When preparing the protein standards, we used a tube (t) without protein, but including bradford reagent and an equivalent volume of 0.15 m nacl instead of protein. can we just use plain water to prepare tube t instead?

I think it the correct answer would be no. We cannot use plain water for this since the preparation of such reagent has certain standards to be followed so as to get the best yield for the experiment. So, it would be best to follow these procedures.