Metrohm tiamo 1.2 Manual - Handbuch (Seite 398)

372 Method
MET
Command for Monotonic Equivalence Point Titrations ( MET ).
Principle
With this command titrations with reagent addition at constant volume increments
are carried out. Measured value acceptance is drift-controlled (equilibrium titra-
tion) or after a waiting period.
Note
This command is suited for titrations with relatively high signal variations or sud-
denly occurring potential jump and for slow titrations or slowly responding e-
lectrodes.
Evaluation
The equivalence points (EPs) are localized by a procedure based on the Fortuin
method which has been adapted by Metrohm for numerical methods. A search is
made for the largest measured value alteration (∆
n
). The exact EP is determined
by using an interpolation factor ρ; this depends on the ∆-values before and after
∆
n
:
V
EP
= V
0
+ ρ ∆V
V
EP
= EP volume, V
0
= total volume added before ∆
n
, ∆V = volume increment, ρ
= interpolation factor according to Fortuin
For the recognition of the EPs found the set EP criterion is compared with the ERC
( E quivalence point R ecognition C riterion) found. The ERC is the sum of the meas-
ured value alterations before and after the jump:
|∆
n–2
|+|∆
n–1
|+|∆
n
|+|∆
n+1
|+|∆
n+2
|
(In certain cases only three or only a single summand are taken into account.) EPs
whose ERC is smaller than the set EP criterion will not be recognized.
Commands
Depending on the measured value one of the following four MET commands can
be chosen:
• MET pH
Potentiometric pH measurement with pH electrodes (measured quantity
pH).
• MET U
Potentiometric voltage measurement with metal electrodes (measured
quantity voltage U).