ALKALINE SOLUTIONS: SIMPLE CHEMISTRY • PREPARATION AND USES
by Cathleen Baker
Alcaline solutions: have many uses in paper conservation: to change and/or buffer the pH of other
solutions; to wash paper in order to enhance removal of soluble acids and dégradation products
(usually following a neutral pH water wash); and to deposit an alkaline reserve in paper. Alkaline
solutions can be divided into aqueous and non-aqueous types. The most common aqueous solutions
used in paper conservation are calcium hydroxide, magnesium bicarbonate, and ammonium
hydroxide: The most common nonaqueous alkaline reserve systems are Wei T´oTM and Bookkeeper.
USING ALKALINE SOLUTIONS
ALKALINE WATER WASHING
If necessary, use an alkaline water wash after first washing the paper with conditioned, neutral pH,
deionized water. Conditioned deionized water is deionized water - which usually has a pH of less
than 7 - which has had its pH raised to 7-7.5 by the addition of either saturated solution of calcium
hydroxide or ammonium hydroxide.
Note: The pH of deionisized or destilled water is slightly acid because of the immediate absorption
of CO2 from the surrounding athmosphere. In contact with air therefore always is a solution of
carbonic acid with a pH raging between 5.7 - 6.0.
The purpose ofan alkaline water wash is to continue the acid neutralization and/or stain removal
processes, but not necessarily to deposit an alkaline reserve in the paper. Therefore, it is not
necessary to use a strong alkaline solution for this neutralization step.
If a dilute NH4OH or Ca(OH)2solution is chosen, the pH should be between pH 7.5 and 8.5. If the
acidity of the paper is very high, or if you want to remove severe stains (either overall or locally), a
stronger alkaline solution, around pH 9-9.5. might be used.
If a fresh Mg(HCO3)2 solution is chosen, the pH should be between 6-7. Noře: Solution is slightly
acid because it is oversaturated of CO2Therefore. this solution would be safe to use on most paper
artifacts. But. remember that this solution potentially contains significant amounts of MgCO3
depending on the concentration chosen. If there is any concern about carbonates forming on the
surface of the image, or of making the paper too alkaline after drying (e.g. concern about paper or
media pH sensitivity), rinse the paper (overall or locally) with conditioned. pH 7 deionized water
immediately following the alkaline water treatment. Drain and air dry.
ALKALINE RESERVES
The purpose behind this treatment procedure is to deposit an alkaline reserve. usually either
magnesium carbonate and probably magnesium oxide and magnesium hydroxide or calcium
carbonate. into the paper. (While these carbonates are often-called "buffers". they are not true
buffers. e.g. sodium carbonate is. as they buffer only acid solutions. not basic ones. This alkaline
reserve should neutralize moderate amounts of acids in the paper which may be present from an
incomplete acid neutralization procedure, from bleaching. and/or from future acid attack from
internal or external sources.
APPLYING THE SOLUTIONS
All of the above solutions can be brushed or sprayed on. or the paper(s) can be immersed into
solutions. Because alkaline reserve solutions are much stronger compared to alkaline water washing
solutions, they could move more degradation products in the paper and might cause tide lines and
staining. Therefore, no matter how the solution is applied, the paper must be drained well so that no
pooling of the solution takes place on the surface. If spraying or brushing on the solution, avoid
uneven wetting or puddling.
NON-AQUEOUS SOLUTIONS
Wei T'o™
Never use a water wet brush with Wei T'o™ or try to deposit it onto water wet or even damp paper.
Ideally, the paper should be absolutely dry. Make sure that the spray apparatus is carefully and
thoroughly cleaned out immediately after use to avoid formation of carbonates in the equipment.
Because Wei T'o™ formulations tend to readily absorb water from the atmosphere. it is very
important that the bottle is kept air- and moisture-tight at all times. In the case of Wei T'o™
solutions sold in bottles, Dr. Richard Smith, Wei T'o™ patent holder, recommends tightly
stretching black electrical tape several times around the cap. If moisture has been absorbed, a
precipitate will be seen in the bottle or the solution will become a gel. If this happens, try pouring
out the amount needed into a beaker adding a small amount of mefhcnol. If this doesn't work, you
will have to discard the solution. Never pour back any Wei T'o™ into the originál bottle or into a
solvent waste container. To dispose of Wei T´o™, in a fume hood, pour it into a homemade Mylar
tray and throw out the tray once the solvent has evaporated.
To deposit a substantial amount of magnesium carbonate in the paper, use solution Wei T'o™ #2 or
#3 undiluted. For smaller amounts of alkaline reserve, dilute either #2 or #3 as necessary with
methanol or ethanol.
To avoid streaking or spotting when either spraying or brushing on the solution, pre-wash the paper
in ethanol first. allow to air dry, and then apply the Wei T'o™ solution.
Bookkeeper Spray Deacidification
This non-aqueous alkaline reserve system does not have the same clogging problems as Wei T'o™.
care should be taken when spraying with the aerosol as the force of the propellant is quite powerful
and could blow away a loose piece of paper or tear a fragile one. The can or pump bottle must be
shaken well before each use as otherwise the micro-particles of magnesium oxide will sink to the
bottom, and application will be ineffective or uneven.
GENERAL DRYING PROCEDURES
Because these solutions, aqueous and non-aqueous, are alkaline in nature, degradation products,
dyes, colors, and media will have a greater tendency to move while drying. To reduce or even
eliminate bleeding, feathering, and sinking, dry the artifact in the following manner:
- Dain the paper well white supported on a polyester woven or non-woven fabric at least 1 inch
larger than the artifact.
- Place the wet artifact, still on its support, onto a drying screen covered with a closely woven
material.
- Using a similar screen, invert it over the first one. if there is any possibly of the two touching, slip
spacers at the corners to elevate the inverted screen up a bit.
- Place a piece of blotter or Hollytex-like material on top of the inverted screen - it should cover the
area of the artifact underneath.
- IF WATER HAS BEEN USED. ENSURE THAT NOTHING TOUCHES THE FACE OF THE
ARTIFACT
DRYING ARTIFACTS TREATED WITH ALKAUNE RESERVE SOLUTIONS
All alkaline reserves are formed when the treated paper is exposed to the atmosphere. Air drying in
a humid environment. >40% RH, is important to ensure that the carbonate, (and in the case of
magnesium solutions, carbonates, hydroxides, and oxides) are deposited into the paper rather than
only on the surfaces.
AQUEOUS SOLUTIONS
If you don't know what the ambient RH is or if it is too low. dry the artifact as outlined above under
General Drying Procedures. This arrangement will create a humid, micro-environment allowing the
carbonate to form in the paper, not only on the surface.
NON-AQUEOUS SOLUTIONS
When using Wei T'o. it is also important to expose the treated paper to a humid environment, >40%
RH while drying. Since you are not using an aqueous solution, the above drying procedure will not
work unless you place a damp blotter on top of the inverted screen (ensure that no dripping can take
place). If you cannot get the room RH above 40%, you could also make a humidity chamber - this is
deal when treating large numbers of artifacts. Or you could dry the paper image side down so that if
carbonates are formed on a surface, they form on the reverse of the sheet. In any case, you should
not expose the treated paper directly to the air if there is any chance that the media could run while
the solvent is evaporating. If you suspect that bleeding might occur, dry between screens. If treating
large numbers and a problem occurs, place clean, dry blotters directly on top of the artifacts,
making layers as the treatment proceeds - use this method with non-aqueously treated artifacts only!
PRECIPITATES ON SURFACE(S) OR "GRITTING"
Occasionally, an excessive amount of the alkaline reserve compounds may form on the surfaces
especially if the paper was in a very dry environment while drying, if this happens and you need to
reduce them, try removing the precipitates with a soft brush.
If that doesn't work, immerse in. or brush on, or spray on a solution of carbonic add (CO2 bubbled
through water) for a few minutes. If a stronger add solution is required, use a 0.1% acetic add
solution for a few minutes - applying overall or locally as required. Follow any acid wash with a pH
7 conditioned deionized water wash for 1 hour, and then repeat the alkaline reserve step using a less
concentrated solution and/or after the drying environment.
PREPARATION OF SOLUTIONS
Calcium hydroxide, Ca(OH)2, (saturated solution - 875 ppm Ca2+
Note: ppm = parts per million.
1.
Pour 1 liter of deionized water into a clean, labeled container. Make sure that the container is
thoroughly clean. If residues of carbonates are evident, clean out the container with a dilute solution
of acetic add. Rinse weil.
2.
Weigh out 5g calcium oxide or calcium hydroxide powder and slowly add the powder to the
water. Never put the powder in the container first, and then add the water otherwise the
reaction between a lot of calcium oxide and a little water gets hot enough to break glass
bottles.
3.
Place the lid on the container, and gentty agitate the bottle to thoroughly mix the caldum
oxide. Allow the solution to sit for about 15 minutes.
4.
Agitate again and allow to sit for at least 1 hour.
5.
Carefully decant off the clear liquor on the top and throw this liquor away. Refill the container
with 1 liter of fresh, deionized water. Agitate the bottle after repiadng the lid, and allow to sit until
the liquor at the top is clear. It is now ready for use. Keep the bottle tightly capped at all times so
that carbon dioxide cannot be absorbed by the liquid to form calcium carbonate.
Magnesium bicarbonate, Mg(HCO3)2, (theoretically 0.1 M solution, - 2400 ppm Mg2+)
Note: Make sure that ail containers are thoroughly clean. If residues of carbonates are evident,
clean out the container with a dilute solution of acetic acid. Rinse well.
Traditional method
1.
Equipment: large glass bottle, stirring magnet, stirrer plate, cylinder of CO2, and dispensing
container with spigot.
2.
Place 4 liters of deionized water, pH 7, into a clean glass bottle.
3. Weigh out 23.32g magnesium hydroxide (use only the best grade available) and carefully pour
the powder into the water. Gently agitate the solution. Theoretically, if all of the Mg(OH)2 went into
solution, this would be a 0.1 M Mg(HCO3)2 solution containing ca. 2400 ppm Mg2+ (MW of
Mg(OH)2 = 58.33, therefore 1M solution = 58.33g/liter, therefore for a 0.1 M solution is required =
5.83g x 4 liters = 23.32g Mg(OH)2 is needed)
4.
Place the stirring magnet into the solution, and position bottle on the stirring plate. Turn up the
power so that the magnet is turning briskly.
5.
Insert the carbon dioxide, CO2. dispersion tube into the bottle, and turn on the CO2 cylinder.
The gas should be bubbling yigorously. Leave this set-up undisturbed, but not completely
unattended until the precipitate is nearly gone, about 4 hours.
6.
First remove the dispersion tube, and for a few seconds, allow the gas to clear out the tube.
Then turn off the gas.
7.
Cover and let sit for a few hours. Carefully decant the clear solution into the correctly labeled
dispensing storage container making sure that the container is air tight. To help maintain the initial
pH 7, store in the refrigerator. If precipitates begin to form in the bottom of the storage bottle, or if a
film develops on the surface, the container is not air tight. If this is the case, the solution strength
has lessened. Always check the strength of the solution by using the-Taylor Magnesium Carbonate
Test as described below.
Soda Siphon Bottle Method:
1.
Refrigerate 0.8 liter of deionized water for several hours. Weigh out and add to the cdd water
4.7g magnesium hydroxide powder.
2.
Pour this solution into a 1 liter commercial soda siphon bottle and following the
manufacturer's instructions discharge one CO2 bottle into the solution. Shake the bottle and
refrigerate for one hour.
3.
Discharge all of the solution into a clean container with a tight fitting lid and open the bottle.
Pour any remaining solution into the container. Race in the refrigerator for an hour and aBow the
solution to clear.
4.
Carefully decant the clear solution into a labeled, air-tight storage container. Store in the
refrigerator. Always check the strength of the solution by using the Taylor Magnesium Carbonate
Test as described below.
Calculations to determine actual strength of a Mg(HCO3)2 solution with Taylor Magnesium
Carbonate Test K-1597
In order to determine the actual amount of magnesium bicarbonate in solution, carry out the Taylor
Magnesium Carbonate Hardness Test K-1597. Using the Test, figure out the amount in g/L of
Mg(HCO3)2 in the solution, and use the following formulas to determine the strength of the
solution:
1.
where a = theoretical amount (g/U of Mg(HCO3)2 in solution when Mg(OH)2 is totally
dissolved into carbonated, deionized water
2.
where b% = percent of Mg(HCO3)2 in actual solution
Dilution guidelines:
To dilute a stock solution of Mg(HCO3)2. first determine the strength of the stock solution by using
the formulas above. Then take the stock solution and add water as in the following example: to
make one liter of a 0.0035M solution from a 0.035M magnesium bicarbonate stock solution,
measure out 10 ml of the 0.035M solution and add to 990ml of deionized water.
Wei T'o: METHOXY MAGNESIUM METHYL CARBONATE
This is a non-aqueous alkaline reserve solution which is manufactured by Wei T'o Associates, Inc.
and is available from distributors such as Conservation Materials, Ltd., Light Impressions Corp.,
and University Products. The Wei T'o solution #2 is methanol-based, other solutions are ethanolbased.
Bookkeeper Deacidification Spray: SUBMICRON MAGNESIUMOXIDE IN INERT SOLVENT
This is a relatively new product. It can be used in much the same way as Wei T'o™ but apparently
is not as toxic or potentially as damaging (color changes, feathering inks, etc.). Contact distributor
or supplier for more information.
CHOOSING THE RIGHT SOLUTION
As suggested in an article by Burgess and Boronyak-Szaplonczay (1992), two aqueous solutions
might be used for types of pcpers listed after them. Potential problems are in italics.
1. fresh magnesium bicarbonate solutions for papers which:
N
- contain lignin, but only if a shift to a darker color with a yellow-red or green tone is acceptable.
(Note: Upon aging, an unprotected paper may darken to a greater extent.)
- do not contain lignin
- are very acidic and/or highly oxidized
- will be returning to a questionable environment and are undergoing only this treatment
- are sensitive to alkaline solutions (includes media)
- cannot tolerate precipitates on surfaces
2. fresh calcium hydroxide solutions for papers which:
- contain lignin, but only if a shift to a darker color with a yellow-red or green tone is acceptable.
(Note: Upon aging, an unprotected paper may darken to a greater extent.)
- do not contain lignin
- are very acidic and/or highly oxidized
- will be returning to a questionable environment and are undergoing only this treatment
- are sensitive to alkaline solutions (includes media)
Burgess et al. (1992) listed the ppm concentrations and corresponding pHs of calcium hydroxide
solutions. These are reproduced in the following table. Use these concentrations in ppm and pH
values to select the most effective alkaline reserve solution for the paper(s) under treatment.
3.
Although not investigated by Burgess et al in their
article, non-aqueous alkaline reserve solution, e.g. Wei T'o, can be used on papers which:
- contain lignin, but only if a shift to a darker color with a yellow or green tone is acceptable.
(Note: Upon ageing, an unprotected paper may darken to a greater extent).
- do not contain lignin.
- are very acidic and/or highly oxidized.
- will be returning to a questionable environment and are undergoing only this treatment.
- are very sensitive to aqueous solutions.
- can tolerate solvent-based solutions (includes media).
- can tolerate precipitates on surfaces (see below for removal).
- can tolerate moderate to high pH after treatment especially if used as alkaline reserve.
Strong Versus Weak Solutions
•
2+
2+
When comparing the concentration of Ca and Mg with the pH of various alkaline reserve
solutions, the conservator notices significant differences. For example, a Ca(OH)2 solution, pH
10.5, has 20 ppm of Ca2+ while a 0.01 M Mg(HCO3)2 solution, pH 7, has 240 ppm Mg2+.
Differences will also occur in treated papers after drying. Papers treated with calcium hydroxide
solutions tend to take up calcium in large quantities. This is because the pH of calcium hydroxide
solutions are quite basic, and basic solutions swell the fibers thus opening the structure of the paper.
This swelling makes the paper able to accept more calcium compounds compared to solutions, such
as Mg(HCO3)2 with lower pHs. Therefore, a paper treated with calcium hydroxide could be
appreciably more alkaline than one treated with magnesium bicarbonate, even if the ppm of Ca2+
and Mg2+ of the two solutions are the same.
CHEMICAL REACTIONS
Aqueous: Calcium Hydroxide, Ca(OH)2:
Notes: - pH of a saturated solution of Ca(OH)2 is ca. 12.4
- saturated solution of calcium hydroxide: 1.85g/L water at O °C
- saturated solution of calcium oxide: 1.31g/L at 10 °C
- saturated solution of calcium carbonate: ca. 0.014g/L water at 25 °C
Aqueous: Magnesium bicarbonate, Mg(HCO3)2
Notes: - pH of a fresh solution of Mg(HCO3)2 should be around 6-7. This is because carbon
dioxide has been dissolved in the solution making carbonic acid. The more carbonic acid present,
the more magnesium hydroxide will be dissolved into the solution.
- saturated solution of magnesium hydroxide: 0.009g/L water at 18 °C.
- saturated solution of basic magnesium carbonate: 0.3g/L cold water
Aqueous: Ammonium hydroxide, NH4OH
Notes: - p H of cone, ammonium hydroxide: ca. 12
- ammonium hydroxide can form ammonium salts of any acids in the paper, and those salts which
are not readily soluble will be left in the paper. Salts of weak adds, such as ammonium carboxylate,
may be left behind as a buffer. They will not make the paper alkaline, and they may function as
alkaline reserves.
- any ammonium salts left behind in paper after drying have varying solubility values, and they are
generally much more soluble than either calcium or magnesium carbonate.
Wei T'o™ solution #2, methoxy magnesium methyi carbonate: