Chemistry – The “Trouble” With pH

Posted on by John Fuchs

In a preceeding blog, I mentioned that pH is a logrhythmic function.  As such, each increase of one pH unit represents a ten-fold change in the measured characteristic.  This alone can lead to considerable confusion.  But it gets worse!

Concentration –

One would think that pH would be directly related to concentration – – meaning that if you have a solution of an acid in water, adding more acid will decrease the pH of the mixture proportionately.  Not True!  Due to the quirks of chemistry, adding acid may decrease the pH, but not always in direct proportion to the ratio of the mixture.  The following ficticious (but typical) pH vs. concentration curves illustrate this effect.

Illustration showing the relationship of pH and concentration for three acids
As acid is added to water (pH7) the pH of the solution changes differently depending on the acid’s properties. Note that in some cases a very small change in concentration can result in proportionately much larger changes in pH.

The same result is common with alkaline (basic) chemicals as well.  The lesson here is that pH is not always a good measure of concentration.  I will address ramifications of this in more detail in an upcoming blog.

Strength –

As it turns out, pH and the “strength” of an acid or base, although frequently thought of as synonymous terms, mean quite different things.  The strength of an acid, for example, refers to its ability or tendency to lose a proton.  A strong acid is one that completely ionizes or dissociates in a solution such that none of the non-ionized acid HA remains.  Examples of strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HClO4), nitric acid (HNO3) and sulfuric acid (H2SO4). In water each of these essentially ionizes 100%.  In contrast, a weak acid only partially dissociates in a solution.  Weak acids include carbonic acid (H2CO3) and acetic acid (CH3COOH).  Thanks Google!

The difference in the strength and the peculiarities of pH make it extremely difficult for one not versed in the fine points of chemistry to predict with any degree of accuracy what will happen when chemicals are mixed with pH or strength as a target.

Illustration showing mixing or an acid and a base in equal quantity.
The above illustration demonstrates that pH can be misleading when mixing solutions with different pH. A true prediction of the outcome involves more than just the pH of the two solutions.

For example, one would intuitively think that mixing a chemistry with a pH of 3 (4 pH units above that of water which is considered neutral at pH 7) and an equal amount of chemistry with a pH of 11 (4 pH units below that of water) would result in a solution that had a pH of 7.  Not necessarily!!!!  It depends on the strength of the acid and base involved.  The blog Neutral Ground gives a good example of how assumptions regarding pH similar to the above can lead to disaster.

And then there are the “special cases.”  For example, concentrated hydrochloric acid increases in strength when mixed in water.  The peak “strength” is at about 14% acid in water.

The lesson here is that one should not assume anything when it comes to chemistry.  Chemistry issues should be addressed by competent professionals to produce meaningful results.

–  FJF  –

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