Spectrophotometric measurements, carbon

The procedure of simultaneous extracting-spectrophotometric determination of nitrophenols in wastewater is proposed on the example of the analysis of mixtures of mono-, di-, and trinitrophenols. The procedure consists of extraction concentrating in an acid medium, and sequential back-extractions under various pH. Such procedures give possibility for isolation o-, m-, p-nitrophenols, a-, P-, y-dinitrophenols and trinitrophenol in separate groups. Simultaneous determination is carried out by summary light-absorption of nitrophenol-ions. The error of determination concentrations on maximum contaminant level in natural waters doesn t exceed 10%. The peculiarities of application of the sequential extractions under fixed pH were studied on the example of mixture of simplest phenols (phenol, o-, m-, />-cresols). The procedure of their determination is based on the extraction to carbon tetrachloride, subsequent back-extraction and spectrophotometric measurement of interaction products with diazo-p-nitroaniline. 

Saunders et al (13) reinvestigated the role of buffer salts in the non-enzymic browning, which term is also in use for the Maillard reaction. He used spectrophotometric measurement of the browning, the evolution of carbon dioxide (resulting from the Strecker degradation), and the change in the pH of the medium as the analytical techniques. The authors confirmed the findings of Schwimmer and Reynolds. 

Figure 4.29. FIA manifold for the determination of trace metals by solvent extraction with dithizone in carbon tetrachloride. The aqueous sample S is injected into the aqueous carrier stream AQ, to which at (a) the organic phase ORG is continuously added and after passing through the extraction coil (b), made of Teflon tubing, the organic phase is again separated at point (c) and carried on through the flow cell for spectrophotometric measurement.

To determine by spectrophotometric measurements the dissociation constant m carbon tetrachloride of the molecular complex formed between acenapthene and s-trinitrobenzene. 

R.H. Byrne, X. Liu, E.A. Kaltenbacher, and K. Sell. Spectrophotometric measurement of total inorganic carbon in aqueous solutions using a liquid core waveguide. Anal. Chim. Acta, 451 221-229, 2002. 

Methods to calculate such charge transfer efficiency of bound enzymes have recently been reported [25], One first calculates a theoretical maximum in current density based on enzyme loading, determined from spectrophotometric measurements (assuming perfect charge transfer efficiency), and then compares the value with the actual measured current density, obtained from direct current (DC) polarization experiments. For example, the direct absorption of Pyrococcus furiosus hydrogenase 1 onto carbon-based electrodes was used as a model redox enzyme system, and Faraday s law was used to calculate the maximum theoretical current density as follows (Equation 12.1) ... 

The various groups of pectic substances are usually characterized by their solubility behavior. Polygalacturonides, like other polyuronides, can be quantitatively estimated by the amount of carbon dioxide formed upon heating with hot acid. Pectic substances of all sorts have been commonly determined by the formation and weighing of water-insoluble pectic acid or calcium pectate but unfortunately it is difficult to get these precipitates free from impurities (particularly other polysaccharides) present in the material analyzed. Today, the most common method of estimation is the spectrophotometric measurement of the colored complex formed upon the addition of alcoholic carbazole to the material to be analyzed after alkali treatment and heating with concentrated sulfuric acid. ... 

The kinetics of this reaction can be very conveniently followed by measuring the volume of carbon dioxide liberated in a given time, and the earlier studies employed this technique, though more recently spectrophotometric (uv) methods have been favoured. Most of the kinetic studies have been directed towards determining whether the SE1 or A-SE2 mechanisms, equilibria (257) and (258) or (259) and (260), respectively,... 

The study of carbonate complexes of Pu is complicated by various experimental difficulties. The low solubility of many carbonates (7), leaving a very dilute Pu concentration in solution, results in difficulties to the experiments with electrochemical or spectrophotometric methods. However, the radiometric method with solvent extraction or solubility measurement is easily applicable for the purpose. Unlike the solution with anions, like Cl, N03 etc., the concentration of which can be varied at a constant pH, the preparation of solutions with varying carbonate concentration accompanies indispensably the change of pH of the solution. As a result, the formation of carbonate complexes involves accordingly the hydrolysis reactions of Pu ions in solutions under investigation. It is therefore prerequisite to know the stability constants of Pu(IV) hydroxides prior to the study of its carbonate complexation. 

For the spectrophotometric method, the evolved carbon disulfide is reacted with copper acetate and diethylamine to form a yellow copper complex which can be measured at 435 nm." The recoveries range between 70 and 90%. Reproducibility of this method was improved by reducing the time and the mode of sample pretreatment. Since all alkylenebis(dithiocarbamates) decompose to carbon disulfide by acid degradation, the above analytical methods are not selective. The result is the measured total residues of all alkylenebis(dithiocarbamates) related products. However, this method is recommended as standard method S15 for alkylenebis(dithiocarbamates) by the German Research Association. ... 

Lehaitre et al. [23] have described a fibre-optic spectrophotometric method for the in situ measurement of biological and chemical species. This instrument can measure phytoplanktonic species, and the potential for chemical measurement such as dissolved carbon dioxide is analysed. 

Kiss [8] examined various techniques for the efficient separation and preconcentration of boron from marine sediments. Alkaline fusion with potassium carbonate was used to render boron reactive, even in the most resistant silicate minerals. Fusion cakes were extracted with water and borate was isolated by Amberlite XE-243 boron-selective resin. Borate was determined spectrophotometrically, following elution with 2 mol L 1 hydrochloric acid. Either the carminic acid complex (620nm), formed in sulphuric acid (94%) or sulphuric acetic acid (1 4), or the azomethine hydrogen ion association complex (415nm) formed at pH5.2, were used for borate measurement. 

The pH of methanol/H20/C02 mixtures was measured spectrophotometrically with CO2 proportions as high as 19.2 mol% using pH indicators [28]. Table 9.1 shows the variation of pH of methanol/H20/C02 mixtures as a function of added CO2. Clearly, the impact of added CO2 on the measured pH was not large. As the proportion of CO2 increases, the dielectric constant of the solution decreases and the there is more dissolved CO2 that could dissociate to carbonic acid. The data clearly show that the... 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

HPLC-based electrochemical detection (HPLC-ECD) is very sensitive for those compounds that can be oxidized or reduced at low voltage potentials. Spectrophotometric-based HPLC techniques (UV absorption, fluorescence) measure a physical property of the molecule. Electrochemical detection, however, measures a compound by actually changing it chemically. The electrochemical detector (ECD) is becoming increasingly important for the determination of very small amounts of phenolics, for it provides enhanced sensitivity and selectivity. It has been applied in the detection of phenolic compounds in beer (28-30), wine (31), beverages (32), and olive oils (33). This procedure involves the separation of sample constituents by liquid chromatography prior to their oxidation at a glassy carbon electrode in a thin-layer electrochemical cell. 

The bromination of dibenzoazepine 63 in 1,2-dichloroethane gives the /raw.v-dibromide 64 as the only product. The reaction was monitored spectrophotometrically and found to exhibit a third-order kinetics (second-order in Br2). A significant conductivity has also been found during the course of bromination. Both spectrophotometric and conductometric measurements are consistent with the presence of Br3- salt intermediates at a maximum concentration of ca 2% of that of the initial reactants. The X-ray structure of dibromide 64 shows a considerable strain at carbons bearing bromine atoms. The strain appears to be responsible for an easy, spontaneous debromination of 64, as well as for high barrier for the formation of 64 from the bromonium-tribromide intermediate. That makes possible the cumulation of the intermediate itself during the bromination of 63119. 

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