I created a 0.1M solution of HCl maybe a week or so ago. I trust my measurements were accurate, but ever since the pH seems to be getting more and more acidic for seemingly no reason. Is there an explanation behind this? Maybe half a week ago I measured the pH of the solution to be around 0.67 instead of the theoretical 1, and today its as low as 0.29. It's been left in an erlenmeyer flask with a rubber stopper, so evaporation shouldn't be the issue here. Perhaps my meter is off? Could it be an error of leaving the solution in a transparent flask? Or could it be the error of meters at high acid concentrations. My meter should be accurate, as it consistently can read the pH of the 7.01 buffer solution it's calibrated to.