Metrohm 840 PC Control 5.0 / Touch Control - Manual (Page 214)

4.1 Titrations
202
PC Control / Touch Control
the last (only the last EP to be found), ascending (only EPs with a
positive titration curve slope), descending (only EPs with a negative
titration curve slope).
Only one equivalence point will be recognized per window. The num-
bering of the equivalence point (EP) is determined by the numbering of
the window (e. g. EP2 in window 2), so that even if EPs are missing the
calculations will still be carried out with the correctly assigned EP vol-
umes.
EP1
EP window 2
EP window 1
EP2
U/mV
V/mL
EP window 1
EP1
l. limit 1
+
U/mV
V/mL
u. limit 1
Fig. 4.6: Equivalence point recognition and numbering in windows
In the first example in Fig. 4.6 two equivalence points have been recog-
nized in two different windows (EP1 in window 1 and EP2 in window 2).
In the second example two equivalence points have been recognized in
a single window, but only the first one will be recognized and the sec-
ond jump will not. In order that the user can recognize that more than
one equivalence point has been found in the set window, EP1 is
marked with a ‘+’ in the result display. In addition, the message Sev-
eral EPs in the window will be entered in the list of messages.
The evaluation of the titration curve and therefore the EP criterion for
the recognition of the equivalence points is different for dynamic
equivalence point titrations (DET) and monotonic equivalence point ti-
trations (MET).
Evaluation and equivalence point criterion in the DET mode
Equivalence points (EPs) are determined in a similar manner to that de-
scribed in the Tubbs method (C.F. Tubbs; Anal. Chem 26 (1954) 1670–
1671, quoted in Ullman 5 (1980) 659). The volume of the equivalence
point (V
E
) is shifted from the point of inflection towards the smaller circle
of curvature for real asymmetric titration curves.