3. PH
pH is a measure of the activity of the hydrogen ion
pure water has a pH very close to 7 at 25°C
solutions with a pH less than 7 are acidic and
solutions with a pH greater than 7 are basic or
alkaline
5. THE PH SCALE
The pH
scale measure
s how acidic or
basic a
substance is.
The pH
scale ranges
from 0 to 14.
6. PH
measurement of pH for aqueous solutions can be
done with a glass electrode and a pH meter, or
using indicators.
pH measurements are important
in medicine, biology, chemistry, agriculture, forestry,
food science, environmental
science, oceanography, civil engineering, and many
other applications.
7. WAYS OF MEASURING PH
Measuring pH Using an Indicator
This category basically includes two methods:
1. comparing the standard color corresponding to a
known pH with the color of an indicator immersed
in the test liquid using buffer solution
2. preparing pH test paper which is soaked in the
indicator, then immersing the paper in the test
liquid and comparing its color with the standard
color.
8. WAYS OF MEASURING PH
This method is simple, but prone to error. A high
degree of accuracy cannot be expected.
Various errors include;
- Error due to high salt concentration in the test
liquid
- Error due to the temperature of the test liquid
- Error due to organic substances in the test liquid
10. WAYS OF MEASURING PH
Natural Indicators
- there are many common household products and
garden plants that can be used as pH indicators
- most plants contain pH sensitive anthocyanins
11. WAYS OF MEASURING PH
Metal-electrode methods (including the hydrogen-
electrode method, quinhydron-electrode method
and antimony-electrode method)
1. Hydrogen-Electrode Method
- made by adding platinum black to platinum
wire or a platinum plate
- it is immersed in the test solution and an
electric charge is applied to the solution and
platinum black with hydrogen gas
12. WAYS OF MEASURING PH
- is a standard among the various methods for
measuring pH
- not appropriate for daily use because of the
effort and expense involved
13. WAYS OF MEASURING PH
2. Quinhydron-Electrode Method
- involves immersing the tip of a polished
antimony rod into a test solution, also immersing a
reference electrode, and measuring pH from the
difference in potential between them
- was once widely used because the
apparatus is sturdy and easy to handle
- results vary depending on the degree of
polish of the electrode, and reproducibility is low.
14. WAYS OF MEASURING PH
3. Antimony-Electrode Method
- involves immersing the tip of a polished
antimony rod into a test solution, also immersing a
reference electrode, and measuring pH from the
difference in potential between them
- the apparatus is sturdy and easy to handle
- results vary depending on the degree of
polish of the electrode, and reproducibility is low.
15. WAYS OF MEASURING PH
Glass-Electrode Method
- pH measurement is applied to
plating, etching of metal surfaces and the
manufacture of batteries
- most widely used for pH measurement
because the balancing time of electrical potential is
short, it has high reproducibility
- it is rarely affected by oxidizing and reducing
agents, and it can measure pH of various solutions.
