THE IDEA OF pH VALUE

Today Alan Mason helps us understand pH.

Ideas of acidity, alkalinity and neutrality have been known about for centuries, hence the common language terms, but it was only in the early twentieth century that the concept was fully understood scientifically, and a number scale was used to describe it. (Søren Peder Sørensen at the Carlsberg Brewery, 1909)

1. PURE WATER

Pure water is a relatively stable liquid, but it does break down very slightly giving rise to positive and negative ions, described by the equations below.


The arrows indicate that the first stage is dynamic and reversible, but because the H+ ion or proton is so reactive, it immediately buries itself in a neutral water molecule to make a positively charged “hydroxonium” ion. There are a variety of other terms for this ion, principally, “hydronium”.

The current usage is to write a single equation:

H2O + H2O = H3O+ + OH-

This break-down is called “dissociation” and is governed by a Dissociation Constant, which is 10-14.

A simple equation can be written:

[H3O+] X [OH-] = 10-14

The square brackets are the mathematical shorthand for the words, “the concentration of”. In pure, neutral water the concentration of H3O+ is 10-7 gram ions per litre, and the concentration of OH- is also 10-7 gram ions per litre.

So the equation above can be expanded.

[H3O+] X [OH-] = [10-7] X [10-7] =10-14

As the equation deals with indices, when multiplication occurs the indices are added. The value 10-7 means “one ten millionth” so it is a very small amount of water that dissociates.

2. DEFINING pH

It would be possible to define acidity, neutrality or alkalinity by reference to either the H3O+ ion or the OH- ion but the former has been chosen. To avoid inconvenient numbers like 10-7 or negatives, pH is defined as, “the negative log of the H3O+ concentration”. So, pure water has a pH of 7.

(See Footnote at the end)

3. ADDING SOLUBLE SUBSTANCES TO PURE WATER

Common Salt

If common salt is added to pure water there is a big increase in positive and negative ions, that is, sodium ions, Na+ and chloride ions, Cl-. This does not affect the concentration of H3O+ or OH- ions so the solution remains neutral, and this can be demonstrated with an indicator.

Alcohol

If ethyl alcohol or ethanol (CH3-CH2-OH) is added to water it dissolves completely, but because it is a covalent or non-ionic material this does not affect the balance of H3O+ or OH- ions, so the solution remains neutral, as shown by an indicator.

Caustic Soda

The material known as caustic soda is highly reactive, dissolving fats, and the materials of human skin. Chemically, it is sodium hydroxide, and is completely ionic as sodium Na+ and hydroxyl ions OH-.

If a small quantity is added to water, the sodium Na+ will have no effect, as was the case with common salt, but let us assume the hydroxyl ions OH- increase by a thousand times. This means the concentration goes up from one ten millionth to one ten thousandth, or from 10-7 to 10-4.

Remember that the Dissociation Constant remains at 10-14.

Now, the equation of ionic balance changes:

[H3O+] X [OH-] = [10-10] X [10-4] =10-14

As the concentration of OH- has increased, the concentration of H3O+ must decrease, from 10-7 to 10-10, so the Dissociation Constant is still 10-14.

The pH of this solution is therefore 10.

Hydrochloric Acid

This liquid is also highly reactive, dissolving most metals, and giving off explosive hydrogen. Chemically, it is a strong solution of hydrogen chloride in water, and is fully ionic, consisting of H3O+ ions and Cl- chloride ions. If a small quantity of the acid is added to pure water, to increase the H3O+ concentration by a thousandfold, from 10-7 to 10-4 the balance will change.

Examining the equation of ionic balance it changes so:

[H3O+] X [OH-] = [10-4] X [10-10] =10-14

As the concentration of H3O+ has increased, the concentration of OH- must decrease, from 10-7 to 10-10, so the Dissociation Constant is still 10-14.

The pH of this solution is therefore 4.

4. A RANGE OF ALKALINE SUBSTANCES

All of the elements in Group 1 of the Periodic Table are called, “The Alkali Metals” because their hydroxides are strongly alkaline; that is Lithium, Sodium, Potassium, Rubidium, Caesium, and Francium.

The Group 2 elements are called, “The Alkaline Earths” and have weakly alkaline hydroxides; that are Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium.

Magnesium hydroxide, Mg (OH)2, is weak enough to be used medically as “milk of magnesia”, a stomach “antacid”.

An alkali can be defined as a substance which gives rise to a large excess of OH- ions in aqueous solutions.

5. A RANGE OF INORGANIC ACIDS

The elements in Group 7, “The Halogens”, (Fluorine, Chlorine, Bromine, Iodine and Astatine) form strongly ionised acids, (hydrofluoric, hydrochloric, hydrobromic, hydroiodic and hydroastatic acids).

Some of the non-metallic elements, including the gas, nitrogen (Boron, Sulphur, Silicon, Phosphorus and Nitrogen) are also associated with acid formation, (Boracic, Sulphuric, Sulphurous, Silicic, Phosphoric, Nitric and Nitrous acids.)

An acid can be defined as a substance which gives rise to a large excess of H3O+ ions in aqueous solutions.

5. A RANGE OF ORGANIC ACIDS

Many organic compounds contain the carboxyl –COOH group which gives them acid properties and a pH below 7. They are called the carboxylic acids as a general name. The simplest organic acid is carbonic acid, resulting from combining CO2 and water.


6. POSTSCRIPT

There is some dispute about the origin of the p in pH. One suggestion is that p and q were simply two contrasting algebraic symbols in Sørensen’s original work. Another idea is that it stands for the “power” of an acid. The most generally held view, and the one I
was taught,
is that it is simply a shorthand symbol for “the negative log of the H3O+ concentration”. It is widely employed in physical chemistry in equations concerned with dissociation constants. It has no connection with partial pressures.

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