Coloured complexes

The thiocyanate ion SCN forms an intensely red-coloured complex (most simply represented as [Fe(SCN)(H20)5] ) which is a test for iron(III). However, unlike cobalt(III), iron(lll) does not form stable hexammines in aqueous solution, although salts containing the ion [FefNHj) ] can be obtained by dissolving anhydrous iron(III) salts in liquid ammonia. 

As esters are usually difficult to detect, this test is of considerable value. In general esters react when heated with hydroxylamine to give a hydroxamic acid (I). The latter gives a coloured complex (II) with ferric salts in acid solution. 

In the absence of ammonia and the concentration of polyamines being > 20 p.M the production of sediments take place. Ethylene diamine reacts with Hg(II) in the form of diimide -HNRNH- to form the insoluble complex IHgHNRNHHgl. In the presence of ammonia the production of sediments having complex composition is also possible. Given concentration of K Hgl 1-2 mM, NaOH 60-120 mM and compai able amounts of ammonia and ethylene diamine the products of reactions ai e only the soluble green-coloured complexes, bearing ammonia in the form of nitride and ethylene diamine in the form of diimide. Those complexes ai e polymers, with their absorption spectmms being different from those of the similar polymeric ammonia complexes. 

The differentiation of analytical signal in the photometry enables one to use non-specific reagents for the sensitive, selective and express determination of metals in the form of their intensively coloured complexes. The typical representative of such reagents is 4-(2-pyridylazo)-resorcinol (PAR). We have developed the methodics for the determination of some metals in the drinking water which employ the PAR as the photometric reagent and the differentiation of optical density of the mixture of coloured complexes by means of combined multiwave photometry and the specific destmction of the complexes caused by the change of the reaction medium. 

The simultaneous determination of Co and Ni is also made at pH 8 in the presence of pyrophosphate. The EDTA is added to the mixture of coloured complexes of these metals to bind the Cu and Zn admixtures into the inactive complexes. The optical density of the solution is measured at 530, 555 and 580 nm. The solution is heated to the boiling point to destmct the complex formed by Ni with PAR, and then is cooled. Again the measurements of optical density ai e performed at the same wavelengths. The Ni concentration is calculated from the variation in the optical density, and the Co concentration is calculated from the final values of optical density. The detection limits for these metals are 4 and 2 p.g/dm, respectively. 

Development of extraction-free photometric procedures for the determination of traces of metals for which hygienic and environmental regulations have been established is an urgent problem. For solution of this problem we used as an organic reagent l-(2- pyridylazo)-naphtol-2 (PAN) which forms intensely coloured complex compounds with many metals and is frequently used for their extraction-photometric determination however these procedures did not find wide application in water analysis due to lack of selectivity and necessity of using organic solvents. 

A new kinetic enzymatic method for the routine determination of urea in semm has been evaluated. This method is based upon an enzymatic reaction and formation of a coloured complex. The method is based on a modified Berthelot reaction. The reaction was monitored specRophotomebically at 700 nm (t = 25 0.1 °C). The optimal pH value, chosen for the investigation of complex, is 7.8. 

Bromopyrogallol red. This metal ion indicator is dibromopyrogallol sulphon-phthalein and is resistant to oxidation it also possesses acid-base indicator properties. The indicator is coloured orange-yellow in strongly acidic solution, claret red in nearly neutral solution, and violet to blue in basic solution. The dyestuff forms coloured complexes with many cations. It is valuable for the determination, for example, of bismuth (pH = 2-3. nitric acid solution endpoint blue to claret red). 

A solution of iodine in aqueous iodide has an intense yellow to brown colour. One drop of 0.05M iodine solution imparts a perceptible pale yellow colour to 100 mL of water, so that in otherwise colourless solutions iodine can serve as its own indicator. The test is made much more sensitive by the use of a solution of starch as indicator. Starch reacts with iodine in the presence of iodide to form an intensely blue-coloured complex, which is visible at very low concentrations of iodine. The sensitivity of the colour reaction is such that a blue colour is visible when the iodine concentration is 2 x 10 " 5 M and the iodide concentration is greater than 4x 10 4M at 20 °C. The colour sensitivity decreases with increasing temperature of the solution thus at 50 °C it is about ten times less sensitive than at 25 °C. The sensitivity decreases upon the addition of solvents, such as ethanol no colour is obtained in solutions containing 50 per cent ethanol or more. It cannot be used in a strongly acid medium because hydrolysis of the starch occurs. 

Discussion. Salicylic acid and iron(III) ions form a deep-coloured complex with a maximum absorption at about 525 nm this complex is used as the basis for the photometric titration of iron(III) ion with standard EDTA solution. At a pH of ca 2.4 the EDTA-iron complex is much more stable (higher stability constant) than the iron-salicylic acid complex. In the titration of an iron-salicylic acid solution with EDTA the iron-salicylic acid colour will therefore gradually disappear as the end point is approached. The spectrophotometric end point at 525 nm is very sharp. 

BASIS OF MANUAL PHOTOMETRIC TITRATION. The determination of anionic surfactants by a photometric titration employs a cationic indicator to form a coloured complex with the surfactant which is insoluble in water but readily soluble in chlorinated solvents (1 ). The end point of the titration occurs when there is a loss of colour from the organic phase. A considerable improvement in this technique is achieved by the use of a mixture of anionic and cationic dyes (4 ), for example disulphine blue and dimidium bromide (Herring s indicator (3)). The sequence of colour changes which occurs during the two phase titration of an anionic surfactant (AS) with a cationic titrant (CT) using a mixed indicator consisting of an anionic indicator (AD) and cationic indicator (CD) is summarised in Scheme 1 ... 

Keeping in mind the above work, experiments were carried out to examine the effects of ultrasound, on the dissolution of zinc metal in an alkaline medium and the decomposition of zinc-dithizone complex in the presence of an ultrasonic field. To examine the effect of power ultrasound on the dissolution of zinc metal in alkaline media, 0.0480 g zinc metal was treated with 10 ml of 5 M NaOH solution. Two samples of this solution were exposed to ultrasound for 15 and 30 min, while, control samples were also kept in the similar condition and for the same duration. To compare their spectra and concentration of dissolved zinc in sonicated and control conditions, zinc-dithizone complex was formed by adding 0.5 ml of 0.005% dithizone solution. The red coloured complex, thus obtained, was extracted in chloroform and made upto to the mark in 25 ml volumetric flask with chloroform. 

To examine the oxidation of Fe2+ to Fe3+, in the second experiment, 10 ml solution of 0.1 M ferrous ammonium sulphate was taken separately in four different beakers and sonicated for 15, 30, 45 and 60 min, before transferring the solution to a 25 ml volumetric flask and adding to it 10 ml of 0.01 M KSCN and making upto the mark with deionised water. The absorbance of these solutions was measured at 4-,iax 451 nm. Sonication of ferrous ammonium sulphate solutions oxidised ferrous ions to ferric ions, which in the presence of thiocyanate ions, produced an intense red coloured complex Fe(SCN)63, in proportions to the oxidation of ferrous ions to ferric ions, as could be seen in Fig. 10.1. 

The DPC first reduces Cr(VI) to Cr(III) and then forms a red-violet coloured complex as under. 

Procedure 10% aqueous solution of potassium iodide, KI, when exposed to sunlight, liberated I2 due to the photolytic decomposition and gave blue colour with freshly prepared starch solution. The intensity of blue coloured complex with the starch increased many fold when the same solution was kept in the ultrasonic cleaning bath. As an extension of the experiment, the photochemical decomposition of KI could be seen to be increasing in the presence of a photocatalyst, Ti02, showing an additive effect of sonication and photocatalysis (sono-photocatalysis) However, the addition of different rare earth ions affect the process differently due to the different number of electrons in their valence shells. 

The technique is used predominantly for the isolation of a single chemical species prior to a determination and to a lesser extent as a method of concentrating trace quantities. The most widespread application is in the determination of metals as minor and trace constituents in a variety of inorganic and organic materials, e g. the selective extraction and spectrometric determination of metals as coloured complexes in the analysis of metallurgical and geological samples as well as for petroleum products, foodstuffs, plant and animal tissue and body fluids. 

Complexes of metals with organic and inorganic ligands which absorb in the visible region of the spectrum are of importance in quantitative analysis. Transitions giving rise to coloured complexes are of three types ... 

The last two types give rise to many strongly coloured complexes suitable for trace analysis. 

More recently Agrawal, Harmalker and Vijayawargiya9 have described the formation of a copper-neomycin complex. With a stoichiometry of 1 1, the blue coloured complex has been made the basis of a spectrophotometric assay method for neomycin(See Section 6.26). 

In nitric acid, Ag2+ forms a brown coloured complex (AgN03)+ via... 

The dye ligand interacts with free or complexed chromium(III) to form 1 1 and 1 2 chromium-dye complexes (Scheme 5.21), mainly the more stable 1 2 complex. These coloured complexes are bound to the wool primarily through van der Waals and electrostatic forces. Any excess chromium (III) will remain linked to carboxylate sites in the wool. 

An example is provided by the work of Valtasaari39 on solutions of cellulose in the coloured complex solvent FeTNa, which necessitates this correction even though the transmittance maximum of this solvent lies conveniently at X0 = 546 nm. In the absence of an absorption correction the molecular weight obtained will be lower than its true value. 

Eq. (c) finally illustrates that the blue-coloured complex shall react quantitatively with sodium tetraphenyl boron to give an insoluble compound. 

The lead content of biological samples is usually very small, rendering gravimetric methods impracticable, and methods have often relied upon the formation of coloured complexes with a variety of dyes. More recently, the development of absorption spectroscopy using vaporized samples has provided a sensitive quantitative method. Oxygen measurements using specific electrodes offer a level of sensitivity which is unobtainable using volumetric gas analysis. 

The methods of measuring the amount of cuprous oxide formed are numerous but the one most frequently used involves the reduction of either phosphomolybdic acid or arsenomolybdic acid by the cuprous oxide to lower oxides of molybdenum. The intensity of the coloured complex produced is related to the concentration of the reducing substances in the original sample. The colour produced with arsenomolybdic acid is more stable and the method is more sensitive than with phosphomolybdic acid. 

An alternative method uses glycerol dehydrogenase (EC 1.1.1.6) to produce NADH. This is measured either by its absorbance at 340 nm or by using a diaphorase (EC 1.6.4.3), which catalyses the reduction of a dye, p-iodonitro-tetrazolium violet (INT) to produce a coloured complex with an absorption maximum at 540 nm ... 

The pyridine-2-carboxyaldehyde imines (49) react with [Cr(CO)6] under u.v. irradiation to afford the intensely coloured complexes (50). Reaction of the... 

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