[Leica] Water stop for printing

[William Gower <w_gower@sympatico.ca>]

On 6/25/02 9:27 AM, "Leica Users digest"
<owner-leica-users@mejac.palo-alto.ca.us> wrote:

> Date: Mon, 24 Jun 2002 21:19:57 -0400
> From: Edward Caliguri <caliguri@ma.ultranet.com>
> Subject: [Leica] Water Stop for printing
> Message-ID: <B93D3AA0.4E73%caliguri@ma.ultranet.com>
> References: 
> 

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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