Chemical Education Today

www.JCE.DivCHED.org • Vol. 83 No. 10 October 2006 • Journal of Chemical Education 1465

Negative pH Does Exist

pH is commonly used as a measure of the hydroniumion concentration in chemistry, biochemistry, soil science,wine science, and other fields:

pH = �log10[H3O+(aq)] (1)

The logarithmic pH scale of eq 1 is open-ended, allowing forpH values below 0 or above 14. Nevertheless, there is muchconfusion about the permissible range of the pH scale (e.g.,1–16 ). The misconception that pH lies between 0 and 14 hasbeen perpetuated in popular-science books (1–3), textbooks(4–8), revision guides (9), and reference books (10–16).

Although many chemistry texts state or infer that nega-tive pH values are possible, typically no examples are given.This may be because negative pH values are difficult to mea-sure experimentally (17, 18) and there has been a lack of suit-able buffer standards for pH <1. It is easier to report H3O+

concentrations or total acid concentrations in the 100–101

mol L�1 range than imprecise negative pH values. Further-more, most texts have a diagram of a pH scale similar to Fig-ure 1A. Comparison of the typical textbook diagram (withoutarrows on the pH-scale axis) to mathematical-textbook dia-grams (e.g., 19, 20, 21), suggests that the pH scale is a closedline interval rather the open-ended scale shown in Figure 1B.

The misconception might be minimized by using pHscale diagrams similar to Figure 1B and listing examples ofsolutions, with pH outside the 0–14 range, in textbooks. Forexample, commercially available concentrated HCl solution(37% by mass) has pH ≈ �1.1, while saturated NaOH solu-tion has pH ≈15.0 (22). Hot springs near Ebeko volcano,with naturally occurring HCl and H2SO4, have estimated pHvalues as low as –1.7 (23, 24). Waters from the RichmondMine at Iron Mountain, CA, have pH = �3.6 (25, 26).

AcknowledgmentKFL thanks Phillip Ponder (Penleigh and Essendon

Grammar School, Keilor East, Australia) and Jeanne Lee( ) for encouraging and helpful discussions.

Literature Cited1. Morgan, N. Chemistry in Action: The Molecules of Everyday Life;

Andromeda Oxford: Abingdon, UK, 1995.2. The Handy Science Answer Book; Bobick, J. E., Balaban, N.

E., Eds.; Visible Ink Press: Canton, MI, 2003.3. Cobb, C.; Fetterolf, M. L. The Joy of Chemistry: The Amazing Sci-

ence of Familiar Things; Prometheus Books: Amherst, NY, 2005.4. Denniston, K. J.; Topping, J. J.; Caret, R. L. General, Organic,

and Biological Chemistry, 3rd ed.; McGraw-Hill: New York, 2001.5. Cochrane, H.; Devlin, J. Heinemann Science Links 2; ,

Heinemann Science Links; Heinemann: Port Melbourne, 2000.6. Timberlake, K. C. Chemistry: An Introduction to General, Or-

ganic, and Biological Chemistry, 7th ed.; Benjamin Cummings:Menlo Park, CA, 1999.

7. Lofts, G.; Merett, M. J. Science Quest 2, 2nd ed.; JacarandaWiley: Milton (Qld), 2000.

8. Kruszelnicki, K. Flying Lasers, Robofish and Cities of Slime; NewMoments in Science; HarperCollins: Pymble, NSW, 1997.

9. Roebuck, C. M. Excel Preliminary Chemistry; Pascal Press:Glebe (NSW), 2000.

10. Thain, M.; Hickman, M. Dictionary of Biology, 11th ed.; Pen-guin: London, 2004.

11. Hayhew, S. The Oxford Dictionary of Geography, 3rd ed.; Ox-ford University Press: Oxford, 2004.

12. Dictionary of Science and Technology; Bloomsbury: 2003.13. Dictionary of Science; HarperCollins: Glasgow, 2003.14. Clarke, A. N. Dictionary of Geography, 3rd ed.; Penguin: Lon-

don, 2003.15. The Oxford Dictionary of Ecology, 2nd ed.; Allaby, M., Ed. Ox-

ford University Press: Oxford, 1998.16. The Hutchinson Dictionary of Science, 2nd ed.; Lafferty, P.,

Rowe, J., Eds.; Helicon: Oxford, 1998.17. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fun-

damentals of Analytical Chemistry, 8th ed.; Brooks/Cole:Belmont, CA, 2004.

18. Harris, D. C. Quantitative Chemical Analysis, 6th ed.; W. H.Freeman and Company: New York, 2002.

19. The VNR Concise Encyclopedia of Mathematics, 2nd ed.; Gellert,W., Gottwald, S.; Hellwich, M.; Kästner, H.; Küstner, H.,Eds.; Van Nostrand Reinhold: New York, 1989.

20. Ganderton, G. D.; McLeod, J. M. Mathematics for AustralianSchools. Year 7, 3rd ed.; MacMillan Education Australia: SouthMelbourne, Victoria, 1996.

21. Evans, M.; Lipson, K.; Jones, P.; Blain, D.; McCoy, T. Essen-tial Mathematical Methods. Units 3 & 4, 2nd ed.; Coghill:Malvern, Victoria, 1996.

22. Dickerson, R. E.; Gray, H. B.; Darensbourg, M. Y. ChemicalPrinciples, 4th ed.; Benjamin Cummings: Menlo Park, CA, 1984.

23. Ivanov, V. V. Geokhimiya 1955, 1, 63.24. Nikitina, L. P. Presented at Proceedings of the Water-Rock In-

teraction Symposium, Prague, Czechoslovakia, 1974.25. Nordstrom, D. K.; Alpers, C. N. Proc. Natl. Acad. Sci. U.S.A.

1999, 96, 3455.26. Nordstrom, D. K.; Alpers, C. N.; Ptacek, C. J.; Blowes, D.

W. Environ. Sci. Technol. 2000, 34, 254.

Kieran F. Lim ( )School of Life and Environmental SciencesDeakin University, Geelong, Victoria 3217, Australialim@deakin.edu.au

Figure 1. Two pH scales: (A) a typical textbook diagram; (B) ar-rows on the axis show that pH is measured on an open-ended scale.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

pH scale

pH scale

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

increasing acidity

increasing acidity

increasing basicity

increasing basicity

15−1

A

B

Letters