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Chemistry Appendix

Collecting Old Maps

Chemistry for Map Collectors
By Dr. Clare Schoene

This appendix presents some basic concepts about acids, bases, and buffers. While it is not necessary to know this material in order to enjoy map collecting, collectors may encounter terms such as, acid-free paper and buffered board and may want to know more about them. This appendix is meant to provide additional background information for completeness.  

Ions and Charge
A hydrogen atom consists of a nucleus which has one proton with a positive charge and one electron with a negative charge. The single positive and negative charges cancel each other out and the atom is electrically neutral. If it loses its negative charge by losing its electron then it is called an ion. The hydrogen ion is a single proton, and has a positive charge.

An acid is a substance, usually in aqueous solution, rich in hydrogen ions (protons). Any solution with excess hydrogen ions is considered an acid since these hydrogen ions are available for chemical reactions. An acid can also be thought of as a proton-donor since there are available protons to donate for chemical reactions. The opposite of an acid is a base. A base can be thought of as a proton-acceptor, or a solution of negatively charged ions available for chemical reactions.  

Chemical solutions are dynamic on the molecular level with molecules coming apart and coming together all the time. Water molecules are continuously coming apart in solution to make ions, only to rejoin and make a water molecule again. This coming-apart is called dissociation. The coming apart and coming together reach an equilibrium. At equilibrium the number of molecules coming apart equals the number coming together and their respective concentrations don’t change.
If a solution contains equal amounts of proton-donors and proton-acceptors, the solution is neutral. For all practical purposes, pure water, with equal amounts of protons and electrons can be considered, neither acidic nor basic, but neutral.

Acidity and pH
The acidity of a solution depends upon its hydrogen ion concentration so chemists express the acid/base characteristics of a solution in terms of its hydrogen ion concentration. This is known as the pH of a solution. pH is a shorthand method of referring to the concentration of hydrogen ions in solution and is not an arbitrary scale, but is based on physical chemistry.

For example, pure water has a hydrogen ion concentration of 1x10–7 moles/L which is equal to pH 7. A more acidic solution would have a higher concentration of hydrogen ions, say 1x10–5 moles/L which is equal to pH 5.

Anything that increases the hydrogen ion concentration of a solution lowers its pH and makes it more acidic. Adding acid to water will increase the number of hydrogen ions and lower its pH. Strong acids are acids that have higher concentrations of hydrogen ions than weak acids which have low concentrations of hydrogen ions. Note that the concept of a strong acid and a weak acid refers to the availability of its hydrogen ions and is not related to the concentration or amount of acid. 

A solution can be neutralized by adding either acids or bases until the pH is neutral (pH 7). The resulting solution can be very sensitive to change and slight alterations in the concentration of proton donors or acceptors can results in large changes in pH. Buffers are substances that can be added to moderate the swings in pH.

A buffer acts a as a sort of chemical sponge that can absorb hydrogen ions without significant changes in pH. This effect is achieved by the buffer reacting with hydrogen ions to create new molecules that do not dissociate readily. So if hydrogen ions are added to a solution containing an appropriate buffer, the concentration of hydrogen ions in the solution will not increase significantly because the hydrogen ions reacts with the buffer and will not be available as proton donors.

Conservators often add a buffer to the final wash water when washing a map. Such aqueous buffers work well and the treatment is not difficult or expensive. If one cannot wet a map because of color or other considerations, it is still possible to apply a buffer to neutralize the pH. Such buffers are applied as complex organic molecules in a non-aqueous base. They are available from specialty dealers in conservation materials.

When selecting mat board and backing board for display it is best to use a neutral board. A buffered board is the next best choice but this means that the paper pulp was acidic to begin with and was buffered during the manufacturing process.

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