Ph after RO treatment

Calibrated my meter and checked the Ph of my RO water. 8.42, meter is a Hannah pHep5. Does this seem right?

No, that doesn't seem right or not usual anyway but if the meter reads 4 in 4 buffer and 7 in 7 buffer then it could be. RO membranes pass hydrogen ions so there is usually a slight downward shift in pH. Plus permeate is usually low enough in ions that the CO2 it dissolves from air is sufficient to overcome the slight buffering capacity of the water and lower the pH. Something around pH 6 might be more typical. OTOH if the feed is at high pH and loaded with alkalinity the permeate could be at higher pH.

What is the feed water like? What's it's pH and alkalinity?

[Edit] I guess I should also add that pH measurement in low ionic strength waters is difficult as pH measurement depends on the completion of an electrical circuit through the water and really pure water is not at all a good conductor. There are special techniques for doing it bight slow response and erratic readings can be expected if they aren't used.

Gases pass through RO membranes fairly easily. CO2 is in most water at modest content and is typically the primary concern in large scale RO treatment. We typically include an air-stripping step following the RO membrane to degas the water and reduce the corrosivity of the finished water.

That indicates that a fresh RO sample should have a somewhat low pH. The 8.42 pH would be atypical. Was the pH meter calibrated and was the water confirmed to have a very low Total Dissolved Solids (TDS) content? If neither of those components are confirmed, there isn't much that can be said of the result other than it shouldn't be that high for typical waters.

My bad. Pooched the calibration. Pre RO is 8.42 and post RO is 6.26. Now to adjust the minerals back, assuming a 90% extraction efficiency of minerals.

That's more like what we'd expect.