[darren69]

How long does pure water last for in 25ltr containers.

week
months
years

[Ian Lancaster]

Funny sort of question

Where would it go?

If you mean could it cease to be pure water, ask yourself where any contamination would come from.

[darren69]

In the right container and air tight it could last for years.

Thank's Ewan 

[Window Washers]

In the right container and air tight it could last for years.

Thank's Ewan 

as long as it is not in sunlight or it will not be pure for very long

[Ian Lancaster]

In the right container and air tight it could last for years.

Thank's Ewan 

as long as it is not in sunlight or it will not be pure for very long

I assume you mean it will develop algae, Ian?  If the container is clean prior to filling, and is then filled to the top and an airtight cap screwed on it will remain pure.  Our definition of 'pure' is that it contains no "dissolved solids".  In order for it to become "impure" i.e. acquire a higher TDS reading there has to be a source of soluble solid material.  The is no such source in the above scenario, so the TDS will remain the same.

[Window Washers]

In the right container and air tight it could last for years.

Thank's Ewan 

as long as it is not in sunlight or it will not be pure for very long

I assume you mean it will develop algae, Ian?  If the container is clean prior to filling, and is then filled to the top and an airtight cap screwed on it will remain pure.  Our definition of 'pure' is that it contains no "dissolved solids".  In order for it to become "impure" i.e. acquire a higher TDS reading there has to be a source of soluble solid material.  The is no such source in the above scenario, so the TDS will remain the same.

Algae yes Ian, I have some out side in the sunshine I will keep a check on them, they are not being used so will be interesting as there is no way I can get them air tight or should I say with no air in the barrell