and the former method attained better sensitivity, selectivity and reproducibility. 

S.SxlO l.mole .sec and 2xl0 1. mole sec , respectively) (c) the Cr(II)-catalysed isomerisation of CrSCN produced in (a) (k = 42 l.mole . sec ). Rate coefficients pertain to 1 M FICIO4 solutions at 25 °C. Thus an inner-sphere mechanism is demonstrated. The S-bonded thiocyanato complex, CrSCN, is not produced when a solution of Cr -FSCN is oxidised by Fe(III). CrSCN can be prepared by the gradual addition of a 5 x 10 M solution to an equal volume of a well-stirred solution of 5.5 x 10 A/ Fe([II) and 4.5 x 10 M SCN . The product solution is green whereas CrNCS solutions are purple. 

Although not strictly relevant to amperometric sensor technology, various metalloporphyrins [Co(III), Mn(III), Fe(III) Fig. 45] have been shown to sense anions potentiometrically with selectivity sequences dependent on the centrally bound metal (Amman et al., 1986 De et al., 1994). For example the anti-Hofmeister selectivity sequence SCN" > I" > CIO4 > N02 > Br > Cl- > NOJ was exhibited by PVC membrane electrodes containing [87]. 

The nitronate anion 414 derived from phenylnorbornene reacts with benzenesulphinate, thiocyanate, nitrite or 4-chlorobenzenethiolate anions in the presence of Fe(III) by the Siwl mechanism to give the norbornenes 415 (R = C SPh, SCN, O2N, or SC6H4CM, respectively). No cyclization occurred450. 

Both Fe(CN)s 4 and Fe(CN)i may be shown, by magnetic measurements (Chap. 25) to be spin-paired type complexes, but most other complexes of Fe(II) and Fe(III), in which, the ligand fields are weaker, are spin free, Other important complexes of Fe(III) are the asymmetric Fe(C204)i 3 ion (formed in removal of rust stains with oxalic acid) and the very deep red Fe(SCN)2" ion. Although the intensity of color of the latter complex could scarcely have been predicted, it is interesting that the azide ion (which is the same shape and has the same number of valence electrons as the 8CN ion) forms a Fe(III) complex whose color is almost as deep. 

As is the case with Fe(III) compounds, the colors of the familiar Ni2+ solutions used as laboratory reagents are not representative of the colors of the salts themselves. The majority of anhydrous nickel salts are yellow but as with copper(II), the color may deepen markedly as the polarizability of the anion increases. Thus NiBr2 is yellow, Ni(SCN)2 brown, and Nil2 black. The green color of aqueous nickel solutions is, of course, that associated with the ion, Ni(H20) +. Although divalent nickel may, in the presence of suitable complexing agents be oxidized to the +3 and +4 or reduced to the +1 state, such reactions are rarely carried out, and the +2 state is by far the most important. 

Figure 2 UV-vis spectra obtained from the time-resolved spectra of the reaction in nonacidified MeOH at —50°C of [Fe(dapsox)(OMe)(MeOH)] (a), intermediate [Fe(dapsox)(NCS)(MeOH)] (b) and [Fe(dapsox)(NCS)2] (c). Experimental conditions [Fe(III)] = 5 X 10 M, [SCN ] = 0.3 M. Inset corresponding kinetic trace fitted to a double-exponential fimction...

Iron(ni) reacts with thiocyanate ion to form the red complex, Fe(SCN) +. Sketch a photometric titration curve for Fe(III) with thiocyanate ion when a photometer with a green filter is used to collect data. Why is a green filter used ... 

In the absence of cyanide, thiocyanate can be determined as the thiocyanate complex FeSCN, formed in acid medium with excess of Fe(III) [41]. Thiocyanate may be extracted and determined as the ion-association complexes formed by SCN with Methylene Blue (1,2-dichloroethane) [42], Rhodamine B (benzene) [43], and ferroin (nitrobenzene) (44). 

In the indirect thiocyanate method (not very sensitive, e 5 10 ) the determination of chloride [20-28] has been based on the displacement of SCN ion from the mercury(II) thiocyanate complex by chloride ions, to give a stable mercury chloride complex. After addition of Fe(III) in excess, the red Fe(SCN) complex is formed, and the absorbance is measured at 480 nm. In the FIA method the UV detection has been applied in the absence of Fe(III) ions [29]. 

Thiocyanate ions react with Fe(III) in a moderately acidic medium to yield a red colour which, for a long time, has been the basis for the determination of iron(III), or total Fe after oxidation of Fe(II) to Fe(IIl) [21-23]. Owing to stepwise complex formation in solution, FeCSCN) ", Fe(SCN)2, and further complexes can be formed. The concentrations of the reagents and the pFl of the medium determine which complexes are prevalent. In general, only FelSCN) " is formed at microgram concentrations of Fe(III). The higher complexes are more intensely coloured. 

When iron is determined in aqueous media or in the presence of acetone, care should be taken that the concentration of thiocyanate is the same in the sample and the standard solutions. The aqueous solution must be sufficiently acid to prevent hydrolysis of Fe(III), which begins even at pH 3. The solution should not, however, be too acidic, otherwise the concentration of SCN may be too small. Iron(III) thiocyanate complexes are not very stable, and can persist only at a relatively high concentration of SCN. The optimum acidity of the solution with HCl, H2SO4, HNO3, or HCIO4 lies within the concentration range 0.05 - 0.2 M. 

The bleaching of Methylene Blue by 8203 affords a sensitive method for determination of thiosulphate [70]. It is possible to determine thiosulphate after extraction of its ion-associates with some basic dyes, e.g., Rhodamine B, Rhodamine 6G, or Crystal Violet [71]. Thiosulphate present in concentrations of the order of 10 M have been determined after the oxidation with iodine by measuring the absorbance due to I3 [72]. Thiosulphate can also be determined in an indirect reaction, in which 8203 reacts with Hg(II) thiocyanate to release SCN which gives a colour reaction with Fe(III) [73]. A method for simultaneous determination of thiosulphate, sulphite, and sulphide has been proposed [74]. 

The ligand concentration affects the fraction of ferric ions that is in the form of the neutral complex Fe(SCN)3 on that basis one might want to use a thiocyanate concentration of about 2.5 M (pL= log[SCN] — 0.4), where [(Fe(SCN)3] goes through a maximum in Fig. 5.1-2. Indeed, Fe(III) can be extracted into oxygen-containing non-polar solvents such as diethyl ether or isobutyl alcohol, and can then be determined spectrometrically in that solvent. 

Fig. 5.3-1 The distribution diagram of an aqueous solution of Fe(III) in thiocyanate in equilibrium with an organic extractant, with the parameter values of Fig. 5.1-2, vKp = 10, and pL = - log[SCN].

Fig. 5.3-2 The corresponding distribution Dof Fe(III) between the organic and aqueous phases pL = - log[SCN].

Iron(III) thiocyanate [Fe(SCN)3] dissolves readily in water to give a red solution. The red color is due to the presence of hydrated FeSCN ion. The equilibrium between undissociated FeSCN and the Fe and SCN ions is given by... 

Fe(III)-cyanide species that according to the literature [10-12] absorbs in the IR above 2100 cm . However, we were not able to detect any traces of Fe either in the filtrate after the exchange procedure or in the FeY zeolite (SCN test). Even when the synthesis is carried out in an air atmosphere, no traces of Fe are detectable, due to the high stability of Fe(NH4)2(SO4)2 employed as the source of Fe(II) ions. Therefore, it is reasonable to attribute the second band at -2115 cm to the presence of other Fe(CN)x species but in different environments. Moreover this species seems to... 

The carrier stream contains Hg(SCN)2 and Fe(III). The chloride of the injected sample reacts with Hg(SCN)2, liberating SCN, which in turn forms with Fe(III) the red-colored complex ion Fe(SCN)2, which is measured spectrophotometrically at 480 nm. The height of the recorded absorbance peak is then proportional to the concentration of chloride in the sample. Besides Fe(SCN)2 , other (higher) complex ions between Fe(III) and SCN might be formed, causing nonlinearity in the cahbration curve at high concentrations. 

Many model studies have focused on methaemerythrin, i.e. the oxidized Fe(III)-Fe(III) form of haemerythrin which contains an 0x0 (rather than hydroxy) bridge. Methaemerythrin does not bind O2, but does interact with ligands such as [N3] and [SCN]. Reaction 28.8 makes use of the trispyrazolylborate ligand, [HBpz3] , to model three His residues the product (28.14) contains antiferro-magnetically coupled Fe(III) centres. 

The manifold depicted in Fig. 5.13 Is used for the determination of chloride Ion In sea and tap water [31]. It uses a dialyser to remove interferents. The reagent Is a mixture of mercury(II) thiocyanate and Iron(III) nitrate, which, in the presence of the analyte, loses the red coloration of the Fe(IIl)-SCN- complex as a result of the formation of the stabler Hg(II)-CI-... 

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