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best pH probe for low alkalinity
Jul 15^th, 2012, 1:24pm

Hello all:

My dilemma involves our pH probe we use in the lab. I have been told that low alkalinity waters can give very unstable pH readings. Our effluent alkalinity is between 10 and 15. The water in the local streams has a very low alkalinity too.

We are using a Thermo Orion 2 Star benchtop meter and a Thermo probe, model 9107BNMD (low maintenance lab model). Our pH readings are always on the low side, and we have new and regularly calibrated meters. When I cross reference these readings with pH strips, the strip readings are always more in the range we expect the samples to be in. Can anyone explain to me more about this problem, and what kind of meter options do you know of that can resolve this without killing our budget?

Thanks -- Phil

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David S.

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Re: best pH probe for low alkalinity
Reply #1 - Jul 16^th, 2012, 9:15am

It isn't the meter that is performing poorly, but the probe. I'm guessing that since this is the effluent, in addition to low alkalinity you have low everything else in solution as well. For weakly buffered dilute solutions you will have problems equilibrating the probe response. Make sure you are calibrated in the proper range for reading your samples. Try pouring up a total of 4 portions of the sample to be tested. Pour up just 3 of them at first and allow the probe to sit for a couple of minutes in each. This will help to condition the probe to the sample ionic strength, or lack thereof. Pour up the fourth portion and use it to record the pH. If that doesn't give you better readings try going up one grade in the pH probe department. The 9107APMD model is only a little more price wise but does have slightly better response systems. The only problem is that it doesn't have the guards around the bulb to prevent breakage. We carry some of the Thermo pH probes including the model you currently use and the other one I mentioned. If you are paying Thermo's list price we do have them for less. Message me if you decide to go that route, but I would definitely try the sample conditioning route first.

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Dedalus Full Member Offline Science is a business of empiricism. Posts: 238 NY Gender: Company or Organization: Dedalus Environmental	Re: best pH probe for low alkalinity Reply #2 - Jul 16^th, 2012, 10:21am They sell kits for the measurement of pH in ultrapure water. I believe that one adds some neutral salt/salts, and that stabilizes the response.
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Victor YaBB Moderator Offline Escherichia coli Happens! Posts: 1269 Company or Organization: www.santacruzproductions.com	Re: best pH probe for low alkalinity Reply #3 - Jul 16^th, 2012, 10:24am Try a pinch of KCl. This is what is present in the probe's internal solution.
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Terry Powers

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Re: best pH probe for low alkalinity
Reply #4 - Jul 16^th, 2012, 12:02pm

The pH electrode you are using is a gel type. It has a very, very, very slow response in cold temperatures and low ionic strength waters. Thermo usually includes them with pH meter kits. We have abandoned them and use only electrodes with a 4M KCl w AG/AgCl electrolyte , usually an Orion 9156-BNWP. We use this electrode in both the field and lab. Other manufactures make something similar. We have to use a separate temperature electrode, but its worth it. Call Thermo-Orion tech support at 800-225-1480 for great help.

Terry Powers
South Tahoe PUD

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