V. V. Kuznetsov

Automated determination of uranium(VI) in seawater using on-line preconcentration by coprecipitation

An automated method is developed for the spectrophotometric determination of nanoamounts of uranium(VI) with Arsenazo III using online preconcentration by the coprecipitation of the uranium complex of the reagent with organic coprecipitants. The collector was an Arsenazo III ion pair with organic cations poorly soluble in water. The concentrate was separated by filtration through a cellulose fiber membrane and dissolved in a flow of an organic solvent; the analytical signal was recorded as the output peak. The developed method is characterized by simplicity, sufficiently high sensitivity, high throughput and rapidity, and can be used for the selective determination of uranium(VI) in complex liquid matrices. The detection limit is ∼0.01 ng/mL (3s, n = 5, P = 0.95). The throughput of the method is 150 samples per hour.

Amylose and amylopectin as reagents for the flow-injection determination of elemental iodine

The complexation reactions of elemental iodine with amylopectin and amylose were studied under flow conditions with spectrophotometric signal detection. It was found that the average hydrodynamic molecular weights of amylose and amylopectin are 260 000 and 430 000, whereas the coefficients of sedimentation are 3.44 × 10−13 and 7.15 × 10−13 s, respectively. It was demonstrated that the optimum configuration of a flow system depends on the properties of polymer reagent solutions, their viscosity, and the different effects of clath-rate formation with iodine. It was found that the reaction somewhat accelerated in the presence of iodide ions, and amylose as a reagent exhibited the best properties, when the calibration function was described by a linear regression equation. The throughput capacity of this technique was as high as 220 or 140 samples per hour with the use of amylose or amylopectin, respectively. The technique is suitable for the direct determination of elemental iodine in seawater and salt. The detection limit of iodine with amylose was as low as 40 ng/mL.

A New Approach to the Estimation of a Stationary State in Flow-Injection Analysis

An approach to calculating the rate of entropy production in a system was developed to characterize the stationary state in flow-injection systems on the basis of principles of nonequilibrium thermodynamics, and its relationship to the parameters of the measured peak was found. Thus, it became possible to make quantitative relative estimations in real systems. By the example of a classical flow-injection spectrophotometric determination of uranium with Arsenazo III, it was shown that the rate of entropy production can be estimated from experimental data. It was found that the peak-averaged rate of entropy production decreases with concentration. It was found that, at the moment of the measurement of the peak maximum, this value is actually minimum and nonzero, and the nonlinear dependence corresponds to the best calibration function because of the effect of nonequilibrium factors.

Coprecipitation of Naphthol Azosulfonates with Organic Coprecipitants and Its Use in Flow-Injection Analysis

The coprecipitation of azo compounds based on 4-nitrophenyl- and 4-sulfophenyldiazonium naphthol sulfonates as their diphenylguanidinium ion-pair complexes with analogous compounds of naphthalene-2-sulfonic acid and 4-phenylsulfodiazonium is investigated. The optimization of the coprecipitation is considered based on the notions of the driving force of the cocrystallization of impurities. The optical characteristics of colored ion-pair complexes are measured. It is found that the water solubility is decreased most substantially for azosulfonates with hydrophobic nitro groups and with no more than two sulfo groups in different aromatic nuclei, which can lead to a decrease in the detection limit due to preconcentration by coprecipitation, which stimulates the thermodynamic flux of concentrate elution. Coprecipitation is combined with flow-injection analysis in the on-line mode, and new procedures are developed for determining naphthol sulfonates and related azo compounds with the detection limit equal to 0.003–0.006 mg/50 mL.

Chemical Amplification of the Signal and the Supramolecular Factor in the Flow Determination of Nanogram Amounts of Arsenic(V)

The reaction of quintuply charged arsenodivanadomolybdate ions with Brilliant Green dye cations results in the formation of an ion pair with the composition of 1 : 5 and provides the effect of the chemical amplification of the spectrophotometric signal from arsenic detected in a flow mode. The analytical species of the element is coprecipitated with the ion pairs of the same cation with protonated anionic species of para- and octamolybdate ions; the concentrate can be separated on-line by microfiltration. Combining the calculated and experimental data makes it possible to interpret the aggregate of particles of the formed concentrate as a supramolecular formation, in which the solvate-accessible collector can be removed by selective washing with isopropanol. The elution of the concentrate with a flow of dimethylsulfoxide results in a spectrophotometric signal proportional to the concentration of arsenic. The implementation of the principle of the chemical amplification of the signal along with preconcentration by coprecipitation provides a limit of detection of 75 pg of As in 2 mL and allows the element to be selectively determined in seawater.

Combination of precipitation preconcentration and chemical amplification of the signal in flow-injection determination of nanogram amounts of phosphorus

Coprecipitation of ion pairs of quadruply charged molybdovanadophosphoric heteropoly anions and cations of Brilliant Green with the ion pairs of the same dye and paramolybdate ions was used for determining nanogram amounts of phosphorus present as orthophosphates. Filtering of the sol through a column with a filter made of fibrous polypropylene provides on-line preconcentration of phosphorus, eliminates the effect of collector, and makes it possible to elute the concentrate for subsequently measuring the signal in the flow mode. A combination of this technique with chemical amplification of the analytical signal lowers the limit of detection to 0.05 ng of phosphorus in 2 mL.

Use of graph theory for evaluating the selectivity of flow-injection analysis based on displacement reactions: Determination of alkaline-earth metals in water-ammonia media

A theory was developed for quantitatively evaluating the selectivity in the determination of metals by the flow-injection technique with spectrophotometric detection. The theory was applied to flow-injection analysis based on the ability of alkaline-earth metals to displace copper(II) ions in the Cu(II)-EDTA-4-(2′-pyridylazo)resorcinol system in water-ammonia solutions. The feasibility of the theory was demonstrated. It was shown how the theory could be used for the selection of justified conditions in the determination of Ca, Sr, and Ba and for the evaluation of the selectivity of the determination. Practical examples of the determination of Ca confirm the usefulness of the approach developed.

Exchange reactions of sulfates and arylazo sulfonates in organoaqueous solutions in the flow

The principles of the method of oriented graphs were used for the theoretical description of the application of heterogeneous exchange reactions of sulfates with poorly soluble barium arylsulfonates in spectrophotometric flow-injection analysis. The capacity of poorly soluble barium salts of colored organic azo-dye sulfonates for participating in exchange reactions with sulfates in a flow system was studied. It was found that the reagent bearing only oneo,o’-hydroxyazosulfonic group possesses the best properties. It was demonstrated how the theory predicts the necessity of using organoaqueous solutions. A method for determining low concentrations of sulfates by flow-injection analysis was proposed based on a heterogeneous displacement reaction.