Hanna Instruments HI 903, Quick Start Manual

Page: 210 / 220

20
TITRATION THEORY
4.3.2.2
Dispensing Errors
Incorrect dispensing due to:
•
Dead valve volume and leaking valve;
• Inaccuracy in motor drive and gear lash/ backlash;
• Poor burette/ piston seal;
•
Non-uniform diameter of burette glass cylinder;
•
Chemical incompatibility with tubing or bubble generation;
•
Density/ temperature changes in titrant.
•
Inadaquate volume to cover electrode.
4.3.3
Chemical Reaction Errors
• Inappropriate solvent or sample resulting in side reactions;
• Poor mixing of the titrant and solvent or sample in the titration vessel;
•
Reaction between titrant and sample is not rapid;
•
Reaction does not go to completion;
•
Reaction has side reactions.
4.3.4
Endpoint Determination Errors
Most manual titrations use a visual indicator to indicate when the endpoint is reached and the
titration should be stopped.
A utomatic titrators can use potentiometric electrodes to determine
the end of a titration and the equivalence point. There are two predominant methods used to
determine the equivalence point, first derivative and second derivative.
The inflection point of a potentiometric curve (m
V vs. V olume) is normally assumed to be
the equivalence point. The first derivative is often used to determine the inflection point. The
maximum value of the first derivative (D m
V vs. D V ) corresponds to the theoretical equivalence
point. During a titration it is rare to have a data point exactly at the first derivative maximum,
the maximum value is determined by interpolating the first derivative data points.
The second derivative (D
2
m V vs. D V
2
) can also be used to determine the equivalence point,
and can offer advantages over the first derivative method. Second derivatives have increased
sensitivity to smaller inflection points and easier numerical evaluation of the actual equivalence
point. The value where the second derivative is equal to zero is the equivalence point. The
second derivative requires fewer points located near the equivalence point, where data is
often not obtained or not as reliable.
Errors in determining the endpoint can result from:
• Incorrect signals from the sensor;
• Sensor drift;
• Sensor or instrument has slow response, keep sensors in good condition;
• Inappropriate setting on the titrator.