Metrohm 796 Titroprocessor - Manual (Page 111)

7. Titration and measuring modes
796 Titroprocessor, Instructions for use 105 105
V [mL]
U [mV]
MET: Monotonic titration
Reagent addition:
constant volume increments
Measurement acceptance:
drift-controlled and/or after a
waiting period
Three predefined sets of parameters, which can be further optimized, are
available in the 796 Titroprocessor for setting the titration control.
7.3.3 Automatic equivalence point recognition
In monotonic equivalence point titrations the equivalence points of a titration
(i.e. inflection points of the titration curve) are evaluated according to For-
tuin.
In order to recognize an equivalence point (
EP) an auxiliary quantity ERC
( Equivalence point Recognition Criterion) is used. In order to avoid inter-
preting background noise of the measuring signal and unwanted interfer-
ence signals as being equivalence points an adjustable threshold value can
be set for the equivalence point criterion. Supposed EPs which have a
smaller equivalence point criterion (
ERC) than this threshold value will not
be recognized as such. The EP criterion ERC is obtained from the 1st de-
rivative of the titration curve and combined with a mathematical function
which detects flat potential jumps more sensitively than steep jumps. The
796 Titroprocessor can process up to 9 equivalence points.
General evaluation range
A common requirement is that the automatic equivalence point evaluation
should only be carried out for a limited range of measurements. For this
purpose a general upper and lower limit for the required range of measure-
ments for the equivalence point recognition can be set.
If several equivalence points are expected in a titration the EP recognition
can be optimized. If only one equivalence point is to be evaluated a selec-
tion can be made as to whether the ‘largest’ (the steepest potential jump) or
the last equivalence point should be evaluated.
EP window
If several equivalence points are of interest then for each expected
EP (EP1
to EP9) an EP window with separate equivalence point criterion ( EPC) can
be defined. An EP window is defined by the limiting values of the measure-
ment range within which limits the equivalence point is to be recognized. In
a single EP window only one equivalence point can be evaluated. The dif-
ferent EP windows must not overlap.
7.3.4 Fixed endpoints
Fixed endpoints (F1 to F9) can be set for the determination of a value for the
volume for a predefined measurement. This function is advantageous for
the application of certain conventional methods; for TAN/TBN deternina-
tions, for example, it is even absolutely necessary. For pH titrations the sen-
sor used must have been previously calibrated.