Colors complexes and

This test utilizes the ability of iron(III) to form colored complexes and precipitates. 

Cadmimn, copper, mercury, silver, and tin form white precipitates with thiourea when present in high concentrations. Only antimony(III) (and vanadate) gives a weak yellow color with thiourea. Hg, SeOj and SeO " are reduced and interfere by forming colored complexes and precipitates. ... 

Molybdenite (M0S2) forms soft, thin flakes, which are very similar to graphite. Molybdenum trioxide (M0O3) is a white powder that is yellow in heat and insoluble in water and most acids. It is easily soluble in alkaline solution, with molybdates being formed. Ammonium molybdate together with phosphates forms a yellow-colored complex and precipitates in nitric acid solutions. If acidified molybdate solutions are treated with reducing agents, they become deep blue (molybdenum blue). Both reactions are used in the colorimetric determination of phosphorus. 

In this chapter, we give some examples of the use of the formation of complexes in qualitative and quantitative organic analysis. It is not surprising that the major part of these examples is in the realm of the characterization of organic compounds by the formation of colored complexes and in the realm of their quantitative analysis by absorption measurements. The formation of complexes permits not only the identification of an organic compound but also the detection of some structural moieties in unknown molecules. In brief, it permits the functional analysis. 

For determination of free PPO activity, different concentrations of catechol were prepared. Solutions contain 1.0 ml citrate buffer, 0.5 ml MBTH (0.3% in ethanol) and 0.5 ml catechol. 1 minute of reaction time was given after the addition of 0.5 ml enzyme solution (O.lmg/ml). 0.5 ml sulfuric acid (5% v/v) was added to stop the enzymatic reaction. Quinone produced reacts with MBTH to form a red color complex and this complex was dissolved by adding 3.0 ml acetone in the test tube. After mixing, absorbance was measured at 495 nm by using a Shimadzu UV-Visible spectrophotometer. 

Chloride is determined by titrating with Hg(N03)2, forming soluble HgCb-The sample is acidified to within the pH range of 2.3-3.8 where diphenylcarbazone, which forms a colored complex with excess Hg +, serves as the visual indicator. Xylene cyanol FF is added as a pH indicator to ensure that the pH is within the desired range. The initial solution is a greenish blue, and the titration is carried out to a purple end point. 

A few substances indicate the presence of a specific oxidized or reduced species. Starch, for example, forms a dark blue complex with 13 and can be used to signal the presence of excess 13 (color change colorless to blue), or the completion of a reaction in which 13 is consumed (color change blue to colorless). Another example of a specific indicator is thiocyanate, which forms a soluble red-colored complex, Fe(SCN) +, with Fe +. 

EDTA forms colored complexes with a variety of metal ions that may serve as the basis for a quantitative spectrophotometric method of analysis. The molar absorptivities of the EDTA complexes of Cu +, Co +, and Ni + at three wavelengths are summarized in the following table (all values of e are in cm )... 

Hydantoin itself can be detected ia small concentrations ia the presence of other NH-containing compounds by paper chromatography followed by detection with a mercury acetate—diphenylcarba2one spray reagent. A variety of analytical reactions has been developed for 5,5-disubstituted hydantoias, due to their medicinal iaterest. These reactions are best exemplified by reference to the assays used for 5,5-diphenylhydantoiQ (73—78), most of which are based on their cycHc ureide stmcture. Identity tests iaclude the foUowiag (/) the Zwikker reaction, consisting of the formation of a colored complex on treatment with cobalt(II) salts ia the presence of an amine (2) formation of colored copper complexes and (3) precipitation on addition of silver(I) species, due to formation of iasoluble salts at N. ... 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

Changes in the backbone of the sulfonic acid azo dyes often produce drastic changes in properties of the materials. The disulfonic acid (5) is somewhat similar to (3), but is used to color leather red (77). More esoteric dyes have also been developed based on sulfonic acid metal complexes and chitosan-derived materials (78,79). 

Chloride. Chloride is common in freshwater because almost all chloride salts are very soluble in water. Its concentration is generally lO " to 10 M. Chloride can be titrated with mercuric nitrate. Diphenylcarbazone, which forms a purple complex with the excess mercuric ions at pH 2.3—2.8, is used as the indicator. The pH should be controlled to 0.1 pH unit. Bromide and iodide are the principal interferences, whereas chromate, ferric, and sulfite ions interfere at levels greater than 10 mg/L. Chloride can also be deterrnined by a colorimetric method based on the displacement of thiocyanate ion from mercuric thiocyanate by chloride ion. The Hberated SCN reacts with ferric ion to form the colored complex of ferric thiocyanate. The method is suitable for chloride concentrations from 10 to 10 M. 

Mordant Dyes. MetaUizable azo dyes are appHed to wool by the method used for acid dyes and then treated with metal salts such as sodium chromate [7775-11-5] sodium dichromate [10588-01-9] and chromium fluoride [1488-42-5] to form the metal complex in situ. This treatment usually produces a bathochromic shift ia shade, decreases the solubUity of the coloring matter, and yields dyeiags with improved fastness properties. The chromium salts can be appHed to the substrate before dyeiag (chrome-mordant or chrome-bottom method), together with the dye ia a single bath procedure (metachrome process), or as a treatment after dyeiag (afterchrome process). 

Borate reacts with curcumin [458-37-7] C2 H2qO, in the presence of a mineral acid to give a colored 1 2 bore acid curcumin complex that has been used to determine microamounts of boron. Carrninic acid [1260-17-9J, C22H2QO23, (98) and azomethine-H (99) also form a colored complex usehil for low level detection of borates. Boron compounds give a characteristic green color when burned in a flame. 

The same color variety is not typical with inorganic insertion/extraction materials blue is a common transmitted color. However, rare-earth diphthalocyanine complexes have been discussed, and these exhibit a wide variety of colors as a function of potential (73—75). Lutetium diphthalocyanine [12369-74-3] has been studied the most. It is an ion-insertion/extraction material that does not fit into any one of the groups herein but has been classed with the organics in reviews. Films of this complex, and also erbium diphthalocyanine [11060-87-0] have been prepared successfiiUy by vacuum sublimation and even embodied in soHd-state cells (76,77). 

Tetracyanoethylene is colorless but forms intensely colored complexes with olefins or aromatic hydrocarbons, eg, benzene solutions are yellow, xylene solutions are orange, and mesitylene solutions are red. The colors arise from complexes of a Lewis acid—base type, with partial transfer of a TT-electron from the aromatic hydrocarbon to TCNE (8). TCNE is conveniendy prepared in the laboratory from malononitrile [109-77-3] (1) by debromination of dibromoma1 ononitrile [1855-23-0] (2) with copper powder (9). The debromination can also be done by pyrolysis at ca 500°C (10). 

Hexacyanobutadiene [5104-27-4] (4), 1,3-butadiene-1,1,2,3,4,4-hexacarbonitrile, is prepared in good yield by a two-step process from the disodium salt of tetracyanoethane (30). It is like TCNE in forming colored TT-complexes and an anion radical. 

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