# pH notation in text

When referring to units of pH in continuous text (i.e. not in a table), which is the recommended usage?

• pH 7 to pH 8
• pH 7 to 8

What about when the term "pH" has already been mentioned in the text?

• the pH was increased from 7 to 8
• the pH was increased from pH 7 to 8
• the pH was increased from pH 7 to pH 8
• the pH value was increased from 7 to 8
• Using pH twice in any of those sentences is redundant. – MaxW Feb 3 '17 at 22:56
• Additionally, would it be correct to refer to pH as a measure of acidity? Or would this be ambiguous with the acid dissociation constant? – Marcel Feb 3 '17 at 23:22
• If your readers are chemists then saying that "pH as a measure of acidity" is ludicrous. If you are writing an article for a newspaper about mine water discharge then through the phrase into the article. – MaxW Feb 3 '17 at 23:36
• In a dilute solution, is not pH an appropriate measure of acidity? Constant references to hydrogen ion activity become tedious. – Marcel Feb 3 '17 at 23:45
• $$\ce{<-->[He went that way!]}$$ The point is that chemists know pH is a measure of acidity. You don't have to explain it every time you use the term. – MaxW Feb 3 '17 at 23:50

E.g. in the ACS style guide, 3rd edition, rather only the pH symbol itself is focused on, with some examples of “half intervals” (or how to name that?) notation:

… Use roman type for pH and pOH; always use lowercase for “p”; always capitalize “H” and “OH”.

Solutions were titrated to pH >11.

The UV spectra were measured at pH 6.

A pOH of <12 was acceptable.

I couldn't find any explicit recommendation for notation of pH ranges. In Measurement of pH. Definition, Standards, and Procedures (IUPAC Recommendations 2002) several “notations” are used in text, e.g.

at p. 2179 (11 of 32):

… between pH 3 and 10 …

p. 2184 (16):

… pH(S) values of … are in the range 3 ≤ pH(S) ≤ 11 …

p. 2199 (31)

… in the pH range 3−10 …

pH 7

The usage is inconsistent with pH being a dimensionless quantity. It would be logical to write "the pH is 7", or "the pH is in the range from 5 to 8". This would correspond to the symbolic notation:

$$\mathrm{pH} = 7$$ $$\mathrm{pH} = [5,8]\quad \text{ or }\quad 5 < \mathrm{pH} < 8$$

where [5,8] indicates a range.

Instead, usage like "at pH 5" are common and sanctioned by the ACS style guide. It's a bit like saying "I am looking for size 11 shoes" instead of "The size of my shoes is 11".

pH 7 to 8

When giving a range of pH values, there is no reason to repeat pH. pH is the quantity, and it is clear that you are giving a range for that quantity. This is different from giving a range of lengths, or a length with a standard deviation. In that case, both lengths, or the length and its standard deviation need to have units, e.g.

$$l_1 = \pu{2.3 cm}\ ..\ \pu{2.4 cm}$$

$$l_2 = \pu{21.7 cm} \pm \pu{0.3 cm}$$

Of course, you can leave out the duplicated unit by using parentheses:

$$l_1 = (\pu{2.3}\ ..\ \pu{2.4})\pu{cm}$$

$$l_2 = (21.7 \pm 0.3)\pu{cm}$$

• Shouldn't it be $\mathrm{pH} \in [5,8]$ ? – Marcel Jun 9 '20 at 22:20