Thursday, December 16, 2010


I should start with an apology for light posting lately. We have a family gathering with folks from overseas, but that wasn't the obstacle. We've been hassling to get some building work finished before the occasion (now done).

This post won't say much about the actual issue of ocean acidification. Rather it's a response to something that seems to always happen when the topic arises. Someone inevitably contends that, no, the ocean can't be acidifying because it's alkaline. pH>7 and all that.

There was a long discussion here, for example. What prompted me to post was seeing it happen recently on Judith Curry's blog. It's a sterile topic - a bit like arguing about whether the greenhouse effect is well named. But there's a bit of science in it. The argument can be answered on four levels.

The practical argument

The meaning is well-known. CO2 in solution will tend to dissolve calcium carbonate, thus disrupting life-forms. It may have other biological effects.

As with the greenhouse effect, human discourse doesn't require literal exactness. We can speak of currency inflation without arguing about whether dollar bills are getting bigger. etc.

The language argument

The objection is that you can't acidify something if it doesn't become "acid". But that just isn't normal usage. If you beautify something, it doesn't have to become beautiful. We can be enriched without becoming rich.

The theoretical chemistry argument

OK, getting more substantive. The notion that acidic is identified with pH<7 invokes an old notion of acidity. Since about 1923, the more general process going on in acid-base chemistry has been recognised as sharing an electron pair, rather than anything specific with protons. When sulphur trioxide reacts with calcium oxide to produce calcium sulphate, it is easy to recognise this as an acid-base reaction, with SO3 as the acid. No hydrogen is involved.

This is pretty much the case in the ocean. The overall reaction is something like:

CO2 + CaCO3 + H2O → Ca++ + 2HCO3-

Water is a reagent, and there may be a role for protons. But it isn't clear why pH 7 should matter in any way.

The aqueous chemistry argument - buffering

pH 7 is the neutral point of a particular acid-base equilibrium - in pure water. It also applies when a strong acid neutralises a strong base. But the ocean is not pure water, and does not have strong acids or bases.

A solution with substantial concentration of a weak acid and its corresponding base is described as buffered. That is because the pH is stabilised near the neutral point (pKa) of that equilibrium. If you add a strong acid, the protons will react with the corresponding base to produce more of the weak acid. Similar if you add a strong base. The pH changes little, even though real acid-base reaction occurs.

That's why pH 7 is irrelevant here. The ocean is dominated by a 3-way buffering involving CO2, bicarbonate and carbonate ions. There is a further solubility equilibrium between Ca++, CO3-- and solid calcium carbonate (eg aragonite). Adding CO2 pushes everything in the acid direction, which reduces CO3-- concentration and tends to dissolve CaCO3. It's a bit more complicated because the solubility equilibrium is not exactly attained - it is fairly easy for CaCO3 to be supersaturated in solution. But that is the direction.


  1. Thanks for this interesting post, Nick.

    In my experience, some people really have a strong allergic reaction to the use of terminology that they perceive as carrying emotional content indicative of "alarm". In both of the threads you linked to, people were particularly irate over the idea that "greenies" are choosing the term "acidification" because it sounds more alarming than, say, "neutralization".

    But that argument cuts both ways equally well. If person A claims that person B is just using the term "acidification" to stir up a sense of alarm among the unwary, person B could just as well claim that person A is insisting on "neutralization" to promote complacency and delay action.

    Let's for the moment assume that both "acidification" and "neutralization" are reasonably defensible descriptions. I would, in general, have no problem with either one being used. It would help to have some kind of consistency, though, so that people looking for information about this phenomenon don't miss potential sources because they used the wrong search term.

    The question then becomes (a) how serious are the biogeochemical consequences of a shift towards lower pH; (b) what level of seriousness does each of the two terms suggest to the general public; and (c) which term provides the best match between the actual and perceived consequences?

    This is *not* an argument for using misleading or incorrect terminology to frighten people. It's an argument for using the term that best satisfies the two criteria of being technically defensible and most comprehensible by the general public.


  2. There are two common variations on the linguistic argument:

    > The word “acidification” is received differently by scientists and by the masses.

    This one would be more potent if it were backed up by empirical studies. And even then, theorical concepts are usually not for public consumption.

    More importantly, the expression “ocean acidification” is already in use in the litchurchur. This is an argument from information science: organizing keywords is already a messy business.

    Replacing this with another might prove difficult. For instance, replacing it with expression “seawater neutralisation” does not work well, as it is already taken elsewhere, for something else.

    I really wonder how the public perceive the concept of acid. Even the history of the scientific notion of acid is as complicated as it is interesting. See **Representing and Intervening**, by Ian Hacking.

  3. Nick,

    Does increased Ca++ solution concentration mean easier biological adsorption?

  4. Nick,

    Doesn't this sulfphur trioxode calcium oxide reation take place in water, with the intermediate formation of SO4-- (same as sulfuric acid)?

  5. Anon,
    I don't think increased Ca++ helps absorption. The product [Ca++]{Co3--] remains constant - it is still in equilibrium. At least, that's the simple version - in fact there is considerable supersaturation. But reduced CO3-- promotes solution.

    The reaction of CaO and SO3 doesn't require water, although water wiuld probably catalyse it. You're right, though that with water it becomes just sulfuric acid and CaO (or Ca(OH)2) - a conventional acid-case reaction.

  6. The two comments above by Anonymous and Willard went, for some reason, initially into the spam filter, which Blogger has recently modified. I've no idea why they should have been seen as spam, and I'll investigate further. Maybe "acidification" really is a bad word :(

    My apologies for the delayed response. I've never had anything appear in the spam filter before, so I had got out of the habit of checking. I have the blog set to no moderation, and I've never yet modified a comment.

  7. It looks to me like they borrowed this terminology from soil science, where it has been used for a very long time. Is that possible? If you search for ocean acidification with Google Scholar, the number of hits starts dropping quickly as you go back by decade. I think it shows up long before ALARMISM could have been the goal.

  8. Thanks for the note about the spam filter, Nick (and for rescuing the lost comments). I was kind of wondering what had happened, but just figured that I had probably failed to click "Post comment" or something, and I was too lazy to retype it all.

    Let's see if this one gets intercepted as well...


  9. Hmmm, that one seems to have gone straight through without any problems. How about this?

    CHEAP REPLICA OCEAN ACIDIFICATIONS! Save $$$ off name-brand ocean acidifications, and your friends won't be able to tell the difference!

    DISCOUNT CARBONATE CHEMISTRY! Our list has 1000s of Canadian pharmacies offering mail-order products for global biogeochemical cycles. Get your Revelle Factors, HCO3-, Henry's Laws, CO3--, and mole fractions here!

  10. You want to see things really fall apart? Start talking about pH in a non-aqueous medium. Getting it right is important for things like liquid chromatography. But most people don't.

  11. How do the public preceive 'acid'?


    'Teenager scarred for life after acid attack'

    'John Christe - The Acid Bath Murderer'

    'Solve your Acid Stomach super fast with xxxx'

    'Protect your teeth from acid erosion with yyyy'

    and so on.

    'Acid' is pretty much a universally 'bad' term. Only among wine buffs who occasionally 'enjoy the acidity of the finish' does it ever get a good press.

    If the collective wisdom of academia cannot come up with a better term I can only imagine it s because they don't wish to.

    1. They are pretty negative about chemicals generally, though that's what they are made from.

      How about "gassing the oceans"?