Classic "probe failing at the upper end" pattern — usually one of three things:
1. Buffer solution age. pH 10 absorbs CO2 from air much faster than 7.0 and goes off in months once opened. If your 10.0 bottle has been open a while, try a fresh single-use sachet — that fixes it more often than people expect.
2. Probe age. Apex pH probes are usually good for 1-2 years before slope drift makes 10.0 calibration unreliable. The probe still reads fine at pH 7 (middle range) but the millivolt response gets compressed at the extremes, so 10.0 fails the slope check.
3. Probe glass cleanliness. Salt creep, biofilm, or even fingerprints on the bulb can throw off the upper end more than the lower. Soak the tip in pH 4 buffer or a commercial probe cleaner for 30 min, rinse with RO, then try again.
Order I'd try: fresh 10.0 buffer first (cheap, fast), then clean the probe, then assume the probe needs replacement.
How old is the probe?
