Water purification

Water Part Deux:

There is one application where distilled water might be desired but
where even it is not pure enough--cold/hot water extraction pH
measurements around +- 1.5-2.0 units off 7.0. (This is especially bad
when conservators (some scientists too) have those meters that give you
3-4 significant figures.)  Most conservators and all scientist should
know this but many may not so it's worth repeating.

Removing all dissolved solids is one thing but one needs to also
recognize the potential need of removing dissolved gases as well.

When water is in equilibrium with ambient concentrations of carbon
dioxide (circa 365 ppm) it's pH hovers around 5.6.  Some of the
dissolved carbon dioxide reacts with water to form carbonic acid.  It is
the basis of this reaction which causes suspended magnesium carbonate to
dissolved under a CO2-rich environments when making magnesium
bicarbonate solutions.

Obviously, if a paper sample is really neutral, it will be unable to
influence it's 5.6 extraction solution and the wrong conclusion can be
derived.  You can avoid this by bubbling nitrogen gas through the water
and keeping a pure nitrogen atmosphere over it for the measurement
(after a two point calibration of the electrode) or you can boil the
water first.  Both methods work (and there are others) but it's probably
not worth the trouble for a treatment lab--better to not worry about
values close to neutral.  When pH values drift very much below this
equilibrium point, the hydrogen ion concentration contributed by
carbonic acid species  becomes insignificant almost immediately.  If the
conservator still wishes to persist the TAPPI T509 om-83 works.

Gases used on the same bench can also create artifacts.  Ammonium
hydroxide is a very good example.

A word on pH meters with more significant figures than are healthy for
the normal world--more than two is too many.  (I've only seen credible
and useful pH's reported (like 4.442)  once before--Liljestrand's
doctoral dissertation on acid rain which started the whole panic of the
1960's for example.)  But even using TAPPI T509 om-83, a really stable
electrode may drift by c. 0.03 in a few hours.

jd

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                  Conservation DistList Instance 8:37
                 Distributed: Sunday, November 13, 1994
                        Message Id: cdl-8-37-008
                                  ***