Colored complexes, titration

Finding the End Point with a Visual Indicator Most indicators for complexation titrations are organic dyes that form stable complexes with metal ions. These dyes are known as metallochromic indicators. To function as an indicator for an EDTA titration, the metal-indicator complex must possess a color different from that of the uncomplexed indicator. Furthermore, the formation constant for the metal-indicator complex must be less favorable than that for the metal-EDTA complex. 

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. 

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. 

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. 

Testing is undertaken by several methods, including chloroform extraction and use of a sulfonphthalein dye (absorbance of yellow-colored complex using bromophenol blue and bromocresol green) or the use of eosin (sodium tetrabromofluorescein) solution in acetone and tetrachloroethane solvent. After shaking with a citric acid buffer and eosin addition, upon standing the lower layer turns pink if filmer is present. Subsequent titration with Manoxol OT (sodium dioctyl sulfosuccinate) quantifies the filmer, with loss of the pink color indicating the end point. 

Lippi et. al (87) and Dirstine (88) circumvented titration by converting the liberated fatty acids into copper salts, which after extraction in chloroform are reacted with diethyldithio-carbamate to form a colored complex which is measured photometrically. While the end point appears to be more sensitive than the pH end point determination, the advantages are outweighed by the additional steps of solvent extraction, centrifugation and incomplete extraction when low concentrations of copper salts are present. Other substrates used for the measurement of lipase activity have been tributyrin ( ), phenyl laurate (90), p-nit ro-pheny1-stearate and 3-naphthyl laurate (91). It has been shown that these substrates are hydrolyzed by esterases and thus lack specificity for lipase. Studies on patients with pancreatitis indicate olive oil emulsion is definitely superior to water soluble esters as substrates for measuring serum lipase activity. 

Volhard titration formation of a soluble, colored complex at the end point... 

NH4 is distilled after alkalinization. Titration with standardized 0.01 M H2S04 and a mixed indicator (methylene blue and methylene red) NHJ is distilled from water after alkalinization. Ammonium reacts with Nessler s reagent (I2Hg—2IK) to form a yellow-brown colored complex (410—425 nm)... 

Formation or disappearance of a soluble colored complex can also indicate an endpoint. Many reagents that form colored complexes with certain metals have been developed only for use as indicators in these titrations. If the cation being titrated produces a color with the indicator, the endpoint will be characterized by the disappearance of this color. When the cation does not give a colored complex, a second cation that does is introduced, and the first excess of titrant then decolorizes this complex. 

Erio-T is a tribasic acid and forms a colored complex with magnesium ions. This is a beautiful example of how a knowledge of equilibrium constants can be used to solve a problem. The formation constants of Ca and Mg with EDTA are 3.0 x 10 and 5.0 x 10. The magnesium complex with the indicator is more stable (lx 10 ) than the complex with the calcium (2.5 x 10 ) but less stable than the Mg EDTA complex. Thus, during a titration, the EDTA reacts first with the free Ca ions, then with the free Mg" ions, and finally with the Mg in the indicator complex... 

The color of the adsorbed indicator is different from that of the unadsorbed indicator, and this difference signals the completion of the titration. A possible explanation for this color change is that the indicator forms a colored complex with Ag, which is too weak to exist in solution, but whose formation is facilitated by adsorption on the surface of the precipitate (it becomes insoluble ). 

A wide range of visual indicators is available for compleximetric titrations. These generally function by forming a colored complex with the metal ion being titrated, which causes a color change when the metal ion is removed from the complex by reaction with EDTA and releases the free ligand. These indicators are described in detail in another article. 

The concentration of iron(III) in solution can also be obtained by titration with EDTA. However, in this case salicylic acid is also added to the titration cell. Iron(III) forms a strongly colored complex with salicylic acid however, the iron-EDTA complex has a much higher stability constant. As the titration is carried out, there is a very sharp disappearance of color at the end-point as the iron-EDTA complex replaces the iron-salicylic acid complex. This titration can be followed photometrically at 525 nm and produces a plot similar to Figure lA. 

Color indicator titration is based on the stability of product of replacement reaction is higher than that of dye-reagent complex. For example, rhodamine 6G and pinacyanol are, respectively, used as the color indicators of fatty amine and sulfonate. 

Spec determination of the colored complex in a phosphoric acid medium Spec A = 513 nm of the red-colored radical cation formed upon the reaction of PPP with cerium(lV) in a phosphoric acid medium Spec titration of the cerium (IV) complex (pH = 8 to 10)... 

Aqueous solutions of tri-iodide ion 13 exhibit a very sensitive yellow to brown color that is detectable by the human eye down to a concentration as low as 5 x 10 mol/L. The solution of 13 may be its own indicator. During a direct iodometric titration, the equivalence point may be detected by the appearance of a persistent yellow color. Another way to detect the equivalence point is to use starch as an indicator. Starch reacts with iodine in the presence of iodide ions to form a blue-colored complex. The coloration is intense. It is still perceptible for a concentration as low as 10 mol/L. The color sensitivity decreases with increasing temperature. 

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