[Leica] pH, activities, etc.

[Edward Caliguri <caliguri@ma.ultranet.com>]

- -- Ok William -
    I was just trying to be simple, OK? Assuming a base is a proton
acceptor, and an acid a proton donor (let's leave multi-protic compounds out
of it) pH is actually the - Log{H+} or the 'activity co-efficient of a given
solution x [H+] = {H+}. There are also STRONG acids and WEAK acids - strong
acids (dissociate completely in solution)  THERE the {H+) = [H+] so pH IS
directly proportional to hydrogen (or hydronium ion, H3O+) concentration
Example -- a 0.01M HF (hydrofluoric acid is a STRONG acid, and NOT because
it dissolves glass or burns fingers) solution has pH=- Log(0.01) or 2.0.
Your actetic acid example is that of a WEAK acid since has what is termed a
pKa, or acid dissociation constant, and is a bit more complicated. But
comparing a pH 4.0 solution of acetic acid (vinegar) and a weak acid to a
solution of HF, a strong acid, at pH 4.0 to support what they DO is
ridiculous - reactivity of the chemical here is what is important! There is
VERY LITTLE HF in a pH 4 solution (0.0001 moles/L)

    And when I mentioned 'pure water' I was not counting the carbonic acid
concentration from dissolved atmospheric CO2 to make Carbonic acid in situ
on purpose. The reaction formula, by the way is CO2 +H20 <=> H2CO3  (not
what you wrote) and in distilled /deionized water it is boiled out as CO2
(LeChatlier's principle pushes the reaction to the left) And so, for the
sake of simplicity, I was starting with pure d/di 18 Ohm WATER, is that OK
with you? And actually, (YES way Jose!) Carbonate buffer systems are GREAT!!
You are alive right now because of one!! The problem with using water as a
'stop' bath is that it quickly looses it's ability to dilute the residual
developer in the print. And you
say "the best, cheapest and odourless stop can be made from a 100 g/L
organic acid like ascorbic or citric. It works like a charm, and not
exceptionally  toxic should you spill it all over the place. "  Well -
Ascorbate will reduce Ag+ to silver metal (that's how XTOL developer works,
best in a basic medium) because it's a reducing agent. Citric acid is also a
reducing agent. Just use a dilute acetic acid solution as has been used for
years, with or without a methylene blue indicator. By the way, Glacial
Acetic Acid is Pure acetic acid with no water in it. Long ago, it was not
uncommon to find it 'frozen' at cold room temperatures when it was kept pure
- - thus the 'glacial' term.
Can we finish this now and talk about photography. PLEASE?
    EC

> From: owner-leica-users@mejac.palo-alto.ca.us (Leica Users digest)
> Reply-To: leica-users@mejac.palo-alto.ca.us
> Date: Tue, 25 Jun 2002 11:58:54 -0700 (PDT)
> To: leica-users-digest@mejac.palo-alto.ca.us
> Subject: Leica Users digest V22 #326
> 
> People banter on about pH without really understanding it - as we should
> understand it, pH is a measure of ion activity, NOT ion concentration. A
> weak solution of vinigar has a pH of < 4 , but so would a weak solution of
> Hydrofluoric aicd. And you wouldn't want to stick your fingers in there, let
> me tell you. 
> 
> Acid or base "strength" cannot be predicted by pH alone. This is an
> important fact.
> 
>> Well -the water bath (assuming it's pure) is pH=7.0.  or alkaline (basic,
> above 7) When you toss a print in from a developer,
>> The developer, which is alkaline mixes with it, making it basic ..
> 
> No way, Jose. Pure water has no buffering capacity to dissolved CO2 from the
> atmosphere, and will quickly acidify due to the formation of carbonic acid
> (H3O+ and CO3)
> 
> Take a beaker of distilled water and measure the pH - it will be close to
> near the point of FMA (free mineral acidity pH = 4.3)
> 
> In ultra-pure water systems for pharmaceutical and semiconductor
> applications, the "pure" water is purged under nitrogen to prevent this
> acidification from happening.
> 
> The problem of using a water stop is the lack of buffering capacity.
> Residual developer will quickly raise alkalinity to the point where it will
> continue to "develop" - albeit slowly. Then more rapidly as the "stop"
> becomes more basic. Carbonic is a weak acid and cannot buffer pH from the
> more alkaline developer.
> 
> And if your water is "hard", the pH will be on the alkaline side, making the
> water stop even more ineffective. Although you can throw it in the fix, the
> fix, if it has to act like a stop bath, the fix rapidly looses capacity. Not
> good.
> 
> The best, cheapest and odourless stop can be made from a 100 g/L organic
> acid like ascorbic or citric. It works like a charm, and not exceptionally
> toxic should you spill it all over the place.
> 
> William
> 
> - --

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