(This page revised October 23, 2016)
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This article is the work of John Smith. My contribution is editorial: rearranging the material, augmenting it slightly for improved clarity to the lay person, and rewriting for stylistic consistency with the rest of this site.
A question that is on the mind of every inkophile who ever lived is “Will this new ink, the one with a color to die for, eat my favorite pen?” The answer is maybe. How acidic or alkaline an ink is (its pH), although this is not the only factor in how corrosive an ink will be, can affect how friendly that ink is to various pens.
Although we often speak casually about pH in various walks of life, pH is actually a complex scientific subject. Moreover, measuring pH accurately requires some fairly expensive equipment and a thorough understanding of how to use that equipment properly.
To the chemist, pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. Stated more simply, it is a measure of the acidity or alkalinity of the solution. pH is measured on a scale from 0 to 14, with 7.0, the pH of pure distilled water, being neutral. The lower the pH, the more acid the solution is; conversely, the higher the pH, the more alkaline the solution is. An alkaline substance is referred to as a base.
The task of measuring pH is far removed from taking a temperature reading or from taking a simple electrical measurement with a voltmeter or ammeter. A pH meter, and more particularly the electrode it uses, is a little miracle of complex science and engineering. Many user-generated errors can operate to make a reading unreliable. As a result, unless the source is known, your first reaction to any reported pH reading should be a sensible suspicion. I trust John Smith, who ran the tests that are reported in this article; and I trust his equipment and his methodology, both of which are described below.
Apart from user-generated errors, many factors can be regarded as contributing to variations between readings from different sources. The following list describes three such factors:
All pH readings are temperature dependent; thus, a pH reading can only be expressed accurately in the form “x pH at y temperature.”
Over time, natural chemical reactions within a bottle of ink can alter the pH of the ink, especially once the bottle has been opened and carbon dioxide has entered.
Not all batches of the same ink are necessarily identical. This is especially important to note in light of the fact that some ink manufacturers introduce slight formulaic variations periodically to aid law enforcement in the forensic analysis of writing samples.
The apparatus used was an SI Analytics Lab 870 benchtop pH meter coupled with a virtually new SI Analytics BlueLine pH 15 all-glass electrode that was fully pressurized before the tests. It has an integrated negative temperature coefficient (NTC) temperature sensor. This is a sophisticated, laboratory quality pH meter with Stability Control, automatic buffer-set recognition, electrode diagnostics, and full spectrum error reporting.
The instrument was calibrated to two points using fresh Schott DIN Technical calibration fluids at pH 7.0 and 4.01 at 25° C (77° F).
All readings were established using the instrument’s Stability Control. Using Stability Control allows the instrument, not the user, to determine when the reading is optimal and stable. In this way, accuracy and reproducibility of readings in chemically complex solutions is greatly enhanced.
During the testing, the calibration was regularly checked against the buffer solutions, and no drift was detected. Standard laboratory practices were followed throughout the testing, and a random selection of inks was retested the following day.
At the time of the tests, none of the inks tested had been open for more than 18 months.
NoteWhile pH is an important factor in the safety of a given ink, it is not the only criterion. There are other factors to consider, such as the ink’s tendency to clog, its ability to lubricate well, and whether it stains and/or damages pens and other surfaces. For more information about inks and their behavior, read Inks: The Good, The Bad, and the Ugly.
In the following table, results are listed from lowest pH (most acidic) to highest (most alkaline/basic). As a basis for comparison, distilled white vinegar, a weak acid, usually has a pH of about 2.4 at the standard strength of 5%. Chlorine bleach, a weak base, has a pH of about 11.
|Manufacturer||Ink||Color||pH at 22° C (71.6° F)|
|Rohrer & Klingner||Salix||1.53|
|Rohrer & Klingner||Scabiosa||1.73|
|Rohrer & Klingner||Magenta||1.99|
|Rohrer & Klingner||Königsblau||2.09|
|J. Herbin||Bleu Myosotis||2.31|
|Rohrer & Klingner||Leipziger Schwarz||2.42|
|Private Reserve||Electric DC Blue||2.65|
|ST Dupont||Royal Blue||3.38|
|Diamine||Eau de Nil||4.09|
|J. Herbin||Rose Cyclamen||4.67|
|J. Herbin||Emerald of Chivor||4.69|
|J. Herbin||Bleu Océan||5.65|
|Private Reserve||Blue Suede||6.02|
|J. Herbin||Ivy Green||6.03|
|J. Herbin||Violette Pensée||6.20|
|Private Reserve||Black Cherry||6.59|
|Rohrer & Klingner||Cassia||6.73|
|Rohrer & Klingner||Alt Goldgrün||6.73|
|J. Herbin||Vert Empire||6.96|
|Neutral Point, Ph 7.00|
|J. Herbin||1670 Rouge Hématite||7.35|
|J. Herbin||Poussière de Lune||7.55|
|J. Herbin||Lie de Thé||7.66|
|Rohrer & Klingner||Verdigris||7.98|
|J. Herbin||Rouge Caroubier||9.42|
|Rohrer & Klingner||Morinda||9.76|
“John Smith” is a pseudonym for a respected expert in the field who has asked to remain anonymous.
The inks listed here are obviously not all the inks that exist. These are the ink that Mr. Smith had available for testing. His contribution has been entirely voluntary, and I do not consider it proper to demand that he purchase more inks specifically to pad this article. Should he test additional inks in the future, they will be added to the list.