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I have a database to classify variables/parameters in water resources as follows:

Variable: Precipitation (P); Quantity: Length; Units of measurement: mm (millimeters).

Following the example, what would be the properly quantity of variable pH?

Variable: Potential Hydrogen (pH); Quantity: ?; Unit of measurement: pHU (pH unit, unit less, dimensionless).

Maybe concentration or activity?

[I know what pH is, It is not necessery explain, I'm looking for the most appropriate word to describe its quantity]

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  • $\begingroup$ My question is about quantity not unit. $\endgroup$
    – Artur_Indio
    Commented Oct 23, 2015 at 19:09
  • $\begingroup$ If you don't want to know what pH is, and you don't want to know what it's units are, then it's not clear to me what it is that you do want to know. $\endgroup$
    – Simon W
    Commented Oct 24, 2015 at 8:31
  • $\begingroup$ @SimonW know the most appropriate physical quantity, if possible, to pH. $\endgroup$
    – Artur_Indio
    Commented Oct 24, 2015 at 8:34
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    $\begingroup$ @Artur_Indio surely, using that definition, pH is a physical quantity, of various substances. So the answer to your question is the same as the answer to "what is pH"? I think you need to (a) think carefully about how to ask the question, as at present it's very unclear; (b) most probably, ask it over at chemistry.stackexchange. $\endgroup$
    – Simon W
    Commented Oct 24, 2015 at 8:42

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I disagree with the answer that says "pH measures acidity or basicity" (no matter what is in Wikipedia).

pH is the negative of the log of the hydrogen ion (H+) activity. Activity is a thermodynamic concept that is related to but not the same as concentration. Activity is similar to chemical potential. The same concentration of hydrogen ions will have different activities, depending on what else is in solution. In weak solutions, the activity coefficient that relates the concentration to the activity depends mainly on the charges of the other ions in solution and not on the particular ion species. But at higher amounts of other species, the particular chemicals and chemical interactions become increasingly important.

On the other hand, acidity is the ability to provide hydrogen ions to a reaction and considers not only the hydrogen activity but also the amount of acid-base reaction that can occur. You may have heard of pH 'buffers'. These are solutions that are designed to have a balance of chemical species so that the pH is very stable, even if small amounts of acid or base are added or if the ionic strength of the solution changes. If you add a bit of base to the buffer solution, the solution reequilibrates to provide acid to react with the base. You have to add a fair bit of base before the buffer capacity is used up and the pH changes very much. Then, with further addition, the pH will change a very quickly until it approaches the pH of the base that is being added. So a pH buffer has high acidity and alkalinity, keeping the pH stable.

In groundwater geochemistry, acidity is measured by reacting the solution with strong base until the acid is essentially used up. This is done by titration either to a fixed endpoint pH or to the inflection point in the curve of base added vs pH curve.

Bottom line, pH doesn't measure acidity. In terms of the original question, I would say:

Variable: pH, Quantity: negative log of the hydrogen ion activity, Units: dimensionless pH units.

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    $\begingroup$ I agree with everything except for the last two words (which should be removed imho). $\endgroup$
    – Jan
    Commented Oct 24, 2015 at 23:01
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    $\begingroup$ I left in 'pH units' because it is often used. One problem with dimensionless numbers in equations (IMO) is that it can be unclear where they came from. For example it is sometimes hard to tell if someone is using a weight ratio or a volume ratio. In the case of pH, you could say that it is based on the ratio of the activity of H+ to the activity of H+ in the standard state (in this case, at infinite dilution, and defined as 1). But yes, pH units is not strictly correct but is descriptive. $\endgroup$
    – haresfur
    Commented Oct 25, 2015 at 21:41
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pH measures the activity (for practical purposes, this is the same as concentration) of H+ cations in an aqueous solution (i.e water).

At standard room temperature and pressure, and at conditions not too far from it, a value of 7 means that the water is neutral. Lower values means acidic (lemon juice, sulfuric acid, rain water). Higher values mean basic (ocean water, lime water).

It is unit less, therefore the term "quantity", as I understand from your question, does not mean much.

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  • $\begingroup$ So, "concentration" seems a good word to be the physical quantity/magnitude of pH. $\endgroup$
    – Artur_Indio
    Commented Oct 24, 2015 at 8:41
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pH measures acidity or basicity: https://en.wikipedia.org/wiki/Acidity_function. There are several acidity measuring scales and pH is one of them. Another example is the Hammett acidity function.

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  • $\begingroup$ So, Quantity: acidity or basicity, Variable: pH, Unit Measument: pH units? $\endgroup$
    – Artur_Indio
    Commented Oct 23, 2015 at 19:31
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    $\begingroup$ Quantity: acidity or basicity, Variable: pH, Unit Measument: pH units (or better, unitless) $\endgroup$
    – arkaia
    Commented Oct 23, 2015 at 19:55

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