Friday, October 28, 2016

Whither pH?

A bit of a while ago, friend of the blog, Simon Donner who studies Pacific Island corals posted a tweet

  to which Eli responded
 and indeed, on the next day when talking with an organiker colleague, Eli mentioned this and in response heard  some, not all in his class thought that the difference in acidity which is the [H+] ion concentration between two acid solutions was the difference between their pHs.

Since pH = -log [H+], this common mistake can have consequences whether one is thinking about organic reactions or ocean biochemistry.

Looking at the chart above, the difference in [H+] ion concentration between pH = 8.2 and pH =  7.7 is

pH= 8.2      [H+] = 10-8.2 = 6.3 x 10-9
pH= 7.7      [H+] = 10-7.7 = 2.0 x 10-8

That's a factor of more than 3 greater

Anyhoo, this set off a train of thought bringing Eli back to yesteryear, actually bask to before his yesteryear and even Mom Rabett's yesteryear.  Why do we use pH rather than just the [H+] concentration.  The Rabett expects that Russell will chime in on this, but what the heck

It goes back to how acidity is measured, specifically the insturment(s) used to measure pH and to more than a little extent to the log tables of yesterday.  Allow Eli to explain

pH is mostly measured through the electrical potential developed across a thin glass window at the bottom of an electrode.  A potential difference is developed between the H+ ions in the solution and the ions on the inside.  The voltage difference between this electrode and a standard electrode is in the mV range over pH scale and is proportional to the pH, that is the logarithm of the H+ ion concentration. 

Fritz Haber and Zygmunt Klemensiewicz were the first to develop a usable pH electrode in 1909, but usable was a stretch because they depended on a a galvanoscope to read out the voltage difference and required patience, luck and skill.  The big step forward was made by Arnold Beckman who hooked up a vacuum tube amplifier to read out the voltage difference and got rich.  He also gave it away to good causes, something that must be mentioned

While vacuum tube amplifiers were pretty good at amplifying, and the scale of the meter could read out the now larger voltage difference, they really could not take antilogs to turn the pH into an [H+] ion concentration.  Besides which pH meters are not very precise, even with care, luck and skill, so there really was no advantage to abandoning pH.

On the other side of this, calculators were at best adding machines, and while there were beasts that could multiply (on the old IBM comptometers you set up the multiplication, walked away and had a cup of coffee before coming back for your result), most did multiplications by hand or converted numbers into logs using log tables, at which point multiplication becomes addition of logs and division subtraction of logs much easier things to do.  Some had nice K&E slide rules, slide rules are essentially log tables on a stick.  A nice stick sometimes, but a stick none the less and if you were lucky two significant figures with a third sometimes possible by squinting.

So there really was no reason to abandon pH for numerical [H+] ion concentrations.

Till today, driven by two things.  First, even refrigerators have implanted digital computers and lcd screens so converting pH to concentrations and displaying them is no longer an issue.  Second, new kinds of [H+] probes are hitting the market which are both precise and accurate

20 comments:

Nick Stokes said...

"Why do we use pH rather than just the [H+] concentration."
I guess, as suggested, because a hydrogen electrode measures chemical potential which is effectively pH. Another version of that is that taking logs converts mass action equilibrium expressions to linear. So in water, pH + pOH=14, which could be handy.

But Moyhu's favorite beef is, why use H⁺ at all? It's just an intermediate which is present in small and hard-to-measure concentration, which is responding to the concentration of some more abundant Lewis acid/base. In this case, it is CO₂ vs CO₃⁻⁻.

Kenneth Johnson said...

Dear Eli

I appreciate your concern about the pH scale (-log(proton concentration) versus plain old proton concentration. Proton concentration in seawater is conveniently about 10 nanomole/kg. On the other hand, pH (proton concentration) depends quite a bit on the temperature and the log scale nicely compresses that temperature dependence. We debate the choice often. No matter what, its easy to convert the concentration now that we don't use slide rules so much anymore.

We're now measuring the vertical profiles of pH (or proton concentration if you wish) throughout the Southern Ocean using your favorite ISFET pH sensors deployed on profiling floats (Deep-Sea DuraFETs - a high pressure flavor of the SeaFET). This is being done through the SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling program - soccom.princeton.edu). You can find the data (in real time) by Googling the word SOCCOMViz - that will get you to the data distribution site for this program. SOCCOM is in the process of deploying 200 profiling floats of the flavor used in the Argo program (0 to 2000 m depth every 10 days), complete with pH, oxygen, nitrate, and biooptical sensors. 58 floats (not all with pH) are now in the water, 44 going in this next winter. For example, at the SOCCOMViz site pick the float 9092, pick Date for X Variable, pick pH_in_situ for y variable and type 30 in the Max Depth box. Click Send and you'll get the pH record near the ice edge for more than 2 years in the Southern Ocean. Want the data? Click the Text box and hit Send again. Want the vertical profiles? Select pH_in_situ for the X variable and Depth for the Y variable and hit Send.

Anyway, we're in the process of accumulating a long-term pH record for the Southern Ocean, which is predicted to have an ecological tipping point in a few decades when the whole water column becomes undersaturated with CaCO3. Not to mention possible changes in CO2 uptake by the Southern Ocean. There are lots of challenges, related to correcting for sensor drifts, etc., but we're meeting the challenges and moving forward. That's what science does.

Thanks for your interest in the esoteric variable of pH.

Ken Johnson

Bernard J. said...


"...or proton concentration if you wish."

The pendants would probably prefer 'hydronium concentration'!

"Anyway, we're in the process of accumulating a long-term pH record for the Southern Ocean, which is predicted to have an ecological tipping point in a few decades when the whole water column becomes undersaturated with CaCO3."

There's a staggering ecosystem implication this, which still seems to be completely missed by most of the governments and businesses of the world. The ecological implications of global warming aside (for just a moment...), it's extraordinary that humanity is choosing to play chicken with its carbon dioxide emissions in the context of the ecology of 70% of the planet's surface and the habitat of 2 million of the planet's species.

There's no point leaving our mitigation action for another few decades, because even getting close to a tipping point is going to be far too late for effective management. And an acidified marine atmosphere combined with a stratified, anoxic oceanic column that arises from the imterplay with temperature, is going to affect the whole planet and not just the coastal communities.

It seems that the only boot up the arse that humanity will notice is the one with "Extinction Co." embossed on the sole.

Fernando Leanme said...

There's too much work being done with that bogus RCP8.5. A more reasonable upper boundary is RCP6.

BBD said...

Fernando

First off, RCP 6 would be horrible in its own right, so quite what you imply might be gained here is a mystery.

Second, we (this means you as well) do not know nearly enough about carbon cycle feedbacks to assert that something like RCP8.5 will certainly be avoided. For example, if the land surface switches from sink to source towards the end of this century, all bets are off.

EliRabett said...

RCP 6, still fantasyland, but at least you are not screwed only your grandkids.

Tom said...

I just love to watch these discussions based on RCPs. They are so fact-free. How would climate change play out on Oz given an estimated frequency of Kansan tornadoes of say, 8.5 per year?

They started with the final stage watts per square meter given to them. The purpose of the exercise was to provide intermediate inputs for computer models. Everyone who has worked on the Representative Concentration Pathway has said repeatedly that they are not predictions, not projections and that scenarios developed to explain a possible method of achieving those goals were done off the cuff and on the fly. Witness the confused mishmash of how the planet could get to 8.5 watts/sq. meter.

Fools fiddling while Rome burns. Or playing the lyre... or liar.

BBD said...

Only contrarians think there is some sort of problem here. Normal, sane people get that it's simple: more CO2 = more warming = bad thing.

And Tom's still accusing other people of being liars.

It'd be funny if it weren't so grotesque.

Tom said...

Grotesque it most certainly is. The reasons why constitute an exercise left to the reader. Or the illiterati, in BBD&O's case.

BBD said...

First you claim that I'm a misinformer but cannot back it up.

Now you claim that I'm illiterate. Can you back that up?

Thought not.

What does that make you, Tom?

Tom said...

A former blogger and commenter who remembers you well, spewing DENIER!!! in all caps and multiple exclamation points in thread after thread with anyone who disagreed with you. Sometimes hundreds of times in the same thread.

Donald Trump, Nigel Farage, Marine LePen and BBD. Too bad I forget the name of the idiot in Hungary.

neverendingaudit said...

Yet another thread where Groundskeeper starts to rip his shirt.

Tom said...

Yet another thread where weewilliewillard whines about irrelevancies. You've trademarked more terms than Trump has. You haven't contributed one word of substance to any conversation about climate change, evah, but you're the champion at characterizing anyone who comes within spitting distance.

Belittle: Objectify and dehumanize
Pigeonhole: Invent a strategy that can sanitize any conversation of content
Dissociate: Decouple comments from the subject of the thread and tie them to an overarching theme of your own invention.
Trademark, rinse and repeat

Cute, weewilliewillard. Not constructive, but cute.

Tom said...

It is said of Ogden Nash that he both created and destroyed American light verse. Weewilliewillard has done the same for ClimateBall.

BBD said...

Tom

A former blogger and commenter who remembers you well, spewing DENIER!!! in all caps and multiple exclamation points in thread after thread with anyone who disagreed with you. Sometimes hundreds of times in the same thread.

This is a lie, Tom.

neverendingaudit said...

> Decouple comments from the subject of the thread [...]

The subject being pH, not Groundskeeper's fantasies about RCPs, otters, or the poor shirt he's trying to rip off again.

8c7793aa-15b2-11e5-898a-67ca934bd1df said...

Yes, but I've been going to night school to learn bug-eye and illucid, and practicing the grammar of the insane is helpful here. That being said, I could use the help of someone fluent in nuts to help translate.

EliRabett said...

It's Halloween

Bernard J. said...

What Eli said.

And TLE, your last was gold!

Russell Seitz said...

Halloween is the season to exhort all receiving CRISPR kits from Santa to try their hand at inserting sequences that express pH paper dye molecules into their favorite mollusc genomes.

Then we can keep an eye on things when we dive, clam, or go scalloping.

Since there's already plenty of bromindigo in wampum, Craig Venter should be able to make money if it works