Evaluation of points of zero charge of aluminum oxide reported in the literature

In 1965 Parks compiled PZC of metal oxides and hydroxides and his paper is still the most frequently cited source of PZC of these materials. The recent compilation (Kosmulski 2001) covers not only oxides and hydroxides, but also mixed oxides, sparingly soluble salts and other materials whose surface charging is pH dependent. One set of data from this compilation (155 PZC of Al2O3 published in 150 papers between 1969 and 2001) was selected to show the effect of crystalline structure on the one hand and of the choice of the experimental method on the other on the PZC. There is no significant difference between the alpha form (32 entries, average 8.2, median 8.55) and the gamma form (58 entries, average 8.2, median 8.4) and the other forms, mixtures of different forms and materials of unknown structure (65 entries, average 8.3, median 8.7). On the other hand, the choice of the method substantially affects the PZC. The isoelectric point IEP from electroacoustic measurements (11 entries, average 9.3, median 9.5) is significantly above the IEP from classical electrokinetic measurements (78 entries, average 8.2, median 8.7) and potentiometic titration at one ionic strength (29 entries, average and median 8). The results obtained by means of two latter methods are much more scattered than in other methods. The lowest standard deviation was observed in the set of PZC obtained by means of combination of titration (the common intersection point of titration curves for three or more ionic strengths is identified with the PZC) and electrokinetic measurements (7 entries, average and median 8.6), and this value is the "recommended" PZC for alumina.