Hanna Instruments HI934 Bedienungsanleitung Seite 129

4.3.4. 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
4.3.5. ENDPOINT DETERMINATION ERRORS
Most manual titrations use a visual indicator to indicate when the endpoint is reached and the titration should be
stopped. Automatic titrators use instrumental methods 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 the titration curve (mV vs. volume) 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 (∆mV vs. ∆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 (∆mV vs. ∆V
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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 (it is recommended to keep the sensors in good condition)
• Inappropriate setting on the titrator
) can also be used to determine the equivalence point, and can offer advantages
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