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Odd Ph Meter Behavior


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#1 DocJavaDude

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Posted 06 January 2012 - 10:34 PM

I posted a note about this in a thread about pH meters in the grape wine making forum, but thought it would be an appropriate posting here.

I've got a Phep 5 meter and have been pleased with its performance. I think...

Something VERY odd happened that I'd love to run by folks here. The meter has seemed to work very well for the few months I've had it -- calibrates easily, settles pretty quickly, etc. That is until I tried to make my setup a bit more sophisticated. I suspended the Phep 5 from a lab stand into my testing beaker which I set on a stirring plate. No physical contact, of course, with the stir rod in the bottom of the beaker. Here's the craziness... With the stirrer off, the meter reads a stable ph level. When I turn the stirring plate on the ph value nearly immediately begins to drop -- and continues to drop until I turn the stirrer off. The meter does not react in this way from agitating the meter itself in the test liquid (e.g. moving it back and forth, twirling it in the wine, etc.). Is it possible that there is some kind of reaction to the magnetic or electrical fields generated by the stirring plate? I was hoping this would be a simple way to free my hands and stir the wine while adding the titration solution. Apparently this method won't work at least with my Phep 5.

Any thoughts? The meter seems to work without flaw in every other setup.
Jeff

Total Depravity Wines
Eat thy bread with joy and drink thy wine with a merry heart!
Ecclesiastes 9:7

#2 giraldestateswines

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Posted 29 June 2012 - 04:24 PM

I know this is an old thread but I thought I'd answer. Agitation from magnetic stir plates should not cause fluctuation in pH readings. I used to do pH measurements in an FDA certified lab and our standard operating procedures required that you use continuous stirring from a magnetic stir plate while the electrode was suspended in the solution being measured.

A couple thoughts:

1) If your using plastic containers to hold your samples then a magnetic stir plate can build up a static charge with the plastic and affect your electrode and cause it to drift. Instead always use glass when using a stir plate.

2) Another possible cause is dissolved CO2 in that when agitated with the stir bar will change the pH of the solution and you'll get drift.

3) Finally you could have a bad electrode, although I think you'd see drift without the stir plate.

Hope this helps and is still useful
-John Giraldes

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"The day you open the bottle, that's the special occasion"

#3 jwalker1140

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Posted 29 June 2012 - 05:02 PM

Funny, I've noticed opposite problem with my new Phep 5 meter where it drifts down when I don't use a stir plate (I haven't used it with a stir plate yet, though I did build one recently with a computer fan). Mine seems to settle into a reading fairly quickly, but after a moment it starts to slowly drift down and continutes to drift down until I eventually get discouraged and turn it off. I use a glass jar and I always calibrate with 4.0 and 7.0 calibration solutions before use.

The next time I use it I'll try it with a stir plate. But if it continues to drift, should I go with the initial reading?

#4 giraldestateswines

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Posted 29 June 2012 - 06:17 PM

You really want a stable reading to have confidence that the pH is accurate; if it hasn't stabilized after about 3 minutes then something's wrong. I'm guessing that your pH meter calibrates OK and doesn't drift in the calibration standards? If it doesn't drift and calibrates, then your electrode should be fine. Again, it could be due to dissolved CO2 as that does affect pH readings and wine usually has a lot of dissolved CO2, depending on the stage.

Here's a couple of article that may provide some further suggestions:

http://www.coleparme...aryArticle/1207 (provides some possible causes for drift)

http://www.eutechins.../tips/ph/14.pdf (Explains how electrodes work and gives some possible causes)

http://www.vl-pc.com...ph-faq-group-2/ (Gives info about cleaning electrodes and possible causes of drift)

Good luck!

BTW nice piece of engineering with the computer fan stir plate!
-John Giraldes

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"The day you open the bottle, that's the special occasion"

#5 jwalker1140

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Posted 30 June 2012 - 08:56 AM

John, thanks for the references. Very helpful and I've bookmarked them for future reference. I conditioned my meter when it was new according to the instructions but I think my first step will be to condition it for a longer amount of time to see if that makes a difference.

I got the idea for the computer fan stir plate on a brewing forum. It was super cheap and it works great (and it was a fun build). I'll post photos and instructions later in the testing section.

Thanks again,
Jason

#6 jwalker1140

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Posted 13 July 2012 - 03:13 PM

Just to follow up, the first time I used my pH meter was in January after MLF finished when I needed to calculate my sulfite addition. Serious drifting. I just used it again earlier this week and it zeroed in on a reading almost instantly and didn't drift at all. Based on what I read from the links that John posted, I assume it was disolved CO2 that was causing the pH of my sample to change, making it appear as if my meter was drifting. Anyway, all seems to be well. smileytoast.gif

#7 giraldestateswines

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Posted 14 July 2012 - 12:52 PM

I'm glad to hear all is working well for you! Cheers friend!
-John Giraldes

www.giraldestateswines.com

"The day you open the bottle, that's the special occasion"




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