Alkaline reserve

Scott: (If I may be so informal.)

The short answer to your question about an easy way to determine
alkaline reserve is "no".  Most field systems such as Gastec's Sensidyne
tubes, Drager tubes or test papers (such as pH papers) require that the
ion or compound of interest be in solution or in gas phase.  Alkaline
reserve is defined by ANSI (ANSI standard Z39.48-2984 - Permanence of
paper for printed library materials) as

     "The presence of a compound (e.g., calcium carbonate) put in the
      paper at a level sufficient to neutralize acid that might, in the
      future, be generated from aging of the paper of from atmospheric
      pollution."

A great deal of chemicals would meet this requirement, but may not be
practical.  Very strong alkalis such as sodium or potassium hydroxide
would absorb acid, but have the disadvantage of being too soluble.  The
alkaline salt would not be very useful if, under humid storage
conditions, it migrated out of the paper and into other porous materials
in contact with the paper. These soluble alkalis also fail to meet the
strict definition of the words "alkaline" and "reserve".  A highly
alkaline paper, (say a pH of 12) could absorb a relatively large amount
of acid before becoming acidic, but this is strictly by virtue of the
pH. To be in reserve, the alkaline salt must be relatively independent
of the pH of the paper (as long as the paper is neutral or alkaline to
begin with.)

This is all a long winded way of saying that the alkaline reserve must
be insoluble in water.  The only way of making some measurable, relative
component soluble is by the addition of acid.  Without performing a
titration (or acidifying and back titrating) the only other way of
measuring the amount of alkaline reserve is to know what chemical is
produced by the neutralization of the alkaline reserve.  A known mass of
paper could then be acidified with a volatile acid (such as
hydrochloric) and then be ashed.  A non-acidified sample of paper (of
known mass) would also have to be ashed.  The difference in ash mass
between the two papers should be the mass of the chloride salt produced
by neutralization (excess acid will evaporate during ashing.)  Obviously
this is not an easy procedure either.

Calcium carbonate, in particular, is a tricky alkaline salt to deal
with.  It can absorb two hydrogen ions and is a weak enough alkaline
salt that it produces a reasonably complex buffer system.  The acid
analog (carbonic acid) really doesn't exist in solution.  Also, in paper
where humidity is the only water source, the small amount of carbonic
acid produced is likely to break into carbon dioxide and water.
Granted, since carbon dioxide is fairly easily soluble in water, the
moisture could be slightly acidic.  To eliminate the problem of how much
carbon dioxide should be left in the water during a titration, the
solution is often boiled to remove all of the CO2.  This leaves a very
dilute calcium bicarbonate in the paper/solution that is titrated at the
end.

In ANSI IT9.2-1989, the paper in water is acidified to a pH of 3.0. (In
the next edition of the standard, the pH will be lowered to 2.0.) Since
the conversion of bicarbonate to carbonic acid occurs at about a pH of
4.0, there is an excess of acid.  The solution is then boiled to remove
the dissolved carbon dioxide (and carbonic acid).  The excess acid
should be enough to eliminate the bicarbonate hopefully leaving us with
an acidic solution free of carbonates, bicarbonates and CO2. The
solution is then back titrated to pH 7.0 with 0.1N NaOH.

The further complication of CO2 with calcium carbonate "buffered" papers
makes the likelihood of ever producing a quick and easy test for
quantifying alkaline reserve very low.

In summary:

1) Color change tests require alkaline reserve in solution.

2) Alkaline reserve by literal definition must be insoluble.

3) The most difficult alkaline reserves to quantify are the carbonates.

4) The most common alkaline reserves used are carbonates.

I hope that this answers your question.

Doug Nishimura
Image Permanence Institute

                                  ***
                   Conservation DistList Instance 4:9
                  Distributed: Tuesday, August 7, 1990
                        Message Id: cdl-4-9-006
                                  ***