If Eliseo used the 6 M HNO3 for the substitution for 6M HCI, thus the product would have yielded a blue solution, whereas there would actually no reaction had he used the 6 M HCI as instructed. Being a good oxidizing agent it oxidizes the compound itself whereas HCL takes part in the reaction and is very volatile, thus HNO3 should have not been used in redox titration.