Teoria De Medicion De Ph

The Theory of pH Measurement
pH measurement has a wide variety of industrial applications in virtually every industry. These range from water conditioning to specific process related measurements to waste treatment. WHAT IS pH? pH is a measure of the acidity or alkalinity of a water solution. The acidity or alkalinity of a water solution is determined by the relative number of hydrogen ions (H+)or hydroxyl ions (OH-) present. Acidic solutions have a higher relative number of hydrogen ions, while alkaline (also called basic) solutions have a higher relative number of hydroxyl ions. Acids are substances which either dissociate (split apart) to release hydrogen ions or react with water to form hydrogen ions. Bases are substances that dissociate to release hydroxyl ions or react with waterto form hydroxyl ions. In water solutions, the product of the molar concentrations1 of hydrogen and hydroxyl ions is equal to a dissociation constant (Kw). Knowing the value of the constant and the concentration of hydrogen ions makes it possible to calculate the concentration of hydroxyl ions, and vice versa. At 25°C, the value of Kw is 10-14 (see Figure 1). The concentration of hydrogen andhydroxyl ions can vary over 15 orders of magnitude in water solutions. Even common household items can reflect this wide variation (Figure 2).

FIGURE 2. pH Values of Acids and Bases pH is strictly defined as the negative logarithm of the hydrogen ion activity (aH): pH = -log10 aH The hydrogen ion activity is defined as the molar concentration of hydrogen ions multiplied by an activity coefficient,which takes into account the interaction of hydrogen ions with other chemical species in the solution. In practice, pH is often assumed to be the negative logarithm of the hydrogen ion concentration: pH = - log10 [H+] In this form, the usefulness of pH as a convenient shorthand for expressing hydrogen ion concentration can be seen on page 2, Figure 3:

Acids:

HA
acid

H+

+

Aanionhydrogen ion

Bases:

MOH
base

OH-

+

M+
cation

hydrogen ion

(H+) x (OH-) = Kw = 10-14 (25°C)
FIGURE 1. Acids and Bases

1 Molar concentration is a measurement of the number of atoms, molecules or ions in a solution.

FIGURE 3. Ion Concentration, MOL/L At 25°C, a neutral solution has a pH of 7.0, while solutions with pH < 7 are acidic and solutions with pH > 7 arealkaline. The normal overall pH range is 0 to 14 pH, although solutions containing non-water solvents can have pH values outside this range. THE MEASUREMENT OF pH pH measurement is based on the use of a pH sensitive electrode (usually glass), a reference electrode, and a temperature element to provide a temperature signal to the pH analyzer. The pH electrode uses a specially formulated, pH sensitiveglass in contact with the solution, which develops a potential (voltage) proportional to the pH of the solution. The reference electrode is designed to maintain a constant potential at any given temperature, and serves to complete the pH measuring circuit within the solution. It provides a known reference potential for the pH electrode. The difference in the potentials of the pH and referenceelectrodes provides a millivolt signal proportional to pH. Most pH sensors are designed to produce a 0 mV signal at 7.0 pH, with a (theoretically ideal) slope (sensitivity) of -59.16 mV / pH at 25°C.

The Need for Temperature Measurement The millivolt signals produced by the pH and reference electrodes are temperature dependent. However, the pH and reference electrode combination exhibits anisopotential point, which is a pH and millivolt potential at which the potential is constant with temperature changes. The isopotential point is most frequently designed to be at 7.0 pH and 0 mV. Using the isopotential point with a theoretical knowledge of electrode behavior makes it possible to compensate (correct) the pH measurement at any temperature to a reference temperature (usually 25°C), using a...