If you dissolve sodium chloride in deionized water and test the pH it will be neutral (pH 7) as will the deionized water! Salt will have NO effect on pH. The saline solution you are talking about is probably buffered and NOT pure sodium chloride to compensate for the acid leached from the PVC bag it is packaged in. Most medical saline is packed in PVC bags and then heat sterlized. The PVC leaches some acids into the saline and lowers the pH. This is why the pH is listed as being in the range of 4.5-7.0!