HOMEBREW Digest #6060 Sat 16 November 2013

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  pH Meters ("A. J. deLange")
  Re: Questions About pH Meters (mossview5)

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---------------------------------------------------------------------- Date: Sat, 16 Nov 2013 09:17:43 -0500 From: "A. J. deLange" <ajdel at cox.net> Subject: pH Meters I would regard a inexpensive pH meter that has never been used and is 2 years old with some suspicion. For general tips on the use of pH meters in brewing see http://www.homebrewtalk.com/f128/ph-meter-calibration-302256/. (1). Inexpensive meters are infamous for lack of stability. The reference above give instructions for checking the stability of your meter (basically measuring the buffers used for calibration over time). A pH meters relies on the flow of electrical current - a very feeble flow perhaps but it is necessary that current flow. Distilled water has a very high resistivity which impedes that current flow, allows local accumulation of charge... For this reason DI water is very hard to measure as is any solution of low ionic strength which may include your tap water. In such cases one uses special buffers and adds ion strength adjusters to the sample which is also protected from atmospheric CO2 (which would cause pH to fall over time). One or more of these effects may be responsible for what you are seeing. My money is on electrode instability. Yes, unfortunately, it is all too common in inexpensive meters. (2) Yes, as noted above, such drift is all too common. What is really telling is to do this with one of the calibration buffers. Cover with Parafilm or aluminum foil to prevent evaporation. (3) It is possible that lactobacillus or yeast started to grow in that time period (this lowering pH) but more probable that your electrode drifted. (4) Yes, but why not just do what the manufacturer recommends? (5) Buffers have, as do most chemicals, finite shelf lives but the main concerns with reuse of buffers is contamination with sample, the other buffer, rinse water (water is a contaminant because it dilutes the buffer). Other advice: be sure to read at the link above. Cheers, A.J. Return to table of contents
Date: Sat, 16 Nov 2013 17:19:53 -0500 From: mossview5 <mossview5 at gmail.com> Subject: Re: Questions About pH Meters I am surprised that they recommend storing the probe in 7 buffer solution. It probably doesn't have a high enough ionic content to keep the bulb saturated. The typical probe storage solution employs something on the order of 1N potassium chloride in the solution. That isn't that saturated a solution, but its got to be better than pH 7 buffer. The climb in pH could be a function of several phenomena. Dissolution of CO2 from the tap water is a possibility, but the fact that the distilled water displays similar response suggests that this is not the mechanism. Since you are essentially soaking the probe in a non-saturated solution, it could be depleting the probe of its ionic balance. I'd say not to soak it like that. A pH reading is obtained in a few minutes, not hours. I literally just wrote about a similar subject on the Bru'n Water Facebook page. You may want to take a peek at that. https://www.facebook.com/pages/Brun-Water/464551136933908?ref=hl Martin Return to table of contents
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