Investigation of stability of 12-tungstophosphoric, 12-tungstosilicic and 12-molybdophosphoric acid in solutions

[D. Bajuk-Bogdanović]

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Fizička hemija - Fizička hemija materijala / Physical Chemistry - Physical Chemistry of Materials

12-volframfosforna (WPA), 12-volframsilicijumova (WSiA) i 12-molibdenfosforna kiselina (MoPA) su heteropoli kiseline (HPK) koje pripadaju velikoj klasi jedinjenja koja se nazivaju polioksometalati. U osnovi struktura ovih kiselina je Keggin-ov anjon (KA) čija se hidrolitička stabilnost razlikuje od kiseline do kiseline, što je utvrđeno ULj, IC, ramanskom i 31P NMR spektroskopijom. Najmanje je stabilna MoPA, kod koje roditeljska Keggin-ova struktura u razblaženim rastvorima (2∙10-5 moldm-3) nije prisutna čak ni pri pH < 1,0, a u koncentrovanim rastvorima (5∙10-2 moldm-3) egzistira samo pri pH < 1,0. WPA je stabilnija, i kod nje je roditeljski anjon prisutan pri pH < 1,0 u razblaženim, odnosno pH < 1,5 u koncentrovanim rastvorima. Najstabilnija je WSiA, kod koje je KA prisutan u razblaženim vodenim rastvorima do pH = 8,5 odnosno do pH = 6,4 u koncentrovanim rastvorima. Korišćenje 50 % metanola kao rastvarača dovodi do stabilizacije KA u ovim kiselinama, tako da se u slučaju MoPA javlja sve do pH = 3,5 i u razblaženim i u koncentrovanim rastvorima. Za WPA u koncentrovanim rastvorima oblast pH u kojoj se javlja KA se proširuje do pH = 2,5, a u razblaženim čak i do pH = 7,0. Najstabilnija WSiA se dodatno stabiliše u prisustvu metanola, pa se KA javlja sve do pH = 11,5 u razblaženim, odnosno pH = 7,8 u koncentrovanim rastvorima. Proces dekompozicije WPA je delimično reverzibilan, a za vodene rastvore je potvrđena vremenska stabilnost u toku jedne godine. Prilikom upotrebe pufera za podešavanje pH vrednosti rastvora HPK, mora se voditi računa o mogućoj interakciji HPK sa puferom, kao što je to slučaj pri primeni TRIS pufera. Dobijeni rezultati se mogu iskoristiti za utvrđivanje oblika HPK prisutnih u različitim sistemima. U slučaju polianilin - WPA mikro/nanostruktura KA je dominantno prisutna vrsta, ali se identifikuju i male količine Dawson-ovog i monolakunarnog KA. WPA i WSiA u inkubacionom medijumu (uz prisustvo enzima Na+/K+_ATPaze i ATP), u obliku monolakunarnog KA, inhibiraju dejstvo enzima Na+/K+_ATPaze, što je od značaja za različite biohemijske procese.

Heteropoly acids (HPA): 12-tungstophosphoric (WPA), 12-tungstosilicic (WSiA) and 12-molibdophosphoric acid (MoPA) belong to the polyoxometalates, the huge and important class of compounds. The main structure of those acids is Keggin anion (KA). The hydrolytic stability of KA differs between acids, and that is confirmed by UV, IR, Raman and 31P NMR spectroscopy. The less stable is MoPA, where parent Keggin structure in diluted solutions (2∙10-5 moldm-3) not exists even on pH < 1.0, while in concentrated solutions (5∙10-2 moldm-3) exists only if pH < 1.0. WPA is more stable; the parent Keggin anion is stable at pH < 1.0 in diluted, and pH < 1.5 in concentrated WPA solutions. The highest stability has WSiA, where KA remains in toto up to pH 8.5 and 6.4, in diluted and concentrated solutions, respectively. The addition of methanol to the HPA solutions (50 % (v/v)) leads to KA stabilization for the investigated acids. As the result, KA remains stable up to pH 3.5 for both diluted and concentrated MoPA solutions. In presence of methanol, WPA structure remains stable till pH 2.5 in concentrated, and even to pH 7.0 in diluted solutions. In case of WSiA, the KA structure is present up to pH 11.5 and 7.8, in diluted and concentrated solutions, respectively. The WPA decomposition process is partly reversible. Further, we confirmed the stability of the water solutions during the period of one year. In the case of buffer using, it is necessary to take into consideration the possible interactions of HPA with buffer’s components (TRIS buffer was tested as an example). The results can be used to determine the form of HPA present in different systems. In the case of polyaniline - WPA micro/nanostructures, KA is dominantly present species, while the traces of Dawson and monolacunar KA were identified by using IR spectroscopy. WPA and WSiA in the incubation medium (in the presence of commercial enzyme and its substrate-ATP) form monolacunary Keggin anion and exibit concentration-dependent inhibitory effect on the activity of Na+/K+-ATPase. This is important for different biochemical processes.

heteropoli kiseline, solvolitička stabilnost, pH vrednost, ULj, IC, ramanska i 31P NMR spektroskopija

heteropoly acids, solvolytic stability, pH values, UV, IR, Raman and NMR spectroscopy

544.344:661.877/.878.271(043.3)

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