Sunday, April 3, 2011

A short Note about washing.......

I did my last film in the SALS 13 mix today, and took the opportunity to measure pH in the mix. It was still at pH 11,7 before being used for the 5th and final film in the mix. More on that later.

What was interesting was what I did AFTER pouring out the developer.

A good while back I stopped using an acid stop bath after Soda-based developers. The reason for that is that acids (like diluted acetic acid stop baths) reacts with the soda and releases CO2 gas. This can easily be observed if one uses an acid stop bath, bubbles and froth will be clearly evident in the tank.

Now this is not so important, except if you think it over, what about the soda sucked up INSIDE the film emulsion layers?  Of course the acids will release CO2 there also, giving rise to small bubbles locked inside the emulsion, these will later show up as small black pinpoints in the pictures....

So I decided to go with plain neutral pH 7 water, relying on the fact that ascorbate developers will stop being active as soon as the pH drops below 9-ish....
Since ordinary caffenol contains large amounts of coffe that quickly makes the fix bath go black, my standard stop bath proceduure has been 3 changes of plain water in the tank.

This should stop both the developing action quite promptly, since the buffer capacity of washing soda is limited, and leave me with just a slight yellowish fixing bath.

Now with SALS 13 - this requirement the blackening effect - just wasn't there, SALS 13 has no coffe in it. So I went with just two changes of stop water....

But hey!  SALS 13 contains NaOH, Lye, and THAT is a far more effective buffer than soda. So I decided to measure just HOW effective my plain water stop was.

I have a very accurate, calibrated pH-meter.  I decided to measure the pH in my stop baths, since that will easily tell me how effective this is.

Pre development, pH in the developer  :  pH 11,7
Post development pH in the second water stop bath change : pH 9,8

Look at that! This means that the concentration of lye in the tank diminished with 1 pH unit per change!

Now this correlated very well with a debate I was in over at APUG about wshing film, and the merits of the Ilford washing procedure.

I calculated this quite precisely at that time and hinted that every water change would reduce fixer level by a factor of 1/10 or thereabouts.
In practice that means that 3 changes of water will give a remaining concentration of 1/1000th, while 6 changes will give 1/ 1000 000th....

Here is the proof! 

One change of water reduces the NaOH concentration with a factor of 1/10th (or 1 pH unit), two changes with a factor of 1/100th (or 2 pH units...)

For the stop bath this means that a Ascorbate/Lye, pH 13 based developer remains somewhat active even after two changes of plain water, pH 9,8 is too high to be a reliable stop.......

For washing the film, rest assured that 6 changes of water will be a wash better than what is possible with running water, running water reduces concentration with 1/10 until the tank is full, after that its a matter of dilution alone, and to reach a concentration of 1/ 1000 000, one would have to use 25 000 litres of water or some insane amount of water like that......

For my own sanity I of course uses 10 changes of water, that is a theorethical concentration of 1/ 10 000 000 000th - not far from washing the film in the entire ocean!

1 comment:

  1. after all, the traditional method of letting water run through for 5 minutes has worked for ages, did you mesure the pH with the runing water method?