Chemistry Reference
In-Depth Information
flows, a pH value is obtained situated between 11.5 and 13. Within this pH
area, the sensor system is useable in its actual configuration, and it is the
basis of the measuring system of the complete setup (analysis).
Some disadvantages are associated with this system. A first disadvantage
is the disappearance of the continuous character of the sensor since a
certain period of time elapses between the measurement of electrical cur-
rent at the sensor surface and the moment in time when the sample to be
analysed leaves the process bath. Whereas the developed sensor is intrinsi-
cally a continuous working system, it is clear that a basically discontinuous
system can be considered as virtually continuous when the duration of the
measurement is situated below a specific critical threshold, the 'dead time'
for every considered application. One of the tasks of the research is to keep
the dead time as short as possible, and if necessary take this into account
when the global process is directed by means of the output signal of the
sensor expanded with a FIA system.
Another problem is waste generation. Since, after detection, the solution
has a pH of 11.5-13, it cannot, depending on the application, be sent back
to the bath where the process occurs at a lower pH. This means that the
waste quantity created by the system must be kept as low as possible and
that one should miniaturise the sensor configuration as much as is practi-
cable. According to a limited survey realised with the users of hydrogen
peroxide, a waste volume of 1 l/h is considered as acceptable. Finally, a dilu-
tion factor should be taken into account, which depends on the volumes of
both liquid flows mixed in the injection area of the FIA system.
5.5.2
The FIA system
The principle of the constructed FIA system is shown in Fig. 5.16.A tank with
a 15 l volume replaces the bath where the actual industrial process occurs.
By means of a peristaltic pump, the process solution is tapped from the bath
and sent to the mixing chamber through flexible tubes having an inside diam-
eter of 1 mm. In the same way, a constant liquid flow of a sodium hydroxide
solution is pumped to the mixing chamber with the same peristaltic pump.
The concentration of this solution is such that, after mixture with the process
solution, a pH of 12.5 is obtained. The concentration of the sodium hydrox-
ide solution is hence adjusted to the pH of the process solution.
The mixing chamber is a T-junction having a 1-mm inside diameter and,
as a result, the two liquid flows (process solution and sodium hydroxide
solution) meet each other under a 90° angle. Subsequently, the mixed solu-
tion is sent to the detection cell (Fig. 5.17). The dimensions of the working
electrode and auxiliary sensors are adjusted in such a way that they strongly
decrease (ca. 50%) the internal volume actually taken in by the solution.
The actual volume of the detection cell amounts to ca. 7 ml. In this config-
