Hach-Lange POLYMETRON 9240 Operator Manual - Manual (Page 25)

23
Analyzer Overview
3.5 pH regulation
3.5.1 Non-cationic applications
In order to ensure the accuracy and the repeatability of low sodium concentrations, pH must be
constant and preferably maintained at or above 11.2 to maintain the lowest proton interference.
The 9240 uses the injection of vapor of diisopropylamine (DIPA) to obtain high pH level without
sodium contamination. Using the siphon effect of the liquid sample column, DIPA is mixed with
the sample without using any pump or pressurized gas. A fine regulation of the siphon effect is
performed using a 2/3-way valve.
The pH is evaluated directly and continuously from the conductivity of the conditioned sample in
the cell. In the solution, the most mobile proton (H
+ ) has the biggest influence on conductivity.
Therefore, by controlling the conductivity, its concentration can be maintained constant. In
addition, the pH is controlled from the conductivity by modifying the gas/liquid ratio thus
simultaneously maintaining a constant pH.
In temperature variations, the system automatically modifies the gas/liquid ratio and then adds
DIPA vapors to compensate the DIPA solubility in the sample.
If the sample is already partially conditioned by a customer system (e.g. with NH
3
up to pH 9.5 -
10.5), the regulation system evaluates the pH and adds the necessary DIPA quantity to reach its
pH
target
± 0.2 and minimizes DIPA consumption.
The pH can be set to between 10.7 and 11.6 pH. The DIPA consumption is 500 mL/month at
25°C for a sample pH
target
of 11.2.
3.5.2 Cationic applications
Instruments to be used in cationic applications are configured at the factory and can be
identified by their model number of 09240=A=1xxx (the 1 identifies it as being for cationic
applications).
Measurement of trace level sodium concentrations after cationic resin processes requires an
assisted conditioning because the pH in the sample to be measured will only be between 2 and
4.
The 9240 analyzer uses a gas pump with no moving parts (based on the piézo principal) to
effect this assisted conditioning. The pump is controlled by the analyzer’s electronics and works
according to the amount of sample conditioning required. The flow rate of the pump is factory
set at 200 cc/min of gas.
An exit tube ensures that any DIPA vapors not dissolved in the sample are recovered and
evacuated to the drain along with the sample.
In a cationic application, the 9240 does not use conductivity to evaluate the pH. However, it can
be adjusted according to the sample pH by a time ratio of the conditioning valve aperture. A
specific T
gas
/T
water
ratio is used for each channel in order to minimize the DIPA consumption
and to adjust a constant pH.