methylene blue and diphenylbenzidine sulfonic acid are redox indicators that tur
ID: 851981 • Letter: M
Question
methylene blue and diphenylbenzidine sulfonic acid are redox indicators that turn from colorless in the reduced state to blue (methylene blue) or violet (diphenylbenzidine sulfonic acid) in the oxidized state. indicate whether each is a suitable indicator for the following redox titrations. (a) reduction of fe3 by sn2 in 1m hcl: methylene blue: suitable or unsuitable diphenylbenzidene sulfonic acid: suitable or unsuitable (b) reduction of ce4 by fe2 in 1m hcl: methylene blue: suitable or unsuitable diphenylbenzidene sulfonic acid: suitable or unsuitable
Explanation / Answer
To do this problem you need to look at the electrical potentials (Eo) given in the hint section. Eo is changes with pH so this could be solved as a pH problem.
Eo for the reduction of Fe is 0.732 V. When compared to our two indicators ethylene blue (Eo = 0.53V) and diphenylbenzidine (Eo = 0.87V) you will notice one is higher and one is lower. To show color they need to be in an oxidized state, that occures when only one half reaction is oxidzed - when both are oxidized the two half reactions offset and it is no longer in an oxidzed state. So you need a situation wher Eo for the indicator is between the E0's for the two half reactions, meaning one will be suitable and the other unsuitable for both.
For the first reaction with Sn2, the Eo for the reduction of Sn2 is 0.139 V. This is lower than 0.732 V and 0.53V so ethylene blue is and suitable diphenylbenzidine is unsuitable.
For the second reaction with Ce4, the Eo for the reduction of Ce4 is 1.47 V. This is higher than 0.732 V and 0.87V so ethylene blue is and unsuitable diphenylbenzidine is suitable.
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