when preparing the protein standards we used a tube tube 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?
Generally speaking, you should use buffer in which your protein is diluted. If it is diluted in 0.15M NaCl, you should use 0.15M NaCl. However, in Bradford method, you measure absorbance at 595 nm. As NaCl does not absorb at this wavelength, you could use water as control. The only reason to use 0.15M NaCl (or any dilution buffer) as a control is just technical – to be sure that your buffer does not contain any substances that could interact with Bradford reagent and give false positive signal.