6.2 Calculable Quantitation Methods
269
If the reaction fails to meet any of the above-mentioned requirements, one can
still use an indirect procedure to determine A (a back-titration). To this end, a
conventional analytical reaction is previously conducted by adding an accurately known excess of a standard solution of reagent R to the sample solution:
A + R H B (excess R remains)
controlled
excess
Then, excess R not reacted with A is titrated by adding a new reagent R' from the
burette until a new end-point is reached:
excess
The analyte concentration is obtained from the difference between the millimoles (volume x concentration) of the two analytical chemical standards used
(R and R'), which are indispensable with a view to establishing the traceability
chain. Box 4.12 describes two typical examples of back-titration.
__ Box6.12
Back-Titrations
A large number of analytes (A) cannot be determined by direct titration with a standard
reagent (B) for a variety of reasons the most common of which are (a) the lack of stable
titrants in solution, (b) slowness in the A + R reaction and (e) the lack (or high cost) of a
suitable end-point indicator. Under these Circumstances, one must resort to an indirect titrimetric procedure based on a preliminary analytical reaction. Below are discussed two typical
examples.
Example 1
When no metallochromic indicator is available to titrate a metal ion M2+ with EDTA or the
chelate formation reaction is too slow, two standard solutions (one of EDTA and the other of
a different metal ion, M, 2+) are prepared and used as follows: (1) an accurately measured
volume of the EDTA standard solution (a controlled excess) is added to the analyte (M 2 )
solution, which is then pH-adjusted with an appropriate buffer - if required, the mixture is
heated -; (2) excess EDTA remaining after reaction with the analyte is titrated by addition
of M, lt standard from the autoburette, in the presence of a suitable indicator for the
EDTA-M, 2 titrimetric reaction. The analyte concentration is obtained by difference as explained in the text.
Example 2
The 1 2 /21 - redox system is widely used to determine both oxidants and reductants; however,
standard solutions of both species cannot be prepared in practice. Instead, an indirect titrimetric procedure is normally employed. One of the most commonplace is that used in the
determination of oxidizing analytes such as dissolved chlorine. Pool water is supplied with an
unmeasured excess of KI to form iodine according to
(1 2 + 21 - H 12 + 2CI -
269
If the reaction fails to meet any of the above-mentioned requirements, one can
still use an indirect procedure to determine A (a back-titration). To this end, a
conventional analytical reaction is previously conducted by adding an accurately known excess of a standard solution of reagent R to the sample solution:
A + R H B (excess R remains)
controlled
excess
Then, excess R not reacted with A is titrated by adding a new reagent R' from the
burette until a new end-point is reached:
excess
The analyte concentration is obtained from the difference between the millimoles (volume x concentration) of the two analytical chemical standards used
(R and R'), which are indispensable with a view to establishing the traceability
chain. Box 4.12 describes two typical examples of back-titration.
__ Box6.12
Back-Titrations
A large number of analytes (A) cannot be determined by direct titration with a standard
reagent (B) for a variety of reasons the most common of which are (a) the lack of stable
titrants in solution, (b) slowness in the A + R reaction and (e) the lack (or high cost) of a
suitable end-point indicator. Under these Circumstances, one must resort to an indirect titrimetric procedure based on a preliminary analytical reaction. Below are discussed two typical
examples.
Example 1
When no metallochromic indicator is available to titrate a metal ion M2+ with EDTA or the
chelate formation reaction is too slow, two standard solutions (one of EDTA and the other of
a different metal ion, M, 2+) are prepared and used as follows: (1) an accurately measured
volume of the EDTA standard solution (a controlled excess) is added to the analyte (M 2 )
solution, which is then pH-adjusted with an appropriate buffer - if required, the mixture is
heated -; (2) excess EDTA remaining after reaction with the analyte is titrated by addition
of M, lt standard from the autoburette, in the presence of a suitable indicator for the
EDTA-M, 2 titrimetric reaction. The analyte concentration is obtained by difference as explained in the text.
Example 2
The 1 2 /21 - redox system is widely used to determine both oxidants and reductants; however,
standard solutions of both species cannot be prepared in practice. Instead, an indirect titrimetric procedure is normally employed. One of the most commonplace is that used in the
determination of oxidizing analytes such as dissolved chlorine. Pool water is supplied with an
unmeasured excess of KI to form iodine according to
(1 2 + 21 - H 12 + 2CI -