STOP BATH QUESTION

Discussion in 'B&W: Film, Paper, Chemistry' started by David Lyga, Mar 16, 2011.

  1. David Lyga

    David Lyga Subscriber

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    I usually use stop bath in a 0.5 % dilution. This is more dilute than most would use but I am liberal with the amount I use and it really is suffient for both BW and color. What this translates to is this: 5 ml glacial acetic acid makes one liter of SB or 18 ml of 28% makes one liter of SB.

    My question is this: Kodak Indicator Stop Bath is ALMOST glacial but not quite. I use 6 ml to make one liter and that works out fine. However, as it is 'indicator' it begins life a pale yellow and turns purple when the Ph gets too high. But I noticed something amazing the other day and I repeated the scenario to confirm (and it confirmed!) I mixed a liter (I use clear, high density plastic bottles from juice, soda, etc) and left it on a shelf for about 5 days. When I looked at it after this time the stop bath was completely clear. The confirmation proved likewise. This clearing did nothing to lessen its realiablity or Ph but I am interested in knowing why it lost its pale yellow color. Actually, I like it better this way, clear. - David Lyga
     
  2. Gerald C Koch

    Gerald C Koch Member

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    Was the stop bath exposed to sunlight? As with any dye the indicator can be decolorized by exposure to light.
     
  3. gordrob

    gordrob Subscriber

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    I use 16ml/L as Kodak recommends for paper. I have had 2Ls stored in a 4L plastic jug for the last month and have not seen any change in the colour. When mixed it is a light yellow colour and appears clear when viewed under a safe light.
    Gord
     
  4. MattKing

    MattKing Subscriber

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    Is there any chance that the Op's water or storage containers are just slightly alkaline?
     
  5. Greg Davis

    Greg Davis Member

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    That would activate the indicator, wouldn't it? If so, it would darken, not lighten.
     
  6. jp498

    jp498 Member

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    I mix it a little thinner for film. It's not a true necessity, as some developers say no stop bath, so I use film stop bath for xtol, but not PMK or caffenol-C. For paper, I do it up at the proper strength as it lasts longer before going purple. A mad printing session will take a lot of life out of a jug of stop bath.
     
  7. MattKing

    MattKing Subscriber

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    With indicator stop bath I've noticed that just before the indicator causes the solution to go dark blue it has a tendency to go slightly blue - just enough to offset the normal yellow colour of the fresh stop bath solution. So it's almost colourless, but generally only for a little while.
     
  8. David Lyga

    David Lyga Subscriber

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    Interesting responses; I wish I could nail it down. The bottle (as I said: clear, high density plastic that is used for Gatorade, juice, etc) was simply on a shelf in room light and not directly exposed to sunlight. I wonder if the dilution (6 ml of K Indicator SB) could have had an effect. It is clear. When first was mixed I put a few drops on a table and added a bit of sodium carbonate and, sure enough, the drops turned from yellow to purple. Twice I did the mixing to prove that I was not imaginging things and both bottles are now clear, fully one and two weeks after initial mixing. As I said previously, I prefer it this way: the clear SB is just as good. - David Lyga
     
  9. nworth

    nworth Subscriber

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    Very dilute dye can be hard to see. Pour a little into a test tube or something similar and compare it to a similar container of water. It the dye has really faded, some oxidizer may have bleached it. Be sure your containers are really clean.

    Your stop bath is really too dilute. The usual recommendation is 2 percent. A very dilute stop bath will not only be less effective (or ineffective) but will have very much less capacity.