Fluorine in high concentrations is known to be quite toxic; its lethal dose is on the average equal to 75 mg/kg of body weight. Fluorine content higher than 1.5 mg/L in drinking water leads to fluorosis. Oxalate-ion in high concentration (more than 350 μmol/L) is known to be toxic to renal epithelium cells. That is why oxalemia can cause several forms of renal disease including both calcium oxalate stone disease and end-stage of renal disease. However, oxalate has been shown to inhibit a variety of enzymes, at least in vitro (lactate dehydrogenase, malate dehydrogenase, pyruvate kinase). In urine, oxalate ion appears to be a nonspecific irritant, and may aggravate vulvodunia. Sensitive, selective, simple and rapid methods of the direct determination of oxalate-ion in urine and also of fluoride and oxalate in water, means of hygiene and biological fluids are of great need today.

PHTALEXONS AS MODIFYING AGENTS:
effective photometric reagents for the determination of anions

Phtalexons, especially methylthymol blue and xylenol orange, are known to be effective photometric reagents for anions determination.

IMMOBILIZATION OF MTB ON QAS-SG:
how to keep a complex-forming ability of the functional groups?

The application of quaternary ammonium salt (QAS)-SG modified MTB (SG1) in analytical practice was limited because of narrow contrast range and, therefore, lower sensitivity of the heterophase reaction in comparison with the reaction in solution. To prevent the interaction of functional carboxyl groups with the surface, MTB was adsorbed at pH <1.0, since monoanionic form of the reagent predominates in solution. However, reagent immobilized in such a way was found to be inactive in the reactions with metal ions. Another way was to immobilize MTB in the form of its complex, in particular, BiH₃R. To activate carboxyl groups of the immobilized reagent the complex on the surface was distracted by treating the sorbent with EDTA solution (SG2). Such approach assures QAS⁺H₅R^- formation on the surface. In this case carboxyl groups do not re-orientate on the surface keeping their complex-forming ability. The long-wave maximum in SG2 absorbance spectra is almost absent in comparison with SG1 (Fig.1, curves 3 and 2) confirming this supposition. That is why, the contrast range of the reaction of MTB immobilized in the form of its complex with Bi(III) is wider than that for the free reagent adsorbed.

COMPETITIVE REACTIONS:
between MTB and ZrOCl₂ immobilized and fluoride or oxalate in solution

Zr(IV) forms a coloured complex with MTB immobilized on QAS-SG. ZrOCl₂ at concentration higher than that of F^- and C₂O₄^2- at pH=1.5-1.6 forms in solution the stable colourless complexes according to the following equations:
ZrO²⁺ + F^- ↔ ZrOF⁺ (lgβ₁=9.8),
ZrO²⁺ + HС₂О₄^- ↔ ZrO(HC₂O₄)⁺ (lgβ₁=10.6)

Competitive reactions in such a medium as "aqueous solution of ZrOCl₂ and anions - immobilized MTB" can be described as follows:

These reactions served as a basis for sorption-spectrophotometric and visual test-methods for fluoride and oxalate determination.

TECHNIQUES:
diffuse reflectance spectroscopy and solid-phase spectrophotometry

The detection limits of solid-phase spectrophotometric of fluoride and oxalate determination are, mg/L: 0.1 and 1.3, respectively. The ranges of linear response for F^- and C₂O₄^2- were observed, up to, mg/L: 3.8 and 8.8.

STANDARD COLOUR SCALE FOR VISUAL TEST DETERMINATION:
MTB immobilized is used as indicator powder

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INTERFERENCE OF FOREIGN IONS:
the developed methods demonstrate high selectivity

The interference of Ві(ІІІ) and SO₄^2-, PO₄^3- is eliminated by the addition of 0.01 mol/L solution of ascorbic acid and 0.01 mol/L of BaCl₂, respectively. To eliminate the interference of fluoride with oxalate determination 1,0 mmol/L solution of Сa(NO₃)₂ at pH=1.5 was added. The anions of the organic acids were destructed prior to F^- determination by ultrasonic exposition (44 KHz, intensity up to 10 W/cm² for 3 min).

APPLICATION:
analysis of foodstuffs, means of hygiene and biological fluids

CONCLUSIONS:

   • The method proposed for fluoride determination is inferior in sensitivity to the known ion-selective electrodes-based methods (DL=0.019 mg/L). At the same time it is proved to be more selective, especially regarding Al(III), Fe(III), Mg(II) and Са(ІІ). The influence of ionic strength of solution revealed to be noticeably lower making possible the application of the method to fluoride determination in highly mineralized media.

   • As the reaction product is concentrated, the sensitivity of the method is 2,5 and 25-fold higher in comparison with photometric methods based on the decoloration of alizarin complexes with Zr(IV) and Се(ІІІ), respectively. Due to the equilibrium reached in only 2 min the proposed indicator reaction allows to decrease the time of one determination from 70-100 to 10 minutes. Moreover, low reproducibility of the measurements possible in solution as a result of varnish formation does not appear when the immobilized reagent is used. The developed method gains in selectivity up to 5-times when compared to diffuse reflectance spectroscopic method. Having the same detection limit it requires drastically less time for the determination in comparison to SS method based on Th(IV) interaction with arsenazo I.

   • The visual test-method comes over the reported methods in sensitivity. In comparison with the dynamic method of fluoride determination using competitive reaction of Zr(IV) with alizarin proposed by the authors earlier, this one has a slightly higher detection limit, being at the same time much more rapid (up to 6 times) and simple. This allows using the developed solid-phase reagent as a ready-to-use analytical form, suitable for the application out of laboratory (on-site).

   • The developed sorption-spectroscopic and visual test methods are simple, cost-effective and may be applied for the determination of C₂O₄^2- in urine. No use of toxic compounds and short time of the procedure (less than 10 min/a single determination) are the main advantages of the proposed methods. The visual test-method of oxalate determination in urine may be applied directly at the site of sampling and does not require expensive equipment to be used.

Key publications:

Цюкало Л.Є., Воловенко О.Б., Запорожець О.А. Іммобілізовані на кремнеземі хелати фталексонів – нові твердофазні аналітичні реагенти // Наукові записки НаУКМА. Хімічні науки і технології. – 2005. – Т.42. – С.37-40.

Спосіб визначення флуориду: Пат. №. Україна. МПК B01J20/283/ О.А.Запорожець, Л.Є.Цюкало.

Спосіб визначення оксалату в сечі: Пат. №. Україна. МПК B01J20/283/ О.А.Запорожець, Л.Є.Цюкало.

Запорожець О.А., Цюкало Л.Є., Мельник Н.В. Сорбційно-спектроскопічне і тест-визначення Pb(II) та F- в харчових продуктах іммобілізованими на кремнеземі хелатами фталексонів // 71-а наукова конференція молодих вчених, аспірантів та студентів "Наукові здобутки молоді – вирішенню проблем харчування людства у ХХІ столітті". – Київ. – НУХТ. – 2005. – С. 81.

Zaporozhets O.A., Tsyukalo L.Ye., Melnyk N.V. New analytical form of methylthymol blue for sorption-spectroscopic and visual test-determination of fluoride // ІІІ спільна наукова конференція з хімії Київського національного університету імені Тараса Шевченка та університету Поля Сабат’є (Тулуза). – Київ. – 2005. – С. 97.

Zaporozhets O.A., Tsyukalo L.Ye., Melnyk N.V. Sorption-spectroscopic and visual test-determination of Pb(II), Zn(II), F^- and C₂O₄^2- in water and food-stuffs // Book of abstracts. International conference "Analytical chemistry and chemical analysis" (AC&CA-05). – Kyiv (Ukraine), 2005. – P. 334.

Цюкало Л.Є. Іммобілізовані на поверхні кремнезему фталексони – твердофазні реагенти для сорбційно-спектрофотометричного і візуального тест-визначення Pb(II), Zn(II), F^- і C₂O₄^2-. Дисертація на здобуття наукового ступеня кандидата хімічних наук за спеціальністю 02.00.02. – аналітична хімія. – Київський національний університет імені Тараса Шевченка, Київ, 2006.