Metrohm 916 Ti-Touch - Manual (Page 304)

28.1 Dynamic equivalence point titrations (DET)
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭
288 ￭￭￭￭￭￭￭￭
916 Ti-Touch
dows (e.g. EP2 in window 2), so that even if EPs are missing, the calcula-
tions will still be carried out with the correctly assigned EP volumes.
EP1
EP window 2
EP window 1
EP2
U/mV
V/mL
EP window 1
EP1
+
U/mV
V/mL
lower limit 1
upper limit 1
Figure 21 Equivalence point recognition and equivalence point num-
bering within windows
In the first example in the above figure, two equivalence points are recog-
nized in two different windows (EP1 in window 1 and EP2 in window 2).
In the second example, two equivalence points are found in one window,
but only the first one is recognized. In order to ensure that the user recog-
nizes that more than one equivalence point was found in the set window,
EP1 is marked with a "+" in the result view. In addition, a corresponding
message is entered in the message list.
Evaluation and equivalence point criterion with DET
The equivalence points (EP) are localized in a way similar to the Tubbs
method [1][2]. The volume value of the equivalence point (V
E
) is shifted
from the inflection point (see arrow) towards the smaller circle of curva-
ture for real asymmetric titration curves.
[1] C. F. Tubbs, Anal. Chem. 1954, 26, 1670–1671.
[2] E. Bartholomé, E. Biekert, H. Hellmann, H. Ley, M. Weigert, E. Weise,
Ullmanns Encyklopädie der technischen Chemie, Vol. 5, Verlag Chemie,
Weinheim, 1980, p. 659.
V
E
pH
V/mL
Figure 22 Tubbs method for determining the equivalence point