Acid-base properties of silica-based ion-exchanger having covalently bonded aminodi(methylphosphonic) acid

Acid-base properties of an organophosphorous silica-based ion-exchanger with covalently bonded complexone groups-N-propylaminodi(methylphosphonic) acid (AdPA-SiO2) were studied. By comparison of AdPA-SiO2 chemical analysis and data of pH and conductometric titration it was demonstrated that the composition of immobilised layer differ from sample to sample and might consist of 60-88% aminodi(methylphosphonic) acid and 12-40% of residual aminopropyl groups. Regardless of the multifunctional nature of AdPA-SiO2 interfacial layer, its acid-base properties can be predicted. All studied AdPA-SiO2 samples have the properties of strong multiprotic H4L acids with pK1 < 2.6. Incomplete transformation of amino groups to aminodiphosphonic acid plays the key role in determination of the conditions of AdPA-SiO2 ionisation in neutral and weak-basic media. Linear correlation was found for pK2 and amine-to-acid conversion degree (ω) with pK2 = 8.8 − 4.0ω. It allows us to determine the conditions of AdPA ionisation for all AdPA-SiO2 samples. The established correlation can be used to check the quality (the degree of interfacial layer heterogeneity) of AdPA-SiO2. The final measurable ionisation constant for AdPA-SiO2, pK3 = 7.9 ± 0.6 is 2-orders of magnitude higher then acidity constant for AdPA homogeneous analogue. It seems that such reduction is caused by increase of electro static interaction between movable counter-ions and negatively charged surface groups. Conductometric study of AdPA-SiO2 also suggests firm fixation of counter-ions in the interfacial layer.