## 2 Materials and Methods  
This clinical trial included 40 patients randomly assigned to one of four treatment groups (10 per group). Each patient received a single in-office whitening treatment of maxillary and mandibular incisors, amines, and premolars (20 teeth in each patient, a total of 791 teeth due to nine missing teeth). Color changes were measured on six guide teeth (maximillary incisors and four canines). A product of different hydrogen peroxide concentration was used in each group: group 1 (G1): HP 35%; group 2 (G2): HP 37.5%; group 3 (G3): HP 38%; and group 4 (G4): HP 40%. The study was approved by the Clinical Research Ethics Committee (CEIH) of the University of Valencia (Spain) with the procedural registration number 1279729. Each patient was provided with full information about the bleaching procedure, the trial's purpose, the structure, the commitment required from the participant, and the possible risks involved in treatment; all patients gave their informed consent to take part. Accepting an alpha risk of 0.05 and a beta risk of 0.2 in a two-sided test, 55 teeth were necessary in each group to identify statistically significant differences between the four groups greater than or equal to 1.5 \(\Delta\)E units. The common standard deviation was assumed to be 2.5. A drop-out rate of 20% was anticipated. Patients fulfilled the following inclusion criteria: aged between 19 and 65 years; absence of oral pathology; maxillary and mandibular incisors, canines, and premolars presenting colors appearing on the Vita Classical color guide (Vita) (guide samples arranged in order of luminosity), with a luminosity greater or equal to A3; teeth free of fillings. Smokers and patients who had received a whiteningtreatment on a previous occasion were excluded from the trial. The 40 patients were assigned to a group, using online randomization software (www.alazar.info). 10 patients in each of the four study groups. A splint was fabricated to position the device used to quantify color modification in the same area of each tooth (maximly and mandibular central incisors and canines) [24]. Treatment began by removing bacterial plaque and any stains on the tooth surfaces with a nylon brush set in a rotary instrument at low speed, with non-abrasive toothpaste. Anterior and premolar teeth were isolated with a double arch rubber dam (OptiDam, Kerr, Orange, California, USA), with the edges of the perforations inverted in the gingival sulci, and with silk ligatures on the tooth necks. Each patient received a single in-office vital tooth whitening session, treating maxillary and mandibular incisors, canines, and premolars with one of the four hydrogen-peroxide-based (HP) products:  
* Group 1: a light-activated hydrogen peroxide at a concentration of 35% (Quick White kit clinica, Quick White), delivered as a powder and a liquid, which when mixed acquires a turquoise-colored gel consistency. Sixty teeth were treated with the gel, carrying out three applications of 10 min each. The gel was activated with a low-temperature LED lamp specifically designed for this purpose. * Group 2: a chemically activated hydrogen peroxide agent at a concentration of 37.5% (Pola Office+ Southern Dental Industries), which is delivered in a double-parallel syringe; when mixed, it acquires a transparent bluish gel consistency. Fifty-eight teeth were treated with the gel, involving three applications of 8 min each. * Group 3: a chemically activated hydrogen peroxide agent at a concentration of 38% (PURE in-office bleaching, Axis Dental). It is delivered in a double-parallel syringe, which when mixed acquires a gel consistency and a red color. Fifty-seven teeth were treated, each receiving three applications of 15 min duration. * Group 4: a chemically activated hydrogen peroxide treatment at a concentration of 40% (Boost. Ultradent), delivered in a double syringe. When mixed, it acquires a gel consistency and a red color. Fifty-six teeth were treated with the product, involving two applications of 20 min each.