Monday, July 22, 2013

Measuring pH

Anybunny who has been following Judith Curry's display of purposeful ignorance, the Weasel's rather stoat like contempt for her and her fans Sargasso Sea like level of stupidity and Willard Tony's braying knows that just about no one offering their opinion has a clue about how pH is measured in the oceans and the precision and accuracy thereof.  Most of the bunnies, of course, at least those who took general chemistry, have some ideas about pH meters, indeed, in one of the later comments at WUWT, Joe, plain old Joe, puts it quite well

pH Meters are often unstable and not always reliable. They are the fussbudgety spoiled brats of scientific instrumentation. As a chemist, it was depressing how difficult it was to get reliable data, for those of us in OSHA who monitored factory operations. The first point is that, the makeup of the solution can radically affect the readings. as there can be surfactants, a large number of ionic species, and organic molecules. So, one would expect that since the ionic and organic constituents of local oceanic waters vary widely from location to location, it will similarly produce variability in pH readings. The second problem is the inherent instability of the pH probes. They can work reliably (perhaps), but then after sitting for any time, not work. It’s in their nature….
Which, to the cautious, might have been a strong hint that maybe pH meters are not used.  So Eli, being Eli, asked over at Curry's and Willard T's what the Dumb Bunnies though was the precision and accuracy of measuring pH in the oceans.  Chirping.  Something that the Some Bunny Eli wrote about earlier might have also  considered.  And indeed this is not the case.

There are two methods, one pretty old, the other newish.  First the newish one, an ion selective field SeaFET, accurate uncalibrated to 0.01 pH units, with a precision of 0.005.  The SeaFET can be left alone to operate and read out later and here is something about a system under use on the California coast.  Basically they are comprised of an ion selective membrane (only lets certain ions pass) on top of an FET. 

Second, the old one.  Like Joe, everybunny knows that pH meters suck.  If nothing else, the damn electrodes always dry out because people don't keep them wet and they cost a fortune.  However there is a better way and quite an old one.  Many chemicals change color when they ionize in solution.  In particular weak acids are happy to do this, and there are shelves in chemistry stockrooms full of different indicators.  The basic reaction is
HIn = H+ + In-
where HIn is a weak acid and In-  is the indicator anion.  The concention, [H+] is simply given by
Ka = [H+][In-]
where Ka is the acid dissociation constant, a weak function of temperature, or, if the bunnies prefer
pH = -log [H+] = log ([In-]/Ka)
determining pH then becomes an exercise in determining the color change as a concentration of  [In-].   In one deployed version, the precision is + 0.0007 pH unit sand an accuracy of  0.0005 relative to a reference system the principal requirement being holding the temperature in the spectrophotometer cell constant.   Indeed, part of the issue with accuracy is that the method really pushes the ability to mix buffers for calibration.  This is OLD technology, although there may have been modern refinements, so Eli would not be surprised to see measurements going back to 1950, if not earlier (stay tuned).

8 comments:

Anonymous said...

The ocean is the ultimate solution.

Anonymous said...

Is this a roundabout way of saying "titration"?

Danger Mouse

Aaron said...

With most any probe, SeaFET, or cuvette in continuous contact with sea water, at some point you end up measuring the pH of the biofilm and resulting metabolic byproducts.

Local environmental effects (e.g., level of light) can rapidly affect the level of CO2 in the biofilm, and hence the measured pH.

Anonymous said...

Of course the pH can be an order of magnitude different away from the sensor, and that's really the issue when trying to measure the pH of the ocean. The ocean is not a beaker.

Hardy Cross

EliRabett said...

Alan, sorry but you are making a Willard Tony Curry mistake, assuming that people who measure the ocean pH as part of their science need to be taught how to piss.

Byrne's student said...

Dr. Robert Byrne of the University of South Florida College of Marine Science pioneered spectrophotometric measurements of seawater pH using indicators. (I utilized this technique way back when.) He made it more accurate by markedly increasing the pathlength of the spectrophotometer cell with fiber optics and by purifying the indicator dyes.

http://www.marine.usf.edu/faculty/robert-byrne.shtml

http://insciences.org/article.php?article_id=8163

Susan Anderson said...

My goodness, what a lovely piece of compound vituperation. Next time I need to escape the nightmares of our disastrous real world polity I'll come for a reality check with your and Stoat's lovely language!

a tonic for tergiversation (with apologies to Horatio A., such a dandy word) ...

Barry said...

EliRabett said...

" Alan, sorry but you are making a Willard Tony Curry mistake, assuming that people who measure the ocean pH as part of their science need to be taught how to piss."

A beautiful comment.

I'd add that if the measurements were that bad, an analysis of the data could find such things.