Monograph - Metrohm

Practical thermometric titrimetry 19
Table 3: Examples of the calculation of the molarity of the titrant from the determination of titrant concentration
by regression analysis.
Analyte Titrant Reaction Molarity
NaOH HCl NaOH + HCl NaCl + H 2 O 1 divided by gradient
H 2 SO 4 NaOH H 2 SO 4 + 2NaOH Na 2 SO 4 + 2 H 2 O 2 divided by gradient
Al 3+ F – Al 3+ + Na + + 2K + + 6F – NaK 2 AlF 6 6 divided by gradient
An example of this approach to the determination of both method blank and titrant
molarity is the thermometric titration of aluminum with fluoride ions. A primary standard
solution of aluminum is made by dissolution of high-purity aluminum metal. Aliquots of
this solution have been taken to prepare the diluted standard solutions. By plotting the
aluminum ion concentration of these standards against the required volume of fluoride
ion titrant, Figure 10 is obtained. The regression was carried out using the linear regression
function in Microsoft Excel.
Fig. 10: Example of a calibration curve.
Results of regression analysis:
Gradient = 6.30520 mL/mmol
Y-intercept (method blank) = 0.0264 mL
Correlation coefficient (R 2 ) = 0.999997
Calculation of NaF molarity:
Molarity = 6 divided by gradient
(because 6 moles fluoride ion react with 1
mole aluminum ion).
Molarity = 6 / 6.30520 = 0.9516 mol/L NaF
Therefore the diluted NaF standard solutions have a titer of:
An advantage of this approach is that not only two critical parameters can be determined
at the same time, but also the analyst has the opportunity to check the linearity
of response over the measurement range in question.