The pH of a solution is a measure of the molar concentration of hydrogen ions in the solution and as such is a measure of the acidity or basicity of the solution. The letters pH stand for "power of hydrogen" and the numerical value is defined as the negative base 10 logarithm of the molar concentration of hydrogen ions.
pH = -log10[H+]
The measurement of the pH of a sample can be done by measuring the cell potential of that sample in reference to a standard hydrogen electrode, as in the accepted procedure for measuring standard electrode potentials. This procedure would give a value of zero for a 1 Molar solution of H+ ions, so that defines the zero of the pH scale. The cell potential for any other value of H+ concentration can be obtained with the use of the Nernst equation. For a solution at 25°C this gives
For this expression, a base change from the natural log to the base 10 logarithm was made in the Nernst equation.
In practice, the pH is not usually measured in this way because it requires hydrogen gas at standard pressure, and the platinum electrode used in the standard hydrogen electrode is easily fouled by the presence of other substances in the solution (Ebbing). Fortunately, other practical electrode configurations can be calibrated to read the H+ ion concentration. Laboratory pH meters are often made with a glass electrode consisting of a silver wire coated with silver chloride immersed in dilute hydrochloric acid. The electrode solution is separated from the solution to be measured by a thin glass membrane. The potential which develops across that glass membrane can be shown to be proportional to the hydrogen ion concentrations on the two surfaces. In the measurement instrument, a cell is made with the other electrode commonly being a mercury-mercury chloride electrode. The cell potential is then linearly proportional to the pH and the meter can then be calibrated to read directly in pH.
Examples of pH Values
The pH of a solution is a measure of the molar concentration of hydrogen ions in the solution and as such is a measure of the acidity or basicity of the solution. The letters pH stand for "power of hydrogen" and numerical value for pH is just the negative of the power of 10 of the molar concentration of H+ ions.
The usual range of pH values encountered is between 0 and 14, with 0 being the value for concentrated hydrochloric acid (1 M HCl), 7 the value for pure water (neutral pH), and 14 being the value for concentrated sodium hydroxide (1 M NaOH). It is possible to get a pH of -1 with 10 M HCl, but that is about a practical limit of acidity. At the other extreme, a 10 M solution of NaOH would have a pH of 15.
In pure water, the molar concentration of H+ ions is 10-7 M and the concentration of OH- ions is also 10-7 M. Actually, when looked at in detail, it is more accurate to classify the concentrations as those of [H3O]+ and [OH]-. The product of the positive and negative ion concentrations is 10-14 in any aqueous solution at 25°C.
An important example of pH is that of the blood. Its nominal value of pH = 7.4 is regulated very accurately by the body. If the pH of the blood gets outside the range 7.35 to 7.45 the results can be serious and even fatal.
If you measure the pH of tap water with a pH meter, you may be surprised at how far from a pH of 7 it is because of dissolved substances in the water. Distilled water is necessary to get a pH near 7.
Meters for pH measurement can give precise numerical values, but approximate values can be obtained with various indicators. Red and blue litmus paper has been one of the common indicators. Red litmus paper turns blue at a basic pH of about 5, and blue litmus paper turns red at an acid pH of about 8. Neither changes color if the pH is nearly neutral. Litmus is an organic compound derived from lichens.
Phenolpthalein is also a common indicator, being colorless in solution at pH below 8 and turning pink for pH above 8.
Measurement of Ion Concentration
Methods have been developed for the measurement of the concentrations of various kinds of ions using the cell potentials of special electrochemical cells. If you form a voltaic cell with two different kinds of metal electrodes, you get a characteristic cell potential for standard conditions where the electrolytes are at the standard 1 Molar concentration. If the ion concentrations are different from 1 M, then there is a change in the cell potential described by the Nernst equation. Measurement of that change is in essence a measure of the ion concentration, and can be exploited for that purpose.
Perhaps best known is the measurement of pH which is a measurement of the H+ ion concentration. Other ion selective electrodes have been developed for the measurement of K+, Ca+, Mg+, NH4+, etc. Ebbing cites an example of a specialized electrode designed for measuring the concentration of urea, an nonelectrolyte. To do this, an electrode sensitive to the NH4+ concentration is coated with a gel containing urease, an enzyme which catalyzes the decomposition of urea, producing ammonium ions. The measured NH4+ concentration can then be calibrated in terms of the urea concentration in the solution.