*Author to whom correspondence should be addressed.  Present address: School of Chemistry, Kingston University, Penrhyn Road, Kingston, Surrey, KT1 2EE, UK. JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 9 (1998) 549 — 554 The interaction of dental cements with aqueous solutions of varying pH J. W. NICHOLSON * , M. A. AMIRI  Department of Biomaterials Science, The Dental Institute, King’s College School of Medicine and Dentistry, Caldecot Road, London, SE5 9RW, UK A study is reported in which a series of dental cements of varying types (zinc phosphate, zinc polycarboxylate, glass-ionomer and resin-modified glass-ionomer) was exposed to aqueous solutions of differing pH for time intervals of a week, after which the pH of the storage solutions was determined. The results showed that all of the acid-base cements altered the pH of their storage solution, regardless of whether that initial solution was weakly acidic, weakly alkaline or close to neutral. All cements were found to act as buffers, because they not only increased the pH of the weakly acidic lactic acid solution, but they also decreased the pH of the weakly alkaline artificial saliva. In deionized water, the zinc polycarboxylate generally increased pH, while all other cements reduced it. In all cases, these results were shown to be repeatable on exposure to fresh-aqueous solutions of the appropriate pH for a further week, such experiments being carried out for up to six weeks. In terms of mass change, in most solutions, there was a modest increase during the first week, after which the mass remained steady. In lactic acid, zinc phosphate and zinc polycarboxylate cements showed a gradual reduction in mass throughout the six weeks, whereas the glass-ionomers showed an initial increase, followed by a much slower decrease in mass. These results confirm that glass-ionomers are the most resistant of the cements towards acid erosion. 1. Introduction The cements used in dentistry fall into the class of acid-base cements [1]. They are formed by reaction of aqueous acids (typically phosphoric or polyacrylic) with powdered solid bases, either deactivated zinc oxide or special degradable glasses. Reaction occurs fairly rapidly after mixing, though not as rapidly as conventional neutralization reactions that take place entirely in the aqueous phase. These cements harden, without separation of water, to give solid masses of good hardness and high compressive strength [1]. There are a number of types, depending on the start- ing materials, as illustrated in Table I; one of them, the resin-modified glass-ionomers, have monomers in- cluded within their formulation [2], which allows the neutralization mode of setting to be supplemented by a polymerization reaction. However, regardless of type, all share similar structures. They consist of a continuous matrix formed predominantly by neut- ralization, together with unreacted filler, the latter acting as reinforcement for the overall structure [1]. These cements have a wide variety of uses in clinical dentistry, including as liners and bases, luting both ceramic and metal crowns, and the bonding of ortho- dontic appliances [3]. In general, their fitness for pur- pose is assessed by their mechanical properties once set, and also through the speed of their setting reac- tion. However, glass-ionomers, including their resin- modified counterparts, also have the important physico-chemical property once set of releasing fluor- ide [4, 5], a phenomenon that probably acts to prevent the occurrence of secondary caries [6, 7]. Recently, a detailed study has been reported of an additional chemical effect that may be of clinical sig- nificance, namely that these cements influence the pH of the solutions in which they are stored [8]. The study employed lactic acid at a concentration of 20 mmol dm, the same as that used in the current standard erosion test [9], and cements were shown to increase the pH of this medium over a week from 2.60 to between 4.50 and 5.90, depending on the type of cement. All cements displayed this effect, and did so without being dissolved or otherwise seriously de- graded in the process. Unlike the zinc phosphate or zinc polycarboxylate cements, the glass-ionomers all increased in mass, though to only modest extents and the full significance of this observation was not clear [8]. The possible clinical benefit of this effect arises from the fact that the pH of active caries is of the order of 0957—4530  1998 Kluwer Academic Publishers 549