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I would like to know (just curiosity) if there is any law for pH conservation in chemical reactions. (like conservation of momentum in dynamics).

EDIT

The reaction I would like to take as example is fuel combustion. Too many sources (mostly media) stating that acid products (NOx and COx dissolved in water) are obtained.

My idea is that acids cannot be created from nothing. Either the inputs of the reaction were acid or the output is not only acid but also basic outputs. Thus the question whether this reasoning is true.

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    $\begingroup$ Welcome to chem.SE! BTW, do you know what pH is, how it's measured and defined? It's very weird to make an analogy of pH and momentum. $\endgroup$ – M.A.R. Sep 23 '15 at 11:51
  • $\begingroup$ sincerely I don't remember from high school $\endgroup$ – Sanandrea Sep 23 '15 at 12:01
  • $\begingroup$ but i remember that base(pH > 7) + acid (pH < 7) neutralises pH (= 7) $\endgroup$ – Sanandrea Sep 23 '15 at 12:04
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    $\begingroup$ The very example you gave as part of your question - formation of acidic products from combustion of neutral fuels - reminds me that the name "oxygen" comes from the Greek, meaning "acid-forming", because that is what happens in combining oxygen with many elements, especially the non-metals. $\endgroup$ – iad22agp Sep 23 '15 at 12:35
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    $\begingroup$ Well, let's take 1l of some strongly acidic solution with pH=0. Dilute it with water till it becomes 10l. Now it has pH=1. What is conserved here, really? Certainly not pH. Number of $\ce{H+}$? Maybe... Then take some water (just neutral, pure water) and add $\ce{SO3}$. All of a sudden it becomes acidic, though we certainly did not add even one atom of H. $\endgroup$ – Ivan Neretin Sep 23 '15 at 13:11
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No, there is no law requiring pH conservation. And such a law would be chemical nonsense. Why? Read on.

You need to recall what pH actually is. It is defined as the negative base-10 logarithm of the activity (similar enough to concentration for almost all practical purposes) of positively charged hydrogen ions in solution. Or in equations:

$$\mathrm{pH} = -\lg [\ce{H+}]$$

Note that most conservation laws in physics somehow relate back to the conservation of energy or something related. However, the pH of a solution cannot be easily traced back to any state function or anything else that would warrant its conservation.

In fact, the only thing conserved is the atom itself. Hydrogen, if $\ce{H+}$ is somehow present in a reaction or solution, does not disappear, it needs to be put in on one side of the equation and turn up somewhere else. In reactions that liberate protons of any kind, usually bases are added to capture them. In those that require protons, acids are added. What (if anything) is conserved is the number of hydrogen atoms but if they are not active as $\ce{H+}$ in solution they do not count towards pH. Therefore, there is no sense in postulating pH conservation.


†: $\ce{H+}$ as is does not actually exist in solution. One can treat it as $\ce{H3O+}$ for all intents and purposes, but actually it is better to think of it as a proton being shuffled back and forth by multiple water molecules. I think I recall four being stated as the lowest count in undergrad education.

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Yes there is a law , but not exactly for pH conservation as pH is just a notation for concentration ( -log of it , and If you've known that log is basically introduced to make calculations and representations simpler). That is the law of chemical equivalence in a chemical reaction. Any reaction in the whole world holds this law. It states that number of equivalents in a chemical reaction are conserved. For more detailed analysis you can search it over the web.

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  • $\begingroup$ But it's not a law of pH conservation - which is what the question is about. $\endgroup$ – bon Sep 24 '15 at 15:30
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    $\begingroup$ I guess I stated it in the starting that pH is basically a notation. It is not an intrinsic property like momentum that it could have been conserved by such law. $\endgroup$ – Akshay Pratap Singh Sep 24 '15 at 15:38

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