pH reading of sample keeps creeping upwards; how long to wait to record reading?

[GlowingApple]

When I started all-grain brewing I bought a cheap $10 pH meter. I never quite trusted it and lately it was giving me readings way off of what I was expecting from Bru'n water.

So I decided to finally upgrade to a Milwaukee 102. This last brew it gave me a reading closer to what I was expecting, but I'm still having the same pH creeping issue... If I leave the pH meter in the sample the reading slowly creeps up over the course of about 15-20 min and only then seems reasonably close to the predicted pH.

This last brew I was expecting a pH of 5.40 (from Bru'n water). Took a sample from the mash at about 15 min in and poured it into a series of ice-cold glasses to cool. When it was about 70 ˚F I dipped in the pH probe. After the reading settled (i.e. the hourglass disappeared) the pH read only 5.19... I left the probe in the room temperature mash sample and over the course of about 15-20 min it rose to 5.38. The reading never jumped, just gradually ticked upwards.

Is the pH of the mash sample just changing slowly at room temperature? Or is it actually taking that long to get an accurate pH reading? I noticed the same thing with my $10 meter and just assumed it wasn't accurate, but I assume I can trust the MW102 much more.

 

[coypoo]

Smarter people will answer but maybe it's actively buffering in your sample and that's what it is measuring? Or perhaps a calibration issue? How old is the meter?

 

[ajdelange]

What you are seeing is completely normal. The acid/base reactions responsible for establishing mash pH take 20 - 30 minutes to approach equilibrium pH.

To be sure you are measuring what is happening in the mash tun rather than the sample jar, stir the mash and take additional samples from there rather than tracking the original sample.

 

[GlowingApple]

That makes sense! I'll record readings once the meter stabilizes but take a few more data points the next time I mash.

Thanks for the replies!

 

[mabrungard]

The pH shift you observed is consistent with what I typically see. pH that starts out at less than 5.4 tends to rise toward 5.4 and pH that starts out above 5.4 tends to fall toward 5.4. This does not mean that the pH will eventually get to 5.4, it just gets closer to that apparently 'magic' value. This tendency is probably due to the buffers in the mash.

 

[ajdelange]

Mash pH buffers to the pH where the protons given up by the individual malts and added acids (if any) exactly balance the protons absorbed by the other malts and added bases (if any). There is nothing magical about 5.4 except perhaps that lots of people try to hit 5.4. If one properly designs a mash for pH 5.3 the pH will move towards pH 5.3. If he designs (properly) for 5.5 it will go to 5.5. The actual trajectory taken by the pH will depend on how it is realized. For example, if one uses acid the early observed pH's will tend to be lower than the final pH and conversely if base is added.