pH Reading and Swirling the Sample

[mainshipfred]

I've always been confused on what reading to use when measuring pH. FWI I use a Vinmetrica 300 which uses a bulb. If the probe is placed in the sample it is always a higher reading than if I were to swirl the sample. In fact the difference is substantial. Once it is swirled to the point the lowering of the reading stops or slows and I stop swirling it the reading increases, but never near to the point before it was swirled. What I've been using is the steady reading after it has been swirled, is this correct or do I use the lowest reading?

 

[CDrew]

I usually do my measurements with a stir bar spinning, so that would have the same effect as swirling the probe. I assume you calibrate it before each use, and that should give an idea about accuracy when still or moving.

 

[mainshipfred]

I usually do my measurements with a stir bar spinning, so that would have the same effect as swirling the probe. I assume you calibrate it before each use, and that should give an idea about accuracy when still or moving.

So from my experience that would be the lowest reading.

 

[BarrelMonkey]

I always stir (with a stir bar) or gently agitate (by hand). Since the meter works by measuring hydrogen ion exchange between the electrode and solution, it always seemed to me that stirring would ensure a constant environment around the probe, dispersing any local accumulation or deficit of H+ ions. But I seem to recall there is someone from Vinmetrica who posts here occasionally and said that it doesn't make a difference?

ETA: Found this thread... https://www.winemakingtalk.com/threads/the-battle-of-the-ph-meter.74850/post-857690

ETA: And it turns out that the instructions for my pH probe say to 'gently stir the probe in the sample for a faster result'

 

[mainshipfred]

I hate when this happens. A few days ago when I took a pH reading I had a huge difference in the readings. I just took a reading on two wines and whether it was agitated or not I got the same reading within a few hundredths. Thanks for your responses.

 

[cmason1957]

I hate when this happens. A few days ago when I took a pH reading I had a huge difference in the readings. I just took a reading on two wines and whether it was agitated or not I got the same reading within a few hundredths. Thanks for your responses.

It was phase of the moon, or perhaps your wine was going through a teen-aged phase and just wanted to be contrary. Who Knows.

 

[Rice_Guy]

@BarrelMonkey stated the logic which I was taught. We are measuring the electrical potential with a small flow of saturated KCl out of a small pore. It is possible to have a concentration build up which we do not want, so mix while reading. This is more important if the sample is thick as checking yogurt.
Practically speaking the last digit has a lot of error. We probably worry about .02 units more than we should.

 

[BarrelMonkey]

Practically speaking the last digit has a lot of error. We probably worry about .02 units more than we should.

Yesterday I had occasion to measure pH for the same sample in two separate labs. I agree that +/- 0.02 units is in the noise - what you would expect given the precision and accuracy of the instruments. What I actually saw was 0.1 unit difference. That indicates a problem in one lab or both; either the standards are expired, the probe needs replacing and/or there is some other systematic error (eg inappropriate temperature compensation).