A Conversation for Units of Measurement

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Post 1

Orcus

An outstanding and scholarly piece of work smiley - biggrin

I'm quite impressed me smiley - winkeye Obviously thoroughly researched and well written. You even got humour in there smiley - ok.

Two minor quibbles.

1. Your definition of pH. I understood from my undergrad days that pH's and pK's were not strictly based on ratios of concentration but on "activity". Ie. the activity of H+ ions in water is close but not exactly the same as the molar concentration. Its a kind of effective concentration and is related to the concentration by the Debye-Huckel Law.
Please correct me if the definition has changed or I'm wrong.
I do appreciate that the article would get mind boggling if it got too technical.

2. I work as an organic chemist/chemical biologist/ biological chemist (depending on which modern buzz word my boss wants to use at any given moment). I am not aware of the enzyme unit being obsolete. I certainly use it and it is continuously published in the literature. Some definitions vary ie. Americans like defining it as nmol/min. But it is used. What else to do use to define the acitivty of an enzyme?


Other comments. Why don't the damn Americans use kJ? smiley - grr I've got loads of sympathy for shopkeepers as it doesn't matter in the slightest bit whether you buy a 1 lb of ham or 0.5 kg but when it comes to science it DOES matter and its irritating having to convert every single time. Its easy enough to do but its not as easy to visualise a kcal/mol as a kJ/mol - imho of course.

I saw a lecture on metrology in our department a few weeks ago. Very interesting stuff. The lecturer was a retiring professor from the Royal Greenwich Observatory and his talk was the history and future of the metre, second and kilogram. I had no idea how amazingly techy these things were. I was very impressed with the latest atomic clocks which slow the caesium atoms down to around a femtoKelvin in order to narrow the spectral line and increase the precision the measurement. Cool.
I'm sure your aware but I like the fact that the standard kilo changes weight with time and they have to use complicated extrapolations from when it was last washed before they can remeasure it. smiley - laugh. Apparently there's a debate about using a new definition. I favoured using E=mc2 and having the kilo defined by the energy of a certain number of photons. Nice and obscure but also very precise.

Anyhow, enough ramblng.

Good Work smiley - ok


Brilliant!

Post 2

Cefpret

As far as pH is concerned: Never heard of 'activity' but I'm not a chemist and it sounds plausible. However the question is: What can you measure directly? I think determining the concentration is more straightforward. However it's a matter of definition and I simply don't know. And I'm too tired for an Internet research. My father should know. I'll ask him this weekend.

The alternative for the enzyme unit is the katal. My brother studies medizine and so I know that this enzyme unit is still widely used. I don't know why I put it there. Probably a slip.

To the kilogram: I mentioned the project 'Avogadro' in the article. The NIST tries something similar with a so-called 'watt-scale'. Very wired, very expensive. In a couple of years we will probably have a new definition of the kilogram and the prototypes can be used as paper weights.smiley - cry Not via photons though.


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Post 3

Orcus

Hi again. I've never heard of the katal which is intereting as I do work with enzymes, not a widely used unit I expect or I would have done I guess. I'm really an organic chemist who occasionally dabbles in the former though so a biochemist may well have done.

Activity is used not only for pH but for any equilibrium constant involving ions/molecules in solution. The reason for the difference is that the actual properties displayed by a solution depend on the number of dissolved species that are available for reaction, this is not the same as the concentration as some species can aggregate in solution for example meaning some are not actually exposed so the solute even when dissolved. You can also use it for surface catalysis I believe in which case only those surface species in contact with the mobile phase (gas or liquid). In this scenario, the concentration certainly is very different to the activity. You are right though, the concentration is usually a known quantity and the activity can only be calculated theoretically but as you said, it is a matter of definition.


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Post 4

manolan


Not a chemist (not since A level, anyway), but surely the concentration is only useful if known, whereas the activity is measurable. Or have I completely missed the point?


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Post 5

Orcus

I've not done any activity stuff since my undergrad days (eight years ago) but I'm pretty sure the activity is a highly theoretical thing. If something is completely ionised in water like HCl then the activity and concentration are so similar I doubt you could actually measure the difference but in general I think the activity can only be calculated using a theory called the Debye-Huckel theory (for ions anyway) and as I recall, that's a pretty hideous mess of complexity.
You don't actually measure a concentration anyway, you tend to dissolve x grams of compund into y liters of liquid and x/y simply is the concentration. If you don;t know it you can only measure pH, uv absorbance or some other property and then use theoretical equations to calculate concentration.

To sum up, its a bit complicated really smiley - winkeye


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Post 6

manolan


Yes, but Cefpret asked the question 'what can you measure directly?' and my answer would have been activity rather than concentration, which you have just confirmed. Concentration is only useful if it is the known quantity.


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Post 7

Orcus

I don't think I did confirm it. I said you could measure an indirect property such as the absorbance of the solution at a certain uv wavelength or the pH for example. That gives exactly that; the pH or uv absorbance. From that you would have to calculate both activity and concentration from equations that only really work on model systems so that really isn't the point.
I really do think its a matter of definition and that is what the article is about, otherwise I wouldn't have quibbled.


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Post 8

Cefpret

Okay, now I checked it on the homepage of the PTB. You are right, it's the activity of the ions that counts.

I read the procedure how pH measurements are calibrated in Germany: The PTB produces in a very costly procedure first-level prototype solutions in Brauschweig that are sent to laboratories of the German Calibration Service all over the country. They use them to create second-level prototype solutions. They can be bought and used for local calibration. Much effort, much money.

Why didn't you complain when it was in Peer Review or on the Coming Up page?smiley - grr

smiley - winkeye


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Post 9

Orcus

Complain? Twas just a comment, a minor foible with, as I said, a quite outstanding article.
Anyway, I didn't see it before, I've only been h2g2ing again since January so it was probably on Peer review before I returned.
Glad to know I can still remember some of my undergraduate stuff. Coiincidentally, I just had to go to our university library to look up something and there was a whole chapter in the book about activity coefficients for ions in solution. Interesting smiley - erm?


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