The Bucket Cell - pH Meters and Probes
Entry posted by Swede
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Very cool - a "Post to Blog" function. I thought I'd give it a try, since this post has some hopefully good information in it.
pH Probes and Meters for the (per)chlorate process
I spent far too much time on pH probes today. Again, the idea behind the simple bucket cell is so one doesn't have to mess with pH probes and meters, but I still need to use them to verify it works without them. That makes no sense at all, does it? 
I've always been a fan of Milwaukee pH stuff. They occupy a niche between school and aquarium junk, and expensive lab stuff. I have a MW-101 equivalent. Look for a meter with a manual temperature adjustment on front. Those with automatic temp compensation inside either require a special probe, or a separate temperature probe, and all of that is a pain. It's easier just to set it manually, because we generally know the temperature of the sample. Look for one with simple calibration procedures, because you'll have to recalibrate often, or it's all junk, because even the best probes drift.
For the (per)chlorate process, don't even bother with a pH pen or some junk from eBay. You've got to pay to play, and a decent setup will run at least $200 for meter and probe. You'll need two buffers for calibration - get the 7.0 and the 10.0, and get the big bottles. They are cheap, and you'll go through it fairly rapidly. The good news is, a nice pH meter can be used for a number of other lab processes. And if a pH meter is out of your price range, pH papers can be used with a bit of care and sneaky manipulation. But the best you'll probably be able to do is pH +/- 1.0 or so. And one last warning, NEVER buy a pH probe from eBay, even one new in box, because they have a shelf life. It might be like buying an alkaline battery with a use-by date of 2002. These things get thrown out all the time from labs, and guys going dumpster-diving get them free and then re-sell them.
A decent meter lasts forever, but probes never do. I have four probes.
http://www.5bears.com/perc/probe.jpg
Two MA911 epoxy gel-filled probes, a higher end MA917, and a cheap aquarium probe off eBay. All had been stored correctly, with the tips immersed in 3M KCl. The two MA911 probes had served me very well, but they are at the end of their lives. You can tell when a probe is about dead when two things happen... the response is really slow, like 30 to 60 seconds, or it cannot be calibrated. My MA911 probes can still be calibrated, but they are a bit slow, so I opened up the NIB MA917.
This probe has a glass body, and is refillable with electrolyte! This makes me happy, because when it begins to sicken, I can replace the internal electrolyte, which is nothing more than 3.5M KCl. A good read on pH probes in general can be found here. I recommend a glass probe because I am hopeful that the toxic and harsh environment it will see, can be repaired with fresh electrolyte. We'll see.
Anyway, the $20 aquarium probe fell apart in my hands. Literally. It was a total waste of money, and even when new, it managed only a dozen pH readings. The gel-filled yellow MA911 probes have hung in there. Both of them calibrated OK, but were just very slow. The glass probe was like a Ferrari... very fast readings. But again, I haven't used it yet with chlorate.
To check cell sample pH readings, get a stack of plastic bathroom cups. Set up 4 in a row, Fill one with your sample, and three with distilled water. Remove your probe from its container, rinse in distilled cup one, then into the liquor. Count to 5, and if it's not settled yet, make your best guess, because if you leave it in there longer, it'll die sooner. Remember, at best, all we need is pH within 0.2 or so, so 6.6 or 6.8, they are all the same. Immediately rinse the probe in water cup one, then two, then three, then back into it's special storage container.
That was my exciting afternoon! I was glad to see the glass probe perform so well, because it had been sitting on a shelf since 2009 or so. I had prepared some ultra-pure 3.5M KCl just in case to refill it, but it turned out to not be necessary.
Tomorrow, I'll do a final set up and hopefully power-on.
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