# Influence of Selected Restorative Materials on the Environmental pH: In Vitro Comparative Study
## 4 Discussion  
Unlike many researchers, we have chosen saline as a research medium to create an oral environment [13; 18; 24]. It should be noted that the use of distilled water, even at human body temperature, does not reflect oral conditions [25]. Several authors have already used saline with interesting findings [26; 27; 28]. Researchers also use artificial saliva to determine the pH of dental materials [29; 30]. However, its composition is very variable; the search for the best composition is still ongoing [31]. Apart from the eating situation, the oral cavity contains maximally about 1.1 mL of saliva as this amount forces the swallowing reflex [32]. For our analysis, we used 5 mL of saline so that the pH electrode was completely immersed in the solution. In our study, we found significant differences in the effects of different materials on the environmental pH. After 15 days, all tested materials reached a pH between 5 and 6 on the pH scale. It seems that this fact may have some clinical implications. It is well known that glass ionomer cements in the initial setting phase are characterised by a low pH that can irritate the pulp [13; 15]. Therefore, it was surprising that they finally reached a pH higher than composites. Interestingly, conventional glass ionomers acidified the environment to a lesser extent than those modified with resin during the entire 15-day observation period. It is believed that this is a consequence of the lack of any buffering capacity of composite materials [4; 33]. Many researchers indicate this fact as the main cause of secondary caries in materials based on synthetic resin [34; 35]. We also observed that a single sample of the GIC lowered the environmental pH less than dual samples. The highest final pH about 5.9 was achieved by Ketac Molar. According to other researchers, it may even reach a pH close to 7 [13]. The first three days of material maturation seem to be crucial, when the pH increase was faster. The pH variation of Riva SC was similar but with a lower amplitude of the pH value at about 0.8 (compared to 1.4). This behaviour can be explained by releasing unreacted acrylic (or another organic acid) and its calcium salt, a weak acid, and conjugate base, which constitute a typical chemical buffer [35]. Some researchers state that it may result from the acid groups being bound to polymer molecules with limited diffusivity [2]. The most acidifying material was Riva LC which had the lowest initial pH of below 4.0 (below the critical point for enamel). In the  
Figure 8: Repeated-measures analysis of variance for pH values in the individual time points depending on the number of the samples for Evetric. case of two samples, it was also the most acidic material after 15 days with a pH of about 5.2. It is commonly believed that RMGICs are more composites than glass isomers [36; 37], which was confirmed by the findings of our experiment. The research methodology on composite materials focuses largely on subjecting them to cyclical changes in pH [38; 39]. However, there is no clear answer as to whether these materials are chemically stable at a neutral pH. Despite the daily and routine placement of composite restorations, dentists' knowledge about the composite properties and the main factors of the polymerisation process is not adequate. It can lead to incorrect curing protocol, resulting in the incomplete polymerisation as well as relying on residual monomers and pH changes [40; 41]. Moreover, chemical degradation of composite resin results in the release of final products, such as methacrylic acid which could acidify the environment [42; 43]. Contrary to other studies [35; 44], composite materials used in our research (Filtek Bulk Fill and Evetric) acidified the environment. Despite the initially relatively high pH of about 6, we observed a significant decrease to about 5.3 during the next 3 days of measurements. Around day 6, the pH started to rise slightly, and on day 15, it reached about 5.5.