ajdelange

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Afraid I don't understand what you mean here.

AZ_IPA

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I think he means this stuff?

ajdelange

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I understand that he is buying buffer solutions but he says he is buying them instead of doing something else or instead of something else and that this is a 'cheat'. My question is "Instead of what?"

DeafSmith

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Thanks for clearing that up. Now I can stop worrying that the two or three times I "slipped up" and turned on power without the electrode submerged might have damaged it. I wondered why the instructions that came with my meter didn't have any cautions about applying power to the electrode without having it submerged.

Biobrewer

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We always calibrate and make sure our percent slope is within the acceptable range.

It is ideal to have the calibrators and the samples at the same temperature, but we have pH meters that correct for variation in temperature. Basically, it takes the temperature the calibrators we run at, and corrects accordingly based upon the temperature of the sample.

ajdelange

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What I am getting at here is that there are really three calibration parameters slope, offset and isoelectric pH (pHi). Modern manufacturers assume pHi = 7.00 and modern electrode manufacturers strive to produce electrodes for which 6.5 < pHi < 7.5. I have checked the 2 electrodes I use most. One has pHi = 6.81 but for the other pHi = 8.38!.

The uncertainty in pHi causes uncertainty in any measurement made with a meter unless the buffer and sample temperatures are all the same. If the buffer temperature is Tb and the sample temperature Ts this uncertainty is sigma_pHi*abs(Tb-Ts)/Tb where sigma_phi is the uncertainty in the pHi of the electrode. This amounts to 0.034*sigma_pHi for each 10 °C in difference between buffer and sample. Assuming that pHi uncertainty is about .3 (for easy math and not terribly inconsistent with 6.5 < pHi < 7.5 or the observed pHi for my 'good' electrode that's 0.01 uncertainty for each 10 °C. If the buffers were at different temperatures when you did the cal there is additional uncertainty associated with this effect from that. If you are taking measurements 20 or 30° away from the calibrating temperature you are kidding yourself if you think frequent calibration is a benefit as the uncertainty in the readings caused by pHi uncertainty is greater than the stability related uncertainty. This is why the calibration instructions contain step 11b in which you check for pHi related ATC problems. This is also why some authors insist that that buffers and sample all be at the same temperature. If Ts = Tb the uncertainty from pHi goes away no matter how large it is.

While I am certainly not familiar with your labs procedures this strikes me as possibly another example of regulations that attempt to suppress one problem without regard to another potentially greater one. I worked in the DC area for years and I've seen plenty of this. Having vented I will say that it is my opinion that where drugs, airplanes and similar things are involved overdoing it a bit on the side of safety is justifiable.

Note that if you calibrate your meter for pHi and insert the proper pHi into your ATC algorithm you can use ATC with confidence. I use the electrode with pHi = 8.38 all the time at varying temperature but ATC is done in the computer - not the meter.

Biobrewer

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Offset error cannot be completely compensated by ATC on a meter. However, the error will be around 1/10 of a pH unit at it's worst for a good pH meter. For a home brewer, that is not a big deal. However, that is with a good pH meter. I happen to have a laboratory-grade pH meter from an old job that is highly accurate over a fairly wide temperature range, though I know that cheaper pH devices, like the ones readily available to home brewers for <$50-100, will not meet that same spec, and will likely vary more with temperature and have a larger error associated with temperature drift.

In industry (and certainly where I work), it is considered good lab practice (GLP) to read a sample within 5 C of your calibrators. In fact, it is mandatory, and any greater temperature disparity will not be accepted and will invalidate any future work completed using that material. These temperatures must be recorded and witnessed by another person, because the tests we make need to be painfully accurate, as people's lives depend on their accuracy.

However, when I am not making clinical diagnostics in a highly regulated environment and am instead brewing beer in my kitchen, I usually won't sweat over 0.1 pH units, though my sample I read is so small it is probably darn near room temp anyway by the time I get it out of the kettle and on the meter.

Either way, cheers! You started a good thread with some sound advice.

ajdelange

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I don't think this is worth belabouring too much more but a couple of things caught my eye. They may be more a matter if terminology that anything else.

A pH electrode produces, when exposed to a solution at pH, a voltage

E = Ei - 58.167*slope*((T + 273.15)/293.15)*(pH - pHi)

'slope' is a number close to 1 and Ei, technically the isolelectric voltage, is a few millivolts at most and is the voltage the meter would produce if pH - pHi. Given that the meter assumes that pHi - 7, it thinks Ei is the offset.

IOW, the meter thinks

E = Eo - 58.167*slope*((T + 273.15)/293.15)*(pH - 7)

and calculates Eo (the voltage at pH 7) and slope based on that assumptions from buffer readings. Presented a voltage E, after calibration, the meter calculates and displays

pH = pHi + (E - Eo)/( 58.167*slope*((T + 273.15)/293.15) )

If pHi is indeed 7 then Ei = Eo is not a function of temperature and ATC has no problem dealing with sample temperatures different from buffer temperatures. But if pHi != 7 then Eo is a function of temperature and the meter has no way of dealing with that because it assumes it is not a function of temperature. OTOH if you know the true value of pHi you can determine, from the calibration, what the actual value of Ei is and from that point on you are fine.

That's a lot of uncertainty. A whole pH unit of uncertainty in pHi and buffer temperatures separated from sample temperature by 32 °C would be required to induce that much pH reading uncertainty from ATC error. A more reasonable level of pHi uncertainty is probably 0.3 as I noted in a previous post (but I do own that rogue electrode) for which the ATC unceratainty is more like 0.03 with the 32 °C temperature spread. That's still enough to dominate the uncertainties associated with temperature and voltage readings and buffer pH uncertainty (± 0.02 in the technical buffers used by most home brewers).

I guess I agree for the average home brewer. But when trying to do things like figure out the titratable acidity of a malt, for example, that's much more error than I am willing to live with.

I'm sure this one is semantics but I'll point out any way that it is not the meter that is in question here but the electrode. The meter simply dumbly plugs readings into the second equation above blissfully assuming that the offset is the isoelectric voltage and pHi = 7. It is the fact that the electrode's pHi != 7 that induces most of the error when ATC is being used. All the meter itself needs to be able to do is measure temperature with rms error less than 0.5 °C and voltage with rms error less than 0.5 mV neither of which are terribly demanding requirements. It is, of course, just good systems engineering that the meter itself produce uncertainties low enough that when rss'd with the uncertainties from buffers and electrode the latter are dominant.

I wonder about that (and I don't mean I'm challenging that statement but literally that I keep thinking about it). From what I have been able to deduce with a small sample size it is often not that cheap meters drift so much as it is that the calibration routines take readings before the results are stable enough. For a calibration I find 10 minutes is a better criterion that just waiting until the change is less than 3 mV/min or whatever the auto cal routines use. Put another way the cheap meters jump too soon and get bal cal parameter estimates so that in reading samples they display stable, but wrong, pH values. This is easily fixed. OTOH some cheap meters are just unstable. That's not fixable.

I will also assert that $ is not a defense against an unusually large pHi deviation. The electrode I have that is way off cost me over $250.

I'll close by noting again that if you know pHi you can (and I do) use ATC confidently at temperatures quite removed from the calibration buffer temperature. The problems with this are 2
1. Meter's won't let you set in a pHi value
2. Since it takes a rather large pHi error (0.3) to induce a fairly small error in pH even at a fairly high temperature excursion (0.03 pH at 32 °C differential) pHi isn't very 'observable'. It thus takes hundreds of buffer readings at various temperatures and some fairly advanced math to estimate pHi from those readings.

Woodbury419

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Mixing my own cal juice solutions is what I meant. I cheat and purchase Hach juice instead.

ajdelange

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Roger that. I do the same. I want my buffers made in a facility that does all that ISO stuff I've been poking fun at.