Hanna Instruments HI934 Bedienungsanleitung Seite 122

4
Electrometric methods result in over-titration or titration past the equivalence point where excess iodine is present in
the titration solution. Titration past the equivalence point is acceptable for two reasons. First, due to the sensitivity
of the electrometric methods, titrations are always carried out to the same, slight excess of iodine resulting in highly
reproducible titrations. Second, the accuracy of electrometrically indicated titrations are not affected by the over-titration
because the slight excess of iodine has been accounted for during the standardization of the titrant.
2.2.2. ACID-BASE TITRATIONS
Acid-base titrations are the most common type of titrations. Acid-base titrations are based upon a reaction between an
acid and a base, a stoichiometric neutralization, or the exchange of protons. Virtually all acid-base titrations are carried
out using a strong acid or a strong base as the titrant. The endpoint of a titration carried out with a weak acid or a weak
base, would be difficult to detect due to a small change in pH at the equivalence point.
Chemical indicators are often used to determine the endpoint. The indicator will change color to signify that the end of
the titration has been reached. When choosing the proper indicator you should select one that has a pK as close to the
a
endpoint of the titration as possible. The color-change region of the indicator is usually ± 1 pH unit around the pK .
a
The theoretical titration curve is useful for illustrating how the solution will change during the real titration, and allowing
the proper selection of an endpoint or an indicator.
Figure 4 shows a traditional titration curve. The curve is obtained by plotting the pH value against the volume of NaOH
added.
Figure 4
2.2.3. ARGENTOMETRIC TITRATIONS
Argentometric titrations use silver (nitrate) as the titrant and are generally precipitation titrations, as many silver salts
are insoluble. These titrations are commonly used to titrate and determine the concentration of bromide, chloride,
cyanide, iodide and sulfide.
Argentometric titrations can be done with Mohr's indicator. After all of the chloride has reacted, a red silver chromate
precipitate is formed or the titration can be easily followed with a silver ISE (or chloride ISE for chloride titrations) and
a reference electrode.
4-8