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  1. #1

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    Water pH - chemistry question

    Can small variations in the pH of the water used for mixing have material/significant effects on film developer activity?

    The question would probably only pertain to dilute, weakly buffered developers. Perhaps even in weakly buffered developers the alkali still provides enough buffering to render small pH variations in water meaningless.

    I’m asking out of curiosity regarding potable tap water (and/or filtered water) vs distilled/deionized – strictly concerning pH. It seems typically, tap and/or filtered water is slightly alkaline and also very weakly buffered due to the presence of dissolved ions etc. My understanding is distilled/deionized/purified water is slightly acid, the pH is inversely related to temperature, and there is no buffering since it pretty much contains only H2O.

    Are these small variations neutralized by the alkali in virtually any developer?

    Thanks
    Michael

  2. #2
    Ian Grant's Avatar
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    You've pretty much answered your own question, developers that might be prone to changes tend to be well buffered and those using just an alkai use enough for the water's pH to have a neglible effect.

    There must be the odd exception with water supplies but then that water wouldn't be potable. Ours in Turkey isn't but it's fine for developing just not a final rinse.

    Ian

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    Assuming that hardness is mostly caused by Ca++ and Mg++, it should be possible to test possible effects, especially with regard to possible pH shift. According to Wikipedia, hard water means you have somewhere around 1.5 mmol/l of these ions combined. This means we should be able to test the highest possible pH effect of hard water on any developer by adding NaOH at twice that molar amount and measure pH difference. Whether that pH effect takes place depends on the anion that comes with the Ca++ and Mg++, e.g. SO4-- should cause less of an pH effect than CO3--.
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  4. #4
    AgX
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    Quote Originally Posted by Michael R 1974 View Post
    My understanding is distilled/deionized/purified water is slightly acid, ...
    Distilled or deionized water is ph-neutral. But this might change by time if aired.

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    Hi Michael,

    To answer your question -- no. This is because water has very little buffer capacity which measures the resistance to pH change.
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    Rudeofus's Avatar
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    Quote Originally Posted by Gerald C Koch View Post
    To answer your question -- no. This is because water has very little buffer capacity which measures the resistance to pH change.
    If you look at some of the ultra low contrast developers in the Film Developing Cookbook, they are very poorly buffered, too ...
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    Thanks everyone for the responses so far.

    Ian, Gerald, Rudeofus - I was originally thinking of something like FX-2 for example. I guess even in a case like that the Carbonate is strong enough to make any pH variation in the water virtually meaningless in practice. Although as Rudeofus points out there are some very dilute special purpose developers which may be affected. Not sure.

    AgX - I guess in theory pH 7 but probably becomes slightly acid almost immediately.

    I suppose where I'm coming from is - could there be any possible downside to using distilled/deionized water to mix developers? The only real benefit I can see when it comes to using distilled/deionized water with film developers would be in a case in which the tap water contains abnormally high levels of Ca, Iron etc. and you are mixing a home-brew developer that doesn't include any sequestering/chelating agents. Pretty much all commercial formulas from say Kodak, Ilford etc. include such compounds.
    Last edited by Michael R 1974; 05-23-2013 at 11:27 AM. Click to view previous post history. Reason: typo

  8. #8
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    Water should be pH 7 but we have discussed this to death in another thread. The CO2 in air lowers pH to about 6 or thereabouts due to the formation of carbonic acid.

    This has no effect on any developer as there is little buffer capacity in water and the alkali in all developers is designed to buffer very well at the designed pH. The hard minerals in some water can cause more of an effect, but the developers that I know of have sequestering agents such as EDTA, Calgon or Quadrofos to eliminate those effects as well.

    If you have hard water and it overwhelms the sequestrant, the developer will either be cloudy or will develop a precipitate in a day or less. If so, use DW or DI water.

    Developers are designed to work with most tap water anywhere in the world. I would make sure that the developer is best in your region though as some countries use Ozone and some use Chlorine in their water. This can make a very small difference in activity I have been told. Personally, I think that there will be no effect.

    PE

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    Thanks. Sorry if the thread was repetitive. I was trying to rule out any possible negative effects of using distilled/deionized.

  10. #10

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    hi michael

    if you have a milk jug filled with water
    leave it over night it finds itself the next morning
    with a neutral ph. some aquaria fanatics love doing this
    when they do a partial water change ...

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