BUCHI KjelSampler K-377 - Manual (Page 36)

4 Description of function
36 K-375/376/377 Operation Manual, Version B
4 .5 Distillation and titration
4 .5 .1 Distillation Mode
Automatic – IntelliDist
This mode eliminates errors caused by a cooled instrument. The countdown of the set distillation
time only starts after operating temperature is attained. With single samples or sample list measure-
ments this mode guarantees result accuracy from the very first run.
Fixed Time
The countdown of the set distillation time starts immediately with the start of the distillation process.
This setting is recommended when a sample changer is used for the analysis of samples in a rack (or
sequence).
4 .5 .2 Titration Type
The built-in titrator is fully controlled via the K-375 software. It is not possible to use the titrator
without the KjelMaster K-375. It can be used for boric acid or back titration. The measuring mode
can be determined as endpoint or startpoint titration by defining the method in the K-375. The soft-
ware of the K-375 allows to choose between standard and online titration. The endpoint detection
can be performed either with a potentiometric sensor or a colorimetric sensor in combination with a
color-indicator.
Boric Acid Titration
Boric acid is used as receiving solution to capture the nitrogen carried over as ammonia during the
steam distillation. The subsequent endpoint titration is performed with an acid titration solution. This
titration type is easy to carry out and does not require an accurate dosage of the boric acid.
Back Titration
The receiving solution is a standardized acid of which an accurate volume is dispensed into the
receiving vessel. After collecting the ammonia the excess acid is titrated with a basic titration solu-
tion. If the use of boric acid has to be avoided the back titration is the procedure of choice.
4 .5 .3 Sensor Type
Potentiometric
Potentiometric electrodes for the pH measurement are commonly used and allow both boric acid
and back titrations. They need regular calibration with buffers and are the most cost-effective solution
for end point titrations.
Colorimetric
Optical sensors used for colorimetric titrations detect the color change of an indicator at the equiva-
lence point of the titration curve. The indicator has to be added to the receiving solution. These
sensors do not require calibration, have an extended life time but are more expensive.
4 .5 .4 Titration Mode
Standard
In the standard mode the distillation and titration are performed sequentially. First the distillation is
completed before the titration starts.
Online
With this mode activated the titration will start while the distillation is still in progress. The start time of
the titration depends on the pH value and is determined automatically. The online mode can only be
used with potentiometric titrations. It helps optimize the speed of measurements as it saves time.