## 4 Discussion  
Tooth discoloration from the intra-coronal bleaching process occurs due to the diffusion of free radicals, such as hydroxyl, superoxide anion, oxygen, and per hydroxyl, produced through the oxidation-reduction reaction of hydrogen peroxide [6; 10; 30; 31; 32; 33]. This oxygen-rich molecule creates peroxide residues in gaseous bubbles that form under the enamel, which can inhibit and prevent adequate infiltration of the bonding agent into the tooth structure [6; 10; 17; 33; 34]. This study showed that all the study groups showed the presence of peroxide residues characterized by the release of oxygen or dissolved oxygen. The high dissolved oxygen in the control group (without antioxidants) was due to the large amount of residual peroxide in the dentin and enamel. In line with our study, [PERSON] et al. (2011) stated that bleaching teeth releases significant amounts of oxygen 24 h after treatment [30]. Studies have also recommended postponing the bonding procedures when bleaching teeth until the remaining oxygen on the teeth surfaces can be removed [31; 33; 35; 36; 37]. Dissolved oxygen in the sodium ascorbate 10% combined with Tween 80 0.2% group and the _Trigona_ spp. propolis 10% group was lower than that in the control group (without antioxidants) because both materials have antioxidant activity. Antioxidants are substances that neutralize free radicals by emitting their electrons, which stop the reaction that causes electron loss [35; 38; 39; 40; 41]. The combination of Tween 80 0.2% with sodium ascorbate causes surface tension and affects the contact angle. In addition, the cohesion force between sodium ascorbate molecules is reduced so that it is easier for sodium ascorbate to penetrate the dentin [42]. _Trigona_ spp. propolis produced via selected extraction has a brown-to-dark-brown color with a soft consistency. Dark-colored propolis contains abundant flavonoids, so it generates more flavonoids than other types [43; 44]. This flavonoid is an active substance in propolis. Moreover, color is a marker of active substance levels in an object or chemical substance in materials [43; 45]. In this study, dissolved oxygen in the _Trigona_ spp. propolisis group was lower than that of the control group (without antioxidants) because _Trigona_ spp. propolisis extracted using ethanol showed moderate antioxidant activity. Ethanol propolisis extract (EEP) has remarkable medical benefits, namely, protection against gamma radiation [46; 47]. The anti-oxidative effects of EEP are associated with free radical absorbability [46; 47; 48]. A study also showed that increasing the amount of EEP inhibits the luminoid of H\({}_{2}\)O\({}_{2}\) chemiluminescence in an in vitro way and suggested that its anti-oxidative capacity is due to the high flavonoid substance [49]. The results of the post hoc test, [PERSON]'s HSD test (\(p<0.05\): significance), comparing the observation times of 1 h, 24 h, and 48 h for each antioxidant group suggested that the duration of the antioxidant application affected the peroxide residue, as indicated by a significant decrease in dissolved oxygen at 1 h, 24 h, and 48 h. Sodium ascorbate 10% combined with Tween 80 0.2% showed lower dissolved oxygen than _Trigona_ spp. propolisis 10% with 48 h of application. Nevertheless, there was no statistically significant difference for both groups. This result is in line with the research conducted by [PERSON] et al. (2016), who showed that sodium ascorbate 10% combined with Tween 80 0.2% has the lowest coronal microleakage after intra-coronal bleaching using 35% hydrogen peroxide and requires a minimum application time of 48 h before composite restorations [50]. Regardless of the peroxide residue, several studies have shown that short application times (2-10 min) for antioxidants can increase bond strength in restored teeth after bleaching, indicating that the short application of antioxidants can reduce free radicals that affect bleaching on dentin bonding [37; 51; 52].