Comments on: Scientists brave ‘world’s worst water’ to watch wild bacteria evolving

By: Mike McD

Mike McD — Sat, 28 Apr 2012 11:36:56 +0000

Pity the comments has devolved in to what pH is or is not possible instead of discussing other more prevalent (IMO) thoughts such as: density dependant controls on population size; resource partitioning and competition by the different species of bacteria.

I can fully understand the reasons for the pH debate, just think that the above I mention are far more interesting topics.

By: Chris

Chris — Fri, 27 Apr 2012 17:43:46 +0000

@18 Chris (addendum)

I should also add that the activity of the H+ is not just dependent on the [H+], but also the concentration of other ions in solution. And with concentrations of several molar, this is a big effect. At low concentrations the Debye–Hückel equation is used, but at these high concentrations, higher order approximations are needed.

By: Chris

Chris — Fri, 27 Apr 2012 15:33:02 +0000

As a chemist I must clear up some misconceptions about pH. The normal way it is presented in gen chem is pH = -log[H+]. This is a good approximation for normal lab acid concentrations. However at very high concentrations the activity of the H+, not the concentration of [H+] becomes important.

pH = -log (activity of H+)

(activity of H+) = (activity coefficient)*[H+]

Think of working in the lab. If you are a dilute solution, you work very efficiently so you have an activity coefficient near one. Now in a concentrated solution you are constantly bumping into other people, waiting for chemicals or to use the scale, so your activity coefficient drops below one. You could also divvy up the work and make you more efficient so your activity coefficient could be greater than one.

Additionally there is a temperature dependence with pH, so at these higher temps, the water acts more acidic. Neutral goes from pH=7 to pH=6.6 at 50 C. Not huge compared to the actual pH but still important.

For very concentrated acids, they tend to have activity coefficients greater than one, giving pH much lower than would be expected. So yes while seeing negative pH values is not common and always worth a double check, it is not impossible.

By: Stephen Curry

Stephen Curry — Fri, 27 Apr 2012 11:07:17 +0000

I’m wondering (a bit superficially, admittedly) if pH meters actually *work* with such contaminated water (200 g/L metals and 760g/L sulfate!?!). I don’t dispute the idea of negative pH (I get the math) but given the chemical constraints (pH -3.6 = 4000 M H+), as pointed out above, I can’t get my head round what it means.

I ‘m guessing there may pH meters that may be specifically designed for very extreme pH values (a bog-standard lab instrument is probably designed to operate from pH 1-10). Does anyone know?

By: Peter

Peter — Fri, 27 Apr 2012 10:58:25 +0000

I never doubted you Ed. Thanks for the link to the PNAS paper. I knew negative pH was possible, just not so negative. It seems pH is more complex than I realised.

By: Jeramia Ory

Jeramia Ory — Fri, 27 Apr 2012 01:27:29 +0000

From the PNAS article:

“The reporting of negative pH values has been controversial, and for several good reasons.”

and

“There is no generally accepted procedure for defining individual ion activity coefficients without some arbitrary assumptions.”

I would add that it causes a great deal of confusion with the general public who have any recollection of their grade school chemistry. It seems sensationalist (the original usage, not yours) with the potential to distance the public from chemistry further.

By: Ed Yong

Ed Yong — Fri, 27 Apr 2012 01:09:27 +0000

Okay, the pH value is back. The source is this paper: http://www.pnas.org/content/96/7/3455.full.pdf

By: Jeramia Ory

Jeramia Ory — Fri, 27 Apr 2012 00:54:52 +0000

I’m guessing they used the methods in this paper to come up with -3.6. Brian’s right, we routinely use [H+] for pH calculation, but it is actually dependent on the activity, not the concentration. At low concentrations and low ionic strength, is is an acceptable approximation. At high ionic strength, it isn’t. This is why I call myself a biochemist, and not a chemist. http://www.onepetro.org/mslib/servlet/onepetropreview?id=NACE-92010035

By: Dan Milton

Dan Milton — Fri, 27 Apr 2012 00:26:16 +0000

The bacteria may have been an isolated underground community for 50,000 years, but they can only have been in the mine since it was opened in the 1860′s.

By: MadGenius

MadGenius — Thu, 26 Apr 2012 23:37:42 +0000

TIL that pH can have a negative value. This might answer Rosie Redfield’s question http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/negative-pH.shtml

(I’m not a chemist either)