When you de-gas the water, cover it after it stops boiling.

    Film   Developing

May 5, 2018


A reader commented about the "C-41 Shelf Life" article, wondering if perhaps I was suggesting you could boil tap water to distill it.

In this article, we'll look at this idea.  And let's compare tap water vs. distilled for film developing.

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In This Article

Distilling vs. Boiling

Distilled or Tap Water?

Bottom Line


Distilling vs. Boiling

If you just boil water on the stove, you're not distilling it unless you collect the steam in some other container.

While you can get better distilled water by making it yourself (the store-bought is dusty), you would have to condense the steam somewhere else. 

In two or three places on this site, I've said to boil water on the stove.  But if you notice, I say to start with distilled water.  We are not trying to de-mineralize the water, because that's already been done for us.

The reason to boil it is very simple.  As I've said before, store-bought distilled water has taken up atmospheric CO2.  This shifts the pH toward acidity.  Distilled water has no buffering capacity;  the water will pH-shift easily.  The dissolved carbon dioxide must be removed to bring it back toward neutral pH.

Table of Contents

Distilled or Tap Water?

Now we get to one of the age-old questions of film developing.

Which is better, distilled water or tap water?

Well, if you use distilled water and you don't de-gas it by boiling, then tap water is better!  That's because the distilled water could be acidic enough to affect the developer.  The place you are most likely to have an issue is with highly-dilute developers, because their buffering capacity would be lowest there.

Basically the developer would have less activity, which means the negatives will be thinner for the same developing time. 

If you do use de-gassed dH2O, then it could be argued that it's a toss-up between that and regular tap water.  As that reader mentioned, he's been developing with tap water for forty years with no problems. 

Except there is one thing:  repeatability between different sources of tap water. 

If you happen to go somewhere else that has the same type of tap water, then great.  But if you go somewhere that has substantially different tap water-- perhaps more chloramines, less calcium, more aeration, more iron, etc.-- that's probably going to change things. 

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Bottom Line

De-gassed distilled water is good for repeatability.

When I'm just wanting to develop some film quickly, I do use tap water.  In no way would I suggest that you couldn't develop with tap water. 

Some film developing chems have some buffering built-in, but that decreases quickly when you start using highly-dilute developers.  Tap water with calcium hardness can actually be a plus here, because the buffering is not tied to the developer strength.

Buffering capacity may also change as developer gets depleted, because some of these proprietary mixtures have rather complex chemistry.  That's where it gets tougher to predict.

So, bottom line:  use tap water if it works for you, but realize that you might have to work up different dev times if you switch to another source of tap water.

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This was a look at tap water vs. distilled water.  There are advantages and disadvantages to both.  Tap water is probably good for the majority of film developing, even if the repeatability is not as good.

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