The effect of ph on the adsorption properties of the phytate molecule

Abstract

The adsorption and elution of phytate to and from Dowex-1-Cl, an anion exchange resin, the exchange site of which does not vary with alteration in the pH, was examined and compared with that of the simplest inositol phosphate, inositol monophosphate (IMP). Phytate at each of three pH levels (2.1,5.6 and 8.6) and IMP at pH 2.4 and 8.6 were applied to a Dowex column and eluted with LiCl under a variety of conditions. Measurement of changes in pH, Na + and Cl − during adsorption of both phytate and IMP showed that ion-exchange adsorption (adsorption arising from electrostatic attraction between oppositely charged ions) was stronger at high than at low pH values. Elution experiments showed, however, that phytate was more strongly bound at low than at high pH whereas the reverse relation held between the binding of IMP and pH. Excessive binding of the weakly ionized phytate molecule at low pH when ion-exchange adsorption is least was attributed to physical adsorption :(adsorption arising from such short-range forces as Van der Waals forces), derived from the large number of polar hydroxyl groups on the phytate molecule. Because IMP contains many fewer hydroxyls than does phytate, physical adsorption of IMP at low pH is weak. The results indicate that the sum of the physical and ion-exchange adsorption forces for phytate is greater at low than at high pH whereas for IMP, the relation between the sum of these forces and the pH is the reverse. The problem of increased adsorption of phytate to the mineral in soil with decrease in pH can be explained from these findings. Also, greater adsorption of phytate to enamel at low than at high pH can be expected.