This document discusses pH determination methods. It defines pH as the negative logarithm of hydrogen ion concentration and describes how Sorensen developed the pH scale in 1909. A pH of 7 is neutral as hydrogen and hydroxide ion concentrations are equal. The document outlines that the glass electrode method measures potential difference using a pH meter to determine pH electrometrically. It also describes colorimetric determination where pH is estimated by comparing color changes of indicators like phenolphthalein in solutions of known pH.

1. Sorensen’s pH scale,
pH determination
by electrometric
and calorimetric
Presented by-
Prof. Mirza Salman Baig
Assistant Professor in Pharmaceutics

2. Dissociation constant (Kw)
• In any aqueous solution at a given temperature (Dissociation
constant = Kw)
• [H+] = [OH-] = 1.0 x 10-7 M
• [1.0 x 10-7][1.0 x 10-7] = 1.0 x 10-14
• Kw = [H+] [OH-]
• Acids
• [H+] increases and [OH-] decreases
• Bases
• [OH-] increases and [H+] decreases

4. Sorensen’s pH scale
• pH = − log [H+]
• where, log is a base −10 logarithm and [H+] is the
concentration of hydrogen ions in moles per litre of solution.
• pOH = − log [OH-]

5. Sorensen’s pH scale
• Danish biochemist Soren Sorensen in 1909 developed the pH scale and
introduced pH definition as minus (−) logarithm of [H+] to the base 10.
• A pH of 7 is considered as “neutral”, because the concentration of
hydrogen ions is exactly equal to the concentration of hydroxide (OH−)
ions produced by dissociation of the water.
• The hydrogen ion concentration in pure water at room temperature is
about 1 × 10−7 M .
• Increasing the concentration of hydrogen ions above 1 × 10−7 M produces
a solution with a pH of less than 7, and the solution is considered as
“acidic”.
• On other hand decreasing the concentration of hydrogen ions below 1 ×
10−7 M produces a solution with a pH above 7, and the solution is
considered “alkaline” or “basic”.

7. Methods of pH
determination
Electrometric method
Colorimetric method

8. Electrometric method
(glass electrode for pH)
• The pH of the sample is determined
electrometrically using either a glass
electrode in combination with a
reference potential or a combination
electrode
• pH is measured with a potentiometric
glass electrode.
• A pH electrode consists of two half-
cells; an indicating electrode and a
reference electrode.

9. Ph electrode/meter
• This creates a change in
electrical potential with respect
to the silver/silver chloride
reference
• This potential difference is
measured by a pH meter
(voltameter) and converted to
a pH output data
• A pH electrode contains a bulb at the end covered with a
thin glass membrane.
• This membrane becomes hydrated in the presence of
water.
• Hydrogen ions can enter the silicon-oxygen structure of
the glass and alter the charge.

10. Colorimetric method
• The basis of colorimetric analysis is the variation in the intensity of the color of a
solution with changes in ion concentration (or pH).
• ​The color may be due to an inherent property of the constituent itself (e.g. is
purple) or it may be due to the formation of a coloured compound as the result
of the addition of a suitable reagent which is called as an indicator.
• ​By comparing the intensity of the color of a solution of unknown pH with the
intensities of solutions of known pH, the pH of an unknown solution may be
determined.
• Certain organic substances change color in dilute solution when the hydrogen ion
concentration reaches a particular value.
• For example, phenolphthalein is a colorless substance in any aqueous solution
with a hydrogen ion concentration while it become pink at pH more than 7
(alkaline)

11. Phenolphthalein pH paper strip
Acidic Basic

12. Thank you