pH meter behavior

[TennBrewer]

I started using a pH meter so I have a Milwaukee M101 meter and it is behaving somewhat unexpectedly. I calibrated the meter at 68 degrees with the 7.0 and 4.1 pH liquids, fine. I collected 5 gallons of filtered tap water at 68 degrees and began checking the pH. Adding phosphoric acid a little at a time (2 ml) I got it down to 6.29. I added 2 ml of acid again and collected another sample. At first, the pH goes up quite a bit to 6.54 then starts dropping. After about 10 minutes I checked the meter and it had dropped to 6.04. Is this normal? When I add more acid to the water and retest, the pH goes UP at first, then drops slowly back down. How long should I wait for the meter to stabilize and give me an accurate (as much as possible for this brand of meter) reading? I have waited as much as 15 minutes for a reading.

 

[mabrungard]

pH meters have a tough time measuring pH if the water has low dissolved solids (ions). If your water supply is relatively free of ions (soft), then it might be causing some of your frustration. But the other thing is that any acid added to the water needs to be well mixed so as to get everything homogenous.

 

[cire]

The pH of my water supply initially rises and also rises at first when titrated with acid. I put this down to CO2 escaping as the water adjusts to atmospheric pressure after being supplied under greater pressure.

Similar effects can occur when CO2 is produced by the reaction between the acid and any alkalinity present.

Acid is normally heavier than water, so sinks to reduce pH more quickly lower in the liquid. A good stir followed by readings top and bottom will show progress, but additional time should be allowed for CO2 produced to come out of solution.

With some exceptions, pH of water is relatively meaningless to the brewer.