Photometric titrations indicators

Determination of iron(III) in the presence of aluminium. Iron(III) (concentration ca 50 mg per 100 mL) can be determined in the presence of up to twice the amount of aluminium by photometric titration with EDTA in the presence of 5-sulphosalicylic acid (2 per cent aqueous solution) as indicator at pH 1.0 at a wavelength of 510 nm. The pH of a strongly acidic solution may be adjusted to the desired value with a concentrated solution of sodium acetate about 8-10 drops of the indicator solution are required. The spectrophotometric titration curve is of the form shown in Fig. 17.23. 

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 ... 

CHOICE OF FILTER FOR AUTOMATED PHOTOMETRIC TITRATION. At the end of a photometric titration using the above two indicators the colour of the chloroform phase changes from pink to blue. To choose a filter to detect this end point the visible spectra of the separated chloroform layers of surfactant titrations were recorded before, at and beyond the end point, see Figure 2. At 580 nm there was a greater change in absorbance than at 440 nm, thus the 580 nm filter was preferred. 

Sulphate has been determined in seawater by photometric titration with hydrochloric acid in dimethyl sulfoxide [223]. The sample (5 ml) is slowly added to dimethyl sulfoxide (230 ml) and titrated with 0.02 M hydrochloric acid (standardised against sulfate) with bromo-cresol green as indicator. Since borate, carbonate, and bicarbonate interfere, a separate determination of alkalinity is necessary. 

Jagner and Kerstein [654,655] used computer-controlled high-precision complexiometric titration for the determination of the total alkaline earth metal concentration in seawater. Total alkaline earths were determined by photometric titration using EDTA with eriochrome Black as indicator. The method yielded 63.32 (xmolekg-1 for the total alkaline earth concentration in standard seawater of 3.5% salinity. The precision was about 0.01%. 

Other reported procedures with minor modifications and use of different indicators have been also reported (156-165). A method was also described for determination of camphor in dilute solutions by photometric titration using bromophenol blue as the indicator (l66). 

For rigorous calculations of equilibrium concentrations of the various species in solution, values of the constants for all the equilibria involved are needed use of a computer may be indicated. For example, SeUers, Eller, and Caruso developed a theoretical model that assumes the existence of a wide variety of species such as can be found in acetonitrile solution. Eleven equations in eleven unknowns are solved with the aid of a computer to give information about conductance, potentiometric, and photometric titration curves in nonaqueous media. 

This system was also used to Implement the photometric titration of calcium in the presence of a higher concentration of magnesium reported by Jagner [78]. Calcium was titrated with EGTA at pH 8.6. A small amount of Zn-EGTA was also used and the decrease In the calcium concentration was followed by the simultaneous titration of zinc with Zincon as Indicator. The experimental data for absorbance vs. titrant volume added (v) were linearized by the Gran function [64]... 

FIGURE 14-13 Photometric titration curves (a) tola hardness of water, (b) determination of sulfate. In (a), total water hardness is obtained by titration with 0.10 M EDTA at 610 nm lor 100 ml of a solution that contained 2.82 mmol/L total hardness. Eriochrome Black T was the indicator, fn (b). 10.0 ml of a solution containing sulfale was titrated with 0.050 M BaCI using Thorin as an indicator and a wavelength of 523 nm. The response shown is proportional to transmittance. (From A. L, Underwood. Ana/. Chom.. 1954, 26. 1322. Figure t, p. 1323. Copyright 1954 American Chemical Society.)... 

The 21 formed in the second reaction is determined either by visual chemical titration with a reagent such as sodium thiosulfate in the presence of a suitable endpoint indicator or by amperometric, coulometric, or photometric titration methods. The most sensitive KF methods for the measurement of iodine are coulometric. For both the volumetric-amperometric and coulometric methods the endpoint is detected by a pair of platinum electrodes called the indicator electrodes. An electrical potential (100-400 mV) is applied across the electrodes to balance the circuit and the endpoint is reached when the concentration of I2 ( 50pmoll ) depolarizes the cathode deflecting a galvanometer. The volumetric method measures the amount of standardized reagent necessary to depolarize the platinum electrodes. The coulometric method utilizes, in addition to the indicator electrodes, a second pair of platinum electrodes (generator electrodes) that electrolytically convert the 1 to I2. The current consumed in this process is used to calculate the amount of water using the equation that describes Faraday s laws of electrolysis. 

Like typical titrations in which electrodes are used, photometric titrations can be carried out by using indicator dyes. Here, the pH is plotted not against the concentration of anions or dissociated components, but against the absorbance at one or more wavelengths [179]. From these diagrams, p/f values of even polyvalent acids and bases or of complex equilibria can be determined. Furthermore. it is possible to linearize the diagram, which allows better control of the model of the number of steps of titration [180], [181]. 

FIGURE 4.6 Schematic diagram of an MCFIA system for photometric titration procedures. C carrier (H2O) MB magnetic stirring bar Mm DC mini-motor PP peristaltic pump Rl dye indicator solution R2 titrant solution S sample TC titration chamber V three-way solenoid valve and W waste. 

Used as ImM soln. in AcOH as an acid-base indicator for photometric titration of bases. Cryst. Sol. AcOH. 

Motomizu, S., M. Oshima, Y. Gao, S. Ishihara, K. Uemura, Indicator system for the photometric titration of ionic surfactants in an aqueous medium determination of anionic surfactants with distearyldimethylammonium chloride as titrant and tetrabromophenolphthalein ethyl ester as indicator, Ana/ysr, 1992,117,1775-1780. 

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. 

In general, the apparatus for titrimetric analysis is simple in construction and operation. A typical analysis procedure would involve measurement of the amount of sample either by mass or volume, and then addition of the titrant from a burette or micro-syringe. Apart from visual indication, the course of a titration may be followed by electrochemical or photometric means in neither is the equipment required complex. A simple valve voltmeter or conductivity bridge will suffice on the one hand, and a simple spectrophotometer or filter photometer with minor modifications on the other. Varying degrees of automation may be incorporated. 

Ionic charges of the polymers were determined by photometric colloid titrations in some instances. A known amount of poly(diallyldimethylammonium chloride) was added to the polymer solution at a pH of 2.5. The excess poly(diallyldimethylammonium chloride) was titrated by poly(vinylsulfate) using the adsorption indicator methylene blue. The end point was detected by the photometric detector as the color of the solution changes from blue to violet. For anionic copolymers the colloid titration was conducted at pH values of 2.5 and 10.0 to determine the extent of modification. 

The coulometric titration of weak bases with electrogenerated H+ has been successful for nicotinamide, sodium salicylate, metronidazole, sodium veronal, isonicotinic acid, and other compounds, in 0.1 M sodium perchlorate in a mixture of acetic acid and acetic anhydride (1 6) [47]. The endpoint of the titration was noted photometrically using a malachite green indicator. Approximately 5 mg of each compound was determined with an average deviation of 0.3%. 

Photometric and spectrophotometric measurements are useful for locating the equivalence points of titrations. This application of absorption measurements requires that one or more of the reactants or products absorb radiation or that an absorbing indicator be added to the analyte solution. 

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