Author Topic: How to use a pH meter  (Read 8301 times)

Offline madscientist

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Re: How to use a pH meter
« Reply #30 on: March 30, 2012, 07:59:45 AM »
I'd imagine you could make a storage solution from NuSalt (potassium Chloride instead of sodium chloride) for a storage solution.
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Offline punatic

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Re: How to use a pH meter
« Reply #31 on: March 30, 2012, 10:31:17 AM »
Unless you contaminated the portion that you removed and returned, e.g. with rinse water from rinsing the pH probe off.

That's what I'm afraid happened. My tap water is really alkaline, so maybe if half a ml of rinse water contaminated it each time I calibrated it, after a long time that could skew the samples. When my new buffers show up I'll compare the results.

Write back with what you find.  It will be interesting to see.  I'm guessing the contaminated buffers will be unchanged.  Good idea to buy new buffers though.
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Offline nateo

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Re: How to use a pH meter
« Reply #32 on: April 03, 2012, 04:52:38 PM »
My new calibration solutions came today. My old calibration solutions were definitely contaminated, I assume from rinse water.
New solution > Old solution
7 > 7.2
4 > 4.4
10 > 10

I didn't use the 10pH solution as often as the other two. So, word of warning. I thought my funky readings meant my meter was busted, but it was just that the solutions were off.
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Offline oceanselv

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Re: How to use a pH meter
« Reply #33 on: April 03, 2012, 05:18:07 PM »
After reading the thread I noticed one piece of advice that has not been given.  The probe should never be wiped with a lab wipe or any thing similar, IE a kimwipe.  Instead gently pat the probe with a damp lab wipe.   Wiping the probe down creates a static charge which can affect the pH of the solution being measured.
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Offline mabrungard

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Re: How to use a pH meter
« Reply #34 on: April 04, 2012, 04:57:46 AM »
I don't ever wipe the bulb on my pH probe.  The whole probe gets a distilled or RO rinse and then I blow out the droplets by mouth.  I suppose it would be even better to blow it off with compressed air, but its not like I'm spitting on it.  The blowing gets the droplets out from around the protected bulb area.  Then I can use a regular paper towel on the exterior of the probe housing to absorb those droplets.  It usually takes a couple of blows and wipes to get the bulb and probe dry.
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Offline bo

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Re: How to use a pH meter
« Reply #35 on: April 04, 2012, 05:21:01 AM »
Can you store them with the probe immersed in distilled water?

Offline jmcamerlengo

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Re: How to use a pH meter
« Reply #36 on: April 04, 2012, 08:57:29 AM »
Can you store them with the probe immersed in distilled water?

I've heard that this is bad practice.
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Offline nateo

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Re: How to use a pH meter
« Reply #37 on: April 04, 2012, 09:54:36 AM »
I've heard that this is bad practice.

Yeah distilled water is corrosive. Storage solutions contain salts that will extend probe life.
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Offline oceanselv

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Re: How to use a pH meter
« Reply #38 on: April 04, 2012, 10:55:01 AM »
Storage solutions are indeed best to store the probes in.  As for drying the probe, there is no need to completely dry the probe. As a bench chemist for 20 years, I have found a good rinse with distilled water and then a very gentle pat down with a paper towel will work fine.  This will not induce a much of a static charge.  I wouldn't use compressed air from a compressor to dry the probe as the compressed air typically has a small amount of compressor oil trapped in it.   Also be careful of blowing on the probe to dry it as our breath also has saliva in it.
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Offline mabrungard

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Re: How to use a pH meter
« Reply #39 on: April 04, 2012, 12:03:31 PM »
Can you store them with the probe immersed in distilled water?

No.  The probe is filled with a ion-saturated (typically potassium) solution or gel.  Its important to keep that solution saturated with that ion.  The glass probe is actually permeable.  Immersing the probe in a solution with very low ionic strength creates an osmotic pressure that draws those ions out of the probe and into the exterior solution.  You end up depleting the ion solution inside the probe.  pH probe storage solutions are typically high in the particular ion and when the probe is immersed in that solution, then there is not the osmotic stress drawing ions out of the probe.  The probe solution stays saturated.
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Offline punatic

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Re: How to use a pH meter
« Reply #40 on: April 04, 2012, 03:20:41 PM »
Yes, probe storage solution is the best way to store your pH probe.

However, if you only have a choice of distilled (deionized) water or leaving the probe in the air to dry out, choose keeping the probe wet.  Allowing it to dry out will ruin the probe quickly.  Parking the probe in deionized water for a few hours to keep it from drying out will not cause problems.  Parking it in a pH buffer temporaily is a better option still.
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Offline redzim

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Re: How to use a pH meter
« Reply #41 on: May 10, 2012, 06:33:00 AM »
Did they try replacing the probe?  I have the MW 102 and haven't really had any problems with it.  When you see the hour glass go away, you should be able to take a reading.  It may drift a little after that, but not by as much as you're seeing.  You should be moving the probe around slowly as you take the reading;  otherwise, diffusion from the probe can affect the sample directly in contact with it.

I pulled mine out of the storage solution after a month and the 4.01 solution reading was dead on, and the 7.01 read around 6.98.

I got to Milwaukee tech support again today and they agree that my meter seems flaky. They are going to replace it free of charge so we'll see if that helps.

I have to give a big +1 to Milwaukee on this one; after a lot of tech calls they agreed to replace the entire meter and probes for free. I must have had a lemon, because the replacement I got has been much more solid; it calibrates a lot quicker (2-3min vs 8-10min) and hardly drifts at all in samples (maybe 0.1pH over a couple minutes max). Also holds its calibration nicely, when I haven't used it for a few weeks I check it in a 4.01 buffer before recalibrating and it is always between 3.98 and 4.04. Which my old meter never was....  so although Milwaukee's QA on their product line might not be the best, their customer service is nice.
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