Volume 3, Number 5
Nov 1990

Comparison of pH Pens on the Market

Abbey Publications has surveyed the various pH pens on the market, in order to evaluate their usefulness for reading the pH of paper outside of the chemistry laboratory. Beside the Abbey pH Pen^TM which contains chlorophenol red, we found five individual pens and a set of five with different pH indicators designed to be used together (Table 1). These are listed below, with their distributors and prices. All the pens had been bought within 12 months of the tests and were unused. We can assume that the pH indicator fluid in them at the time of testing was at the original concentration. At least one of them (No. 1) is known to be strongly affected by the concentration of the indicator fluid, so our findings apply only to unused pens, not those that may have dried out with use. 1 Readings obtained from Pen No. 6 at a given pH are the same over a wide range of fluid concentrations. The effect of fluid concentrations for the other pens has not been examined.

Table 1. Pens Tested

Pen No. 1 has a green barrel, with the pen name and supplier printed on it. It comes attached to a card, which has comprehensive instructions for its use. Pens No. 2 and 3 have identical barrels (gray) with the label being the only difference between then. The label on Pen No. 2 gives the name of the pen, while both labels give the supplier's name and telephone number, but no information regarding their use. We bad to rely on the ads from which we had ordered these two pens for information on use. Pens No. 4 and 5 also have identical barrels (purple with a white top), with a silver stick-m label carrying the pen name. The label on Pen No. 5 also has a guide to the color results and the supplier's name and address. Except for the name, the label on Pen No. 4 is entirely in Japanese; however, it does come with a general English-language information sheet on the pens distributed by the supplier, which includes a brief explanation of this pen and its use. Pen No. 6 has a gray barrel with a purple top, printed with the pen name, brief instructions for use, and the supplier's name and telephone number. More detailed instructions for use are sent with the pen. The set of five pens (Pens No. 7a-e) are small and color coded to match the chart which is attached to the box they come in. Pen No. 7a is white, while the others are black with a colored ring identifying each. Like Pen No. 2 from the same distributor, there are no instructions for use. Pen No. 7a was dry when opened, and had to be remoistened using a few drops of distilled water.

Method

To test the pens, we prepared test papers at pH levels 2.0 through 9.0. We used Fisherbrand Filter Paper (coarse porosity grade P8) from Fisher Scientific, soaked in pHydrion buffer solutions from Micro Essentials laboratory, then dried. We numbered all the pens, and marked each paper with each pen. The colors are identified both by names and by Pantone color code (Tables 2 and 3).

After completing these tests at ambient humidity, we conducted dry, humid and wet tests on Pens No. 1-6 on papers buffered to pH 7.0; and on Pens No. 7a-e on papers buffered to pH 5.0 and 8.0 (Table 4). For the dry test we placed the papers in an oven at 200°F for a period of two hours. For the humid test we subjected the papers to steam and then immediately conducted the same test.

(Since the results of these tests for Pew No. 1-6 on Wet Paper Buffered to pH 5.0 and 8.0 contributed no new information, we decided not to do them on Pens No. 7a-e.) For the wet test we used papers saturated with tap water but not dripping. We noted the reading obtained both while they were wet, and again after they had dried.

Table 2. Color Comparison of Pens No. 1-6

Pantone Color Code, a swatchbook available from print shop suppliers

Table 3. Color Comparison of Pens No. 7a-e at Ambient Humidity

Table 4. Color Comparison of Pens No. 7a-e

* These were too faint to properly identify

Results

Testing at Ambient Humidity. Pen No. 1 appears to contain a modified bromcresol green, and changes from a bright yellow to a bright blue. It begins to change from yellow to green at pH 4.0, is a very definite green at 6.0 and is a bluish green at 7.0. Above this, it is a definite blue. (Unmodified bromcresol green changes from yellow to green at pH 3.8 and from green to blue at 5.4.)

Pens No. 2-5 all performed in a very similar fashion, and appeared to contain the same indicator. At low pH levels the color was a bright orange-yellow. It gradually darkened to a brownish yellow by pH 6.0 and a greenish brown at 7.0.

At pH 8.0 the brown took on a purple hue and at 9.0 all four were a strong purple color. The changes in all four pens were gradual, although Pen No. 5 was slightly brighter; however, their performance was at times very different from the suppliers' information. In all cases the colors at pH 6.0 through 8.0 had not been correctly identified by the suppliers. They all eventually reached the color purple, but not until pH 9.0. The supplier for Pen No. 5 had indicated that a mark on alkaline paper should be blue, but this color was never obtained at any pH.)

Pen No. 6 showed a pale yellow below pH 6.0, turned to a very light yellow (almost clear) at 6.0 and to a pale purple at 7.0, after which there was no change. One notable difference between Pen No. 6 and the others tested was the intensity of the mark on the paper. While No. 6 was very pale, the others all gave a definite bright mark.

Pens No. 7a-e gave varied results. Table 3 shows both the indicated and actual colors of each mark. In over half of the cases the color was not as the supplier had predicted. A comparison of the marks with those from Pens No. 1-6 suggests that Pen No. 7c has the same indicator as Pen No. 1 (bromcresol green), Pen No. 7d has the same indicator as Pens No. 2-5, and Pen No. 7a has the same as Pen No. 6 (chlorophenol red).

Dry, Humid and Wet Tests. All the pens tested on the very dry paper gave exactly the same results that they gave at ambient humidity.

When we tested Pens No. 1-6 on the humid paper, the results were very similar. After a short wait (two minutes), the results were identical to those obtained in the dry paper test.

The results for the tests for Pens No. 1-6 on the Wet paper at pH 7.0 were different. Most, except for Pen No. 6, gave color readings as if the pH level had been raised (i.e. the wet reading at 7. 0 for Pen No. 1 war, the same as that for the dry reading at 8.0). Pen No. 6 showed a pinkish purple instead of a purple color. Again, it was very pale.

When the wet paper was dry 55 minutes later, we took further readings. All of the colors had paled, and except for Pen No. 5, gave results similar to our test at ambient humidity. The only noticeable difference was an inconsistency in the color of the marks on the paper. For example, the brown of Pen No. 3 had flecks of yellow in it, and the blue of Pen No. 1 had flecks of green. Pen No. 5, on the other hand, dried to a yellowish green, and the color was very uneven, in contrast to its brown color at pH 7.0 at ambient humidity.

The results of the wet tests conducted on Pens No. 7a-e (at pH 5.0 and 8.0) were confusing and difficult to draw conclusions from . In some cases the marks on the wt paper were closer to the supplier's predictions, in others they bore no resemblance, while in some cases the results were the same as at ambient humidity, and in some cases not. The marks on the test papers faded dramatically on drying and were difficult to identify. The mark from Pen No. 7a had virtually disappeared. Pens No. 7b-e dried to shades similar to those they had showed at ambient humidity.

Conclusions

The effects of different concentrations of indicator fluids in the pens, and of different types of paper, were not investigated. Because of the demonstrated unreliability of the suppliers' guides, it is highly recommended that when using these pens, they first be tested on papers of known pH levels (such as buffered papers), which could then serve as a color chart for future use.

Pen No. 1 is good over a wide range of pH, and is particularly useful in distinguishing different degrees of acidity.

Pens No. 2-5 are good for distinguishing neutral and very alkaline paper. One problem encountered with Pens No. 2-5 was that the results did not match the suppliers' instructions. They all gave good strong colors, but these were not the colors predicted by the suppliers for any given pH. For accuracy a color chart should be used. It is also noted that the marks from these four pens took some time to reach their eventual colors, and the higher the pH, the longer they took. It is recommended that readings be taken at least two minutes after application.

The paleness of the mark made by Pen No. 6 makes it useful when identifying materials of value.

The pen set (Pens No. 7a-e) gives various and often inconclusive results. The supplier's color-pH chart is rather vague, and uses some unusual color descriptions, such as purple yellow. Many of the marks do not match those indicated by the supplier. If using these pens, a color chart would have to be constructed by the user before trying to identify pH. The pH levels that the five pens are supposed to indicate overlap a great deal, and it is difficult to see why so many different pens are necessary. A combination of only two or three of the individual pens tested would identify a wide range of pH levels just as easily, while giving more consistent results at less cost to the customer.

Bibliography

1. "Experiments with the 'Archivist's Pen'," by Ellen McCrady. Abbey Newsl. 14/3, p. 45, June 1990.