Agriculture Reference
In-Depth Information
CHAPTER
6
TITRIMETRIC MEASUREMENTS
Titration
is a general word used in many different disciplines. Any time a
solution of known concentration is used to find the amount of an unknown
component in another solution, it can be called a titration. Although this
type of analysis is very old, it still finds widespread use in chemical
analysis. Titrations are used in soil analysis to determine soil acidity, soil
organic matter, and various constituents isolated from soil, particularly
ammonia.
Common chemical titrations are acid-base, oxidation-reduction, precipita-
tion, and complexometric and have been in use for a very long time. In all
cases the basic concepts are contained in a classical acid-base titration. A
known amount of acid is placed in a flask and an indicator added. The indi-
cator is a strongly colored compound that is one color at acid pHs and a dif-
ferent color at basic pH. A base of precisely known concentration, the titrant,
is then added to the acid until the pH becomes neutral as indicated by the
indicator changing color. From the amount of titrant added, the amount of
acid originally present is determined. This is a typical titration; however, a pH
meter can also be used to detect the endpoint using a pH electrode, by fol-
lowing the electrode potential. Meters, usually pH meters, capable of measur-
ing millivolts (Eh) or ion-selective electrodes (ISE), can also be used in a
similar same way.
The endpoint in a titration is a little bit different from the end of a reac-
tion. What is desired is to know when all the acid is titrated. This happens when
the titration curve, shown in Figure 6.1, changes from acid to base. This change
occurs when it passes pH 7 (for a typical strong acid-base titration), neutral,
pH and when a color indicator is used, when there is a change in color. If the
milliliter of acid added is plotted against pH, then a titration curve such as that
shown in Figure 6.1 is obtained. The endpoint is the portion of the curve with
the greatest slope.
In most cases a curve is not drawn and the endpoint is taken as the milli-
liters (mL) used just before the color change takes place. There should be a
half-drop of titrant difference between the change from one color to the next.
In many cases the color change is very light but distinctive. If the titration
curve is plotted, then the endpoint can be determined by inspection or by
