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142
Fig. 138
Fig. 139
Replacing the titration solution
5.2.2
If titration solutions are to be changed, since differing analysis methods are used, one should first consider
whether the time required for frequent changes is not more expensive than the acquisition of another dosing unit.
As a principle and in the case of all piston/cylinder- systems, a substitution of the titration solution by another one
involves mixing and carry-over processes. The reason for this is the dead volume above the piston in the cylinder
and in the hoses. The disturbances to be anticipated are the greater, the more the new solution differs from the
previous type and concentration. In the case of highly different solutions, the first substitution liquid (rinsing)
should be distilled water, and the new titration solution should be filled in only subsequently.
The possible disturbances are very much different in the individual cases and cannot be predicted without
knowledge of the specific case. Therefore the replacement of titration solutions must always be performed under
the supervision of experts who ensure the correctness of the future analyses.
If the decision to change the titration solution has been made, the first thing to do is to remove the dosing unit as
it is described in 5.2.1. If possible, the residue of the titration solution should be removed by hand by carefully
pushing the projecting piston rod towards the hoses. When doing so, more liquid will leak out of the titration tip,
and the residual volume is furthermore reduced. Removing the old titration solution can be accelerated by moving
the piston rod of the dosing unit positioned top down. The suction hose is then immersed in the new solution or in
water as intermediate liquid. By moving the piston several times in both directions (pumping) the previous liquid is
gradually replaced by new liquid. Subsequently, the dosing unit is set on again according to the description in
5.2.1.
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