[Polleke]

I don't think its unreasonable to specify distilled or deionized water for ethanol-based disinfecting solutions.  Tap water is an undefined product, it may contain random dissolved salts, depending on the time of the year, or the location.  Above, someone has stated that it might be more microbiologically clean that the DI system, but even if that is true sometimes, its not under the facilities control, so if its important, you never know if its always the case, for all times at all facilities.  You're not going to be allowed to use tap water, if there is an auditing process, it just looks sloppy.  And being slightly sloppy, when it doesn't really matter, doesn't make you look ingenious, it makes people wonder, "What other corners do they cut, 'round here."  c.f. : http://www.chemicalforums.com/index.php?topic=29560.msg112551#msg112551

I can see your reasoning, but still find it weird to use a pure product (or more expensive product) like demiwater to make 70% ethanol solution that you use to clean your working bench for example.
I really find it weird.
I also dont see the impact of the random salts, dont see how that would matter for 90% of the usage of the 70% ethanol.

You said that the tap water is not under the control of the facility, but be honest: how many facilities test their demiwater on a regular base to check if its still up to standards?
I have worked in a lab for a few years now and there was never (!) a real control on the demiwater. And its in many labs the same thing.
Another thing: many labs have a system that uses a plastic tube (that is attached to the tap (valve)) to fill their bottles with demiwater, this plastic tube is often used for many months/years without ever being cleaned. I am not sure you know a lot about biofilmformation, but I can tell you that those plastic tubes are often filled with bacteria and other "crap".
(I have also read literature in which is stated that in some labs they even found algea(!) growing in the systems that were used for demiwater or even worse, miliQ water systems.

If you indeed use a regulary checked demiwater system I can see your point, but for many of the labs this is not the case.

About using distilled water: is that not really really expensive?

[discodermolide]

If you work in a cGMP environment quality control and following procedures are paramount, even for labs. Our water quality sometimes gave us more problems than anything else and was constantly monitored. Everything is documented for the batch records. The slightest variation is noted and investigated.
If this is not done you can get all sorts of warnings from the governmental authority (if they find out and they can do snap unannounced inspections) even up to banning the sale of your products, costs a lot of money and usually heads roll.
This is not usually the case in normal research labs where you do not have the same degree of quality control.

[Polleke]

If you work in a cGMP environment quality control and following procedures are paramount, even for labs. Our water quality sometimes gave us more problems than anything else and was constantly monitored. Everything is documented for the batch records. The slightest variation is noted and investigated.
If this is not done you can get all sorts of warnings from the governmental authority (if they find out and they can do snap unannounced inspections) even up to banning the sale of your products, costs a lot of money and usually heads roll.
This is not usually the case in normal research labs where you do not have the same degree of quality control.

I understand this, but I doubt majority of us is working in such labs with strict protocols.

[discodermolide]

It might be more people than you think. In the company I was in there were more people in development labs than in med. chem.
Also consider hospital labs, forensic labs, food testing labs, etc etc etc.

[Polleke]

It might be more people than you think. In the company I was in there were more people in development labs than in med. chem.
Also consider hospital labs, forensic labs, food testing labs, etc etc etc.

Indeed.
Thats true.

I can understand it in such labs, but other labs...

Its not about not being prepared to use the products as described, its more about the idea behind why you use things.
Eg. to clean a bench in a lab where you simply use it to disinfect.

[Arkcon]

Eg. to clean a bench in a lab where you simply use it to disinfect.

Why are you disinfecting it at all, if it isn't also important that that also be clean, and dust, and mineral scale free as well?  If iron oxide builds up, and the surface ends up rust colored, would you actually say, "Yeah, yeah, I know, but its disinfected even though it looks tatty."?  What if some day you read in the newspaper about a serious sewage failure in your municipality, perhaps caused by a natural disaster.  Probably your work surface status is the last thing you'd worry about, but after the disaster is taken care of, isn't some of your work now in doubt?  And which ones?  Will you just dump all of it, just to be sure?  Or pass all of it, until someone notices a problem?

[Polleke]

Eg. to clean a bench in a lab where you simply use it to disinfect.

Why are you disinfecting it at all, if it isn't also important that that also be clean, and dust, and mineral scale free as well?  If iron oxide builds up, and the surface ends up rust colored, would you actually say, "Yeah, yeah, I know, but its disinfected even though it looks tatty."?  What if some day you read in the newspaper about a serious sewage failure in your municipality, perhaps caused by a natural disaster.  Probably your work surface status is the last thing you'd worry about, but after the disaster is taken care of, isn't some of your work now in doubt?  And which ones?  Will you just dump all of it, just to be sure?  Or pass all of it, until someone notices a problem?

You are speaking of very extreme examples.

The tab water here is very clean.
And iron oxide builds up and causing rust on a working bench? A little bit extreme... 

And about the sewage failure in general: I dont get your point because the demiwater is also made with tapwater.. And the system that generates the tapwater for you is not a system that removes bacteria really..

I really dont see your point (microbiology wise) here.

I can understand the part of water that is free of minerals, but this is often not a necessity for the usage of the ethanol.