PH Meters

[albada]

After 6 months of moderate use, my Hanna pHep-5 meter developed a problem in its electronics making the display hard to read. Hanna has a couple of aspects of poor design, so I've decided to buy another meter from a different maker. I'll first buy a new meter, and then return the Hanna for warranty-replacement so I won't lack a meter during the return process. I'll end up with two meters.

I'd like a meter that has an accuracy of +/-0.02 pH, and that uses a standard BNC connector. Price around US$200-300. Any recommendations?
There are many meters on the market, and here are a couple that appeal to me:

Cole-Parmer meter: http://www.coleparmer.com/Product/Co...al/WU-05996-60
This meter lacks bells and whistles, but looks solid with good resolution. Probe must be purchased separately.

Sper 850056 kit: http://www.sperdirect.com/cgi-bin/it...d-pH-Meter-Kit
More features than the Cole-Parmer, and includes a probe, for a lower price. But I wonder if the quality of this is good enough. I'm leery of anything that looks like mass-produced junk.

Oakton sells many lower priced meters, but my concern as always is the quality.

Opinions are encouraged!

Mark Overton

[RattyMouse]

Why in the world do you need that extreme level of accuracy?

[albada]

Why in the world do you need that extreme level of accuracy?

Because I'm creating new developers, and I need better accuracy than +/-0.1 pH to insure good consistency and repeatability.
When I mix developers, I am consistently within +/-0.02 pH. When the difference reaches 0.05 pH, I know something went wrong. But that requires a two-digit meter.

Mark Overton

[jochen]

Hello,
if you really want to use the extreme accuracy of +/- 0.01 pH-values, you have to use the best possible electrode for your work. You should contact a laboratory pH-meter manufacturer or a supplier for laboratory equipment for the best recommendation for your work. Only professional laboratory pH-meters (several haundred $ or €) can use different types of dedicated electrodes. Furthermore it is important that your meter is temperature-compensated and can be calibrated by at least two, better three calibration buffer solutions. The shelf-life of these calibration standards is limited. When not in use the electrode has to be stored in a specified storage-solution in order not to get dried.

[albada]

The cheap Hanna meter broke! I wanted to be able to run experiments this weekend, so I built my own based on this web site that describes a simple DIY meter. Its heart is a TL082 op amp. After spending a few USDs at Radio Shack, I built this (inside view):



Here it is taking a measurement of a developer. Connect a voltmeter to it, and then calculate pH based on millivolts.



You calibrate it any way you'd like, which gives you a slope and offset. Having those, the actual pH calculations are quick; much quicker than the time it takes to measure and mix the chemicals and wait for the probe to settle.

QUESTION

I want a better probe than the one that came in the Hanna (pictured above). Do you suggest single- or double-junction? Refillable versus gel? I've heard that refillable probes are faster than gel-electrodes. Mine takes about 5 minutes for the tenths of millivolts to stabilise -- that's slow! jochen pointed out that good probes are important. Any opinions about these two:

http://www.amazon.com/Thermo-Scienti...H+probe+thermo

http://www.amazon.com/dp/B004G8PUGG/ref=biss_dp_t_asn

Mark Overton

[Mr Bill]

I don't have any personal knowledge about the items you linked, but I read quite a few photo chemical samples in my younger days - beyond 20 years ago. We used info from Kodak for our guidelines. Similar to what Kodak's motion picture group has available here (if you haven't already seen it): http://motion.kodak.com/motion/uploa...3_ulm191-2.pdf

I think you'd be fine with a single junction reference. Double junctions are mainly for special cases where something you're doing will cause interference. We typically used separate electrodes - this lets you rotate out the pH electrodes. And if something goes bad, you only replace that electrode.

I never personally used gel electrodes; they were either frit or anular (sleeve) junctions. A frit junction is harder to troubleshoot for clogging; with sleeve junctions, you can just open them up to drain, then refill. So if there are any clogging issues, a sleeve junction is good. Note that we were testing replenished and regenerated solutions; if you only test scratch-mixed B&W developers, I suspect that these are less problematic.

Whatever you use, you'll probably get a little faster response by gently stirring the sample while reading. It should be a constant speed, so a small magnetic stirrer works great. Calibrating (with buffers) should be done the same way.

[Michael R 1974]

Because I'm creating new developers,
Mark Overton

Mark, are you taking requests?

[albada]

Mark, are you taking requests?

At least you didn't ask me to build a pH meter for you.

[kreeger]

Try looking for Beckman Zeromatic model. Again the accuracy as someone mentioned is in the probe itself. Get your buffer solutions to calibrate and you're set.
When I was doing photo chemical analysis in the 80s under military spec requirements, we used Beckman pH meters as the standard. Lots of Beckman stuff on eBay cheap.

http://www.ebay.com/itm/Beckman-Zero...item3cba92e07b

And, in best practice you leave them running all the time with the probe in a proper buffer solution.

[albada]

Try looking for Beckman Zeromatic model. Again the accuracy as someone mentioned is in the probe itself. Get your buffer solutions to calibrate and you're set.
When I was doing photo chemical analysis in the 80s under military spec requirements, we used Beckman pH meters as the standard. Lots of Beckman stuff on eBay cheap.
http://www.ebay.com/itm/Beckman-Zero...item3cba92e07b
And, in best practice you leave them running all the time with the probe in a proper buffer solution.

Thanks for the suggestion. I'm realizing that I can't go cheap on the probe, so I'm looking at a refillable model from Thermo Scientific. Interesting that the Beckmans are cheap on eBay. I'd guess those meters were well north of US$1000 when new.

Mark Overton