Archived posting to the Leica Users Group, 2002/06/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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 - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html