## 1 Introduction  
Treatments that improve dental aesthetics increase the patient's quality of life and psychological state, as a harmonious smile is positively related to high tests of social, intellectual and occupational competence [1; 2; 3; 4; 5; 6; 7; 8].  
Dental dyschromias vary in aetiology, location and severity, and their origin may be related to extrinsic causes, intrinsic causes or a combination of both [9; 10; 11; 12; 13]. When tooth discoloration occurs, a detailed study of the case must be carried out in order to make an accurate diagnosis, since the success of the treatment and the accuracy of the results will depend on it [9; 14; 15].  
Cases in which only a single tooth in the anterior sector is affected are relatively frequent, and when this occurs the negative effect becomes even more pronounced because the color does not match the rest [2; 5; 16].  
Internal or intra-coronal whitening is a therapeutic option that involves the application of an oxidizing chemical agent, which removes intrinsic stains via chromogenic degradation, and by breaking down the larger pigments into smaller ones the color of the teeth is lightened [17; 18].  
The most widely used bleaching agents for decades in this type of bleach for devitalized teeth were hydrogen peroxide (HP) and sodium perborate (SP). Recently, carbamide peroxide (CP), which decomposes into HP itself and urea, has also been used.  
HP is a powerful, unstable oxidizing agent that causes irritation to the skin, eyes and mucous membranes upon exposure to high concentrations. It decomposes into water and reactive oxygen radicals. The maximum reported concentration of HP without causing mucosal irritation has been estimated at 5%, and harmful effects have been observed from 8% onward [19]. SP is an effective and relatively inexpensive intra-coronary whitening agent used on devitalized teeth in cases where there is intrinsic discoloration [20; 21; 22]. It is a white powder that is usually mixed with HP 30% or distilled water when used for internal bleaching purposes [23; 24].  
The walking bleach technique is generally used by placing the bleaching agent inside the pulp chamber of the devitalized and darkened tooth [10; 11]. The cavity is temporarily sealed, leaving it to act for about 7 days between sessions, where the bleaching agent is changed and renewed until the tooth reaches the desired shade.  
The reactive oxygen released during the whitening process has been associated with both the whitening capacity and the toxicity of the agents, depending on the concentration of HP and the duration of the treatment [25]. It has been shown in some studies that there is microleakage of the temporary coronal sealant after internal bleaching, which has certain drawbacks, such as conditioning the desired results and the leakage of HP and reactive oxygen into the oral cavity [26; 27; 28]. If the expected success is not achieved during the first session, the treatment time is prolonged and the possibility of coronal leakage may increase due to inserting another temporary restoration when renewing the bleaching agent. This may compromise the endodontic treatment and the viability of the teeth with related aesthetic consequences, since it is generally performed on the teeth of the anterior sector [29]. Furthermore, it has been shown that leakage of HP in high concentrations into the oral cavity can easily cause burns to mucosal and periodontal tissues [30]. The aim of this in vitro study was to evaluate and compare the oxygen expansion that takes place after the reaction of the bleaching agents HP 30% and SP with dental tissues from inside the pulp chamber.