[blakeb]

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.

[AWK]

pH of 0.100 M HCl measured by pH-meter should be close to 1.1.

[Borek]

Single buffer is not enough to calibrate the meter, if you are 6 units from the calibration buffer your meter can be seriously off and drifting.

[blakeb]

I do calibrate it to both a 7.01 buffer and a 4.01 buffer, I just retest the 7.01 buffer after a few reads to see if it is still reading accurately.

[Borek]

OK. Still, at pH 1 you are outside of the calibrated range which never helps.

Not that I have any better explanation to what you observe. HCl solutions at these concentrations are quite stable and their pH doesn't change by much, so the meter/electrode/measurement are my first suspect.

[Corribus]

As Borek suggested, my guess is that it's a calibration issue. You are 3 pH units away from your lower calibration point. Too far to extrapolate accurately.

Buy a pH = 2 buffer. At least that's a lot closer. You should be calibrating to at least 3 points anyway.

If you trust your ability to accurately dilute, you could dilute to what pH4 should be and measure, see how close you get.