pH Measurement Theory Guide

 Why do we classify an everyday liquid like vinegar as being acidic? The reason for this is that vinegar contains an excess of hydronium ions (H3O+) and this excess of hydronium ions in a solution makes it acidic. An excess of hydroxyl ions (OH–) on the other hand makes something basic or alkaline. In pure water the hydroniumn ions are all neutralized by hydroxyl ions and this solution is what we call at a neutral pH value.  H3O+ + OH–  2 H2O Figure 1.  The reaction of an acid and a base forms water.
If the molecules of a substance release hydrogen ions or protons through dissociation we call this substance an acid and the solution becomes acidic. Some of the most well-known acids are hydrochloric acid, sulfuric acid and acetic acid or vinegar. The dissociation of vinegar is shown below: CH3COOH + H2O  CH3COO– + H3O+ Figure 2.  Dissociation of acetic acid.
Not every acid is equally strong. Exactly how acidic something is, is determined by the total number of hydrogen ions in the solution. The pH value is then defined as the negative logarithm of the hydrogen ion concentration. (To be precise, it is determined by the activity of the hydrogen ions. See chapter 4.2 for more information on the activity of hydrogen ions). pH = –log [H3O+] Figure 3.  The formula for calculating the pH value from the concentration of hydronium ions.
The quantitative difference between acidic and alkaline substances can be determined by performing pH value measurements. A few examples of pH values of everyday substances and chemicals are given in figure 4: 

For optimal pH measurements, the correct electrode must first be selected.

The most important sample criteria to be considered are: chemical composition, homogeneity, temperature, pH range and container size (length and width restrictions). The choice becomes particularly important for non-aqueous, low conductivity, protein-rich and viscous samples where general purpose glass electrodes are subject to various sources of error.

The response time and accuracy of an electrode is dependent on a number of factors. Measurements at extreme pH values and temperatures, or low conductivity may take longer than those of aqueous solutions at room temperature with a neutral pH.

The significance of the different types of samples is explained below by taking the different electrode characteristics as a starting point. Again, mainly combined pH electrodes are discussed in this chapter.

Figure 14. Electrode with ceramic junction.

a) Ceramic junctions

The opening that the reference part of a pH electrode contains to maintain
the contact with the sample can have several different forms. These
forms have evolved through time because of the different demands put
on the electrodes when measuring diverse samples. The ‘standard’ junction
is the simplest one and is known as a ceramic junction. It consists
of a porous piece of ceramic which is pushed through the glass shaft of
the electrode. This porous ceramic material then allows the electrolyte to
slowly flow out of the electrode, but stops it from streaming out freely.
This kind of junction is very suitable for standard measurements in aqueous
solutions; the METTLER TOLEDO InLab®Routine Pro is an example
of such an electrode. A schematic drawing of the principle of this junction
is shown below in figure 14.