Hanna Instruments HI931, User Manual

Page: 206 / 218

12
TITRA
TION THEORY
2.2.8. KARL FISCHER TITRATIONS
This method is based on a well-defined chemical reaction between water and the Karl Fischer reagent. The chemistry provides
excellent specificity for water determination. The method can be used to determine free and bound water in a sample matrix. The
Karl Fischer method is widely considered to produce the most rapid, accurate and reproducible results and has the largest detectable
concentration range spanning 1 ppm to 100%.
The determination of water content is one of the most commonly practiced methods in laboratories around the world. Knowledge
of water content is critical to understanding chemical and physical properties of materials and ascertaining product quality. Water
content determination is conducted on many sample types including pharmaceuticals and cosmetics, foods and natural products,
organic and inorganic compounds, chemicals, solvents and gases, petroleum and plastic products as well as paints and adhesives.
The KF method is verifiable and can be fully documented. As a result, Karl Fischer titration is the standard method for analysis of
water in a multitude of samples as specified by numerous organizations including the Association of Official Analytical Chemists, the
United States and European Pharmacopoeia, ASTM, American Petroleum Institute, British Standards and DIN.
2.3. TITRATIONS ACCORDING TO THE TITRATION SEQUENCE
2.3.1. BACK TITRATIONS
Back titrations are generally used when a reaction is too slow to be directly accomplished using a “direct” titration, where the
reaction goes to completion within a few seconds. In a back titration, a large excess of a reagent is added to the sample solution,
helping a slow reaction to go to completion. The unreacted, excess reagent is then titrated. The difference in the total volume of the
first reagent added and amount determined from the second titration is the quantity of reagent required to complete the first reaction.
2.3.2. MULTIPLE ENDPOINT TITRATIONS
Under certain conditions, some titrations can exhibit more than one equivalence point and be titratable to the individual endpoints
to determine the concentration of each individual component. Examples of these types of titrations include acid-base (where different
strength acid or bases are in a mixture), redox (where each species has a different reduction potential), complexometric (where
different species are separately titratable), and acid-base using polyprotic acids (the pK
a
of the different protons varies enough to
separate them).
Figure 9 shows three different types of multiple endpoint titrations. “A” shows the titration of a polyprotic acid. The different acid
strengths of the first and second proton can be determined. “B” illustrates a mixture of two different metal redox species, where the
different redox potentials allow the species to be separated. “C” is the titration of a solution containing strong, weak, and very weak
acids.
Figure 9