When instructions specify "water," without qualification,
it is usual to use purified water. If tap water (whatever
might be dissolved in it) is OK, the instructions will 
usually say "tap water" explicitly.

EDTA has various uses; in histological work they mostly
involve removal of calcium ions. If your tap water is
hard (= contains calcium ions), it will use up some of
the EDTA in the solution. If you know the Ca content of 
the tap water you can calculate the equivalent amount
of wasted EDTA. One mole of calcium ions (40 grams) 
combines with one mole of disodium EDTA dihydrate (372
grams).

For example, if a litre of tap water contains 100mg of
calcium, that will consume 100X372/40 =930mg of EDTA.
This would reduce the available EDTA in a 1% solution to
about 0.9%. I do not know if my guess of 100ppm Ca is
anywhere near reasonable for hard water, or if a
certificate of tap water analysis reports the calcium
content in this way. If my guesstimate is too low, 
hard water may eat up more than 10% of the EDTA in
a 1% solution. 

A simple test for minimally "clean" lab water is to mix 
it with an equal volume of 1% aqueous silver nitrate.
Any precipitate or opalescence indicates that the
water contains anions with insoluble Ag salts or
organic compounds that reduce Ag+. 

John Kiernan
Anatomy, UWO,
London, Canada.
_______________________________________

Trisha Emry wrote:
> 
> Is it necessary to use distilled H20 for EDTA or can you use tap?
> 
> Is anyone using heat with EDTA?  Please send details.
> 
> Thank you,
> 
> Trisha Emry
> U of WA, Seattle
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