Volume 14, Number 3
Jun 1990

Experiments with the "Archivist's Pen"

Bromcresol green, or bromcresol green, is a pH indicator fluid with a visual transition interval of pH 3.8 or 4.0 to 5.4 or 5.6, being yellow below that interval and blue above it. It is advertised as the indicator fluid in the pH pen sold by Light Impressions to identify acid-free paper, but if so, the producers have found a way to raise the top value of the interval a full point to about pH 6.4, where the mark on the paper turns from a blue-green to a bright blue.

Bromcresol green is not a reliable indicator, though, when used to mark paper rather than to show the pH of solutions. The concentration of the solution can make a great deal of difference. For instance, if the cap is left off and tire pen dries out a bit, it may still make a mark that is yellow, green or blue, but it is likely to show the paper as more acidic than it really is. If water is added (by dropping it on the tip and letting it soak in and equalize overnight), the color will move one or two pH points up the scale toward blue.

Experimental

Filter paper from a chemical supply house was buffered to pH 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0 and allowed to dry. A partly dried out Archivist's Pen was used to mark each sheet. After the solution had been diluted by adding water as described above, the colors on the sheets were somewhat lighter in tint and one or two steps further toward blue an the pH scale. The colors after dilution were:

Recommendations

In order to get reliable readings from this pen, the user should periodically calibrate the pen on a chemically buffered sheet of paper kept for this purpose. If the reading is too far to the yellow side, and the pen seem to be too dry, a few drops of distilled water should be added and the concentration of the fluid cautiously brought back to normal. (It does not matter what color or concentration you call "normal" as long as you know what pH it corresponds to.)

Buffering solutions may be purchased from Micro Essential Laboratory, Inc., 4224 Avenue H, Brooklyn, NY 11210. Coffee filter paper would probably work as well as filter paper made for use in a chemistry lab.