THE SOLID-PHASE REAGENTS FOR THE DETERMINATION OF PHOSPHORUS AND ORGANIC REDUCTANTS

O. Zaporozhets, I. Kachan, L. Zinko

The solid-phase reagents obtained in such a way are ready-to-use analytical forms (indicator powders).


IMMOBILIZATION: ADSORPTION ON SILICA
a procedure takes 20 min/100 portions of a sorbent

Tetradecyl ammonium nitrate (QAS) sorption onto silica gel from chloroform-hexane (1:18) solutions has been studied.
Silica gel with the particles sized from hundred to two hundred micrometers was chosen as a matrix due to its high values of a sorption constant and sedimentation rate.
QAS immobilized (QAS-SG) was obtained easily by its adsorption from chloroform-hexane (1:18) solution.
Lucigenine (Lc) immobilized (Lc-SG) was obtained easily by its adsorption from aqueous solution.


DETERMINATION OF PHOSPHORUS
what is the problem?

Phosphorus is one of the most active biogenic elements, and its content is the major criterion of natural waters trophic status. Therefore, concentration of various forms of phosphorus is an important index of natural waters quality. In addition, special requirements to phosphorus content control in reservoirs exist because of stormy reproduction of water-plants and reservoir eutrophication as a result of phosphorus concentration increase (more then 2 μg/L). An international standard recommends spectrophotometric techniques for the monitoring of phosphorus compounds content in natural waters and the evaluation of water and wastewater quality. The methods based on heteropoly compounds formation in the solution are still actual and widely used for phosphate determination. However, the sensitivity of such methods is not satisfactory for analysis of waters, where concentration of phosphorus or its coexisting forms could be lower than 0.01 μmol/L.

This technique can be improved by preconcentration of heteropoly acid via its extraction or by adsorption recovery using different sorbents. The main advantages of adsorption techniques are high distribution coefficients, cost-effectivity and ecological safety. Moreover, using of silica gel as matrices for the recovery of an analyte allows to create test-methods suitable for semi-quantitative analysis "on-site".

PHOSPHORUS:
sorption-spectroscopic and test determination


We have studied the interaction of QAS adsorbed onto silica surface with heteropoly acids (HPA) in order to obtain the solid-phase reagents for sorption-spectrophotometric and test-determination of phosphorus (V) speciation in natural waters.
The kinetics experiment has shown that equilibrium of Red-Ox reaction in solution is reached in 4 min at room temperature.
H2-type of the isotherm of HPA adsorption onto QAS1-SG may testify ion associate formation on the surface of a sorbent. The curve can be formally linearized in Langmure coordinates (R2=0.989) with k=9.3·105 L/mol. The maximum adsorption capacity of SG1 was 7.6 μmol/g HPA at aQAS1=25 μmol/g that points out the formation of ion associate with the ratio QAS mmobilized : HPA = 3 : 1.


The parameters of HPA recovery with modified SG and analytical signal do not change up to V/m, mL/g: 700 (QAS1-SG) and 200 (Lc-SG).


The detection limits were: 1.9 μg/L (SG1) and 20 μg/.L (SG2), where σ is standard deviation of blank estimation. The linearity of calibration graphs were observed in the ranges of phosphorus concentration (2.5–124) μg/L-1 (QAS1-SG) or (90–500) μg/L1 (Lc-SG).
When 4·10-6 mol.L-1 phosphorus was determined no interference was observed for such ions or elements (mol/L): alkaline and alkaline earth metals < 2·10-2, NH4+ < 1·10-3, Co(II) < 1·10-6, Cu(II) < 1·10-4, Mn(II) < 1·10-5, Fe(III) < 3·10-4, NO3- < 2·10-2, ÑÎ32- < 1·10-2, As < 2·10-5, Si < 1·10-3.


The advantages of the method:

The important advantage of proposed technique is the possibility of VT monitoring of phosphate "on site" with clarification of obtained results in laboratory using SSF.

The proposed techniques are simple, fast, ecologically safe and cost-effective.
The time of one element determination is less then 15 min. The developed technique does not require using of expensive equipment and highly skilled personnel attraction. Because of high sensitivity and selectivity proposed sorbents were found to be applied to speciation analysis of phosphorus in natural waters, in particular orthophosphate and polyphosphate determination.


Conclusion:

The new effective anion-exchangers on the base of silica loaded with high-molecular weight aliphatic (QAS1) and heterocyclic (Lc) quaternary ammonium salts for HPA recoveries are proposed. The QAS1-SG is characterized by the higher affinity to HPA anions as a result of ion-associate formation on the surface. QAS1-SG may be proposed for the effective quantitative recovery of phosphorus in the form of HPA at ppb levels.
The correlation of QAS-SG absorbance with HPA concentration in solution and the colour change of the modified sorbent from white to dark blue enables QAS-SG application for simple, fast, ecologically safe and cost-effective solid-phase-spectrophotometric and visual-test determination of phosphate. Moreover, the low interference of a number of ions and elements from natural waters allows to use the anion-exchanger proposed for the sensitive and selective determination of coexisting forms of P(V) in natural waters.



REDUCTANTS:
sorption-spectroscopic and test determination


Determination of ascorbic acid (Asc)



Determination of catecholamines

The spectrophotometric methods based on the interaction of catecholamines with molybdoarsenic heteropolyacid (HPA) in solution were recommended for their determination. However, this technique can be applied only for analysis of the pharmaceutical species (epinephrine, norepinephrine, dopamine and dihydroxyphenylalanine) because of low selectivity, unsatisfactory stability of the reagent, long-term and labour-consuming procedure.

Approach 1. Preconcentration of the reduced heteropolyacid (HPA) with QAS-SG


The conditions of Red-Ox interaction between epinefrine and HPA in aqueous solution were optimized. The recovery of reduced HPA with Silica gel SG 60 (Merck) modified with heterocyclic (lucigenine, Lc) and aliphatic (tetradecyl ammonium nitrate, QAS1) QAS have been investigated. QAS1-SG was found to have better affinity to HPA anion, however, the sorption was non quantitative. The equilibrium of reduced HPA recovery from an aqueous solution with QAS1-SG is reached in 15 min. Maximum recovery was found to be 50% only. The increase of epinefrine concentration in solution is accompanied with the growth of the intensity of a long-wave maximum in the spectra of concentrates. However, QAS1-SG is not recommended for the determination of epinefrine because it does not provide the required sensitivity and reproducibility.

Approach 2. Heterophase redox reaction


Silica gel SG 60 (Merck) modified with heterocyclic (lucigenine, Lc) and aliphatic (tetradecyl ammonium nitrate, QAS1) QAS and then modified with oxidized HPA (HPA) by its adsorption from aqueous solution have been investigated as indicator powders. The modification procedure is simple and rapid (taking 30 min per 20 portions of a sorbent), and the solid-phase reagents obtained in such a way (HPA-QAS1-SG and HPA-Lc-SG) were stable for more than 6 months when stored dry. The immobilization of HPA on the surface of QAS1-SG and Lc-SG is a result of ion-associate formation on the surface.


The result of HPA immobilisation on the surface of QAS1-SG and Lc-SG is its higher stability in comparison with that of HPA in solution (indicator powders are stable for more then six months when kept dry).


The conditions of Red-Ox interaction between catecholamine (epinefrine and dopamine) and HPA immobilized were optimized. The equilibrium of a heterophase redox reaction is reached in 15 min.


The increase of catecholamines concentration in solution is accompanied with the growth of the intensity of a long-wave maximum in the spectra of HPA immobilized. This allows to use HPA-QAS-SG absorbance as an analytical signal for the determination of catecholamine concentration in solution. The equations of calibration graphs for epinefrine and dopamine determination were the same, so total catecholamine concentration may be determined.

The increase of catecholamines concentration in solution was accompanied with the colour change of a sorbent from yellow to green (HPA-Lc-SG) and from yellow to dark blue (HPA-QAS1-SG). On this base colour scales for visual-test catecholamines determination were developed.


A solid-phase spectrophotometric method and a colour scale for the determination of total catecholamines were proposed. The detection limit was 5 μmol/L of catecholamine. The proposed method was applied to the determination of catecholamines in standard solutions, pharmaceuticals and urine.


Conclusion:

The new effective solid-phase reagents on the base of silica loaded with HPA in the form of its ion-associates with high-molecular weight aliphatic (QAS1) and heterocyclic (Lc) quaternary ammonium salts for the determination of individual catecholamines in pharmaceuticals and their total content in urine (after their previous separation using alumina oxide) have been proposed. HPA immobilized on QAS1-SG and Lc-SG demonstrates higher stability in comparison with that of HPA in solution. The correlation of HPA-QAS-SG absorbance with catecholamines concentration in solution and the colour change of the modified sorbent from from white to dark blue (HPA-QAS-SG) and from yellow to green (HPA-Lc-SG) enables the application of the indicator powders for simple, rapid, ecologically safe and cost-effective solid-phase-spectrophotometric and visual-test determination of catecholamines.



Key publications:

Test method of ascorbic acid determination in fruit juices. Pat. ¹19303 of Ukraine, 8 G 01 N 33/02 C 07 D 307/ O.A. Zaporozhets, L.S. Zinko, I.A. Kachan. - ¹ u 2006 06114; declared 02.06.2006; publ. 15.12.2006. bull. ¹12, 4 p.

A way of sorption-spectroscopic determination of ascorbic acid. Pat. ¹19302 of Ukraine, 8 G 01 N 21/31 A 61 K 31/375. / O.A. Zaporozhets, L.S. Zinko, I.A. Kachan. - ¹ u 2006 06113; declared 02.06.2006; publ. 15.12.2006, bull. ¹12. 8 p.

Zaporozhets O.A., Ivanko L.S., Kachan I.A. A solid-phase reagent on the base of molybdophosphoric heteropolyacid for sorption-spectroscopic determination of ascorbic acid // Voprosy khimii i khimicheskoy tekhnologii (Questions of chemistry and chemical technology). - 2005. - ¹1. - P.9-13.

Kachan I.A., Ivanko L.S., Zaporozhets O.A. Determination of organic reductants with immobilized onto silica molybdophosphoric heteropolyacid. // Magisterium "Natural sciences". - Êyiv: "Kyiv Mohyla Academy" publishing house, 2006. - V.24. - P.51-53.

Kachan I.A., Zaporozhets O.A., Zinko L.S., Koval A.A. Solid-phase spectrophotometric determination of the reductants in solution by the reaction of a "blue" heteropolyacid forming // Metody i obiekty khimicheskogo analiza (Methods and objects of chemical analysis). - 2006. - V.1, ¹2. - P.127-131.

Zaporozhets O.A., Kachan I.A., Zinko L.S., Yevlash Yu.P., Levchenko S.I. Solid-phase analytical reagents on the base of heteropolyacids and their ion associates immobilized on silica // Visnyk Kharkivskogo universyteta (Bull. of Kharkiv university). - In press.


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