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Unformatted text preview: rothermally treated at moderate
temperature (60-70°C) for 2 hours. 52 2. Experimental For coprecipitation at low supersaturation a system was build, which is depicted in
Figure 6. pH-meter with automatic temperature
compensation Mechanical stirrer Peristaltic micro pump
(4ml/min) pH controlled diaphragm pump
(activated at pH≤7.9 and
deactivated at pH≥8.1) thermoelement
pH electrode 0.074 mol/l
NaHCO3 aq. pH 8.0
Σ > 1mol/laq.
Heating plate NaOHaq.
2 mol/l Fig. 6
Set up for the preparation of hydrotalcite-type solids at low supersaturation The following preparation procedure was developed and strictly followed for every
preparation of HTs. The reaction vessel was filled with 500 ml of a freshly prepared 2. Experimental 53 ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
0.074 mol/l NaHCO3 solution (p.a., POCh, Poland) providing the carbonate ions
necessary for the synthesis of carbonate containing HTs. Considering the general
formula of the desired HTs M(II)2.6M(III)1.0(OH)3.6CO3 the solution contained a slight
excess of carbonate of about 5% . It was also found that the solution of NaHCO3 in
the reaction vessel before the coprecipitation is advantageous in order to keep the pH
value stable in the beginning, when the first precipitate is formed. The freshly calibrated
pH meter (Eurosensor, Poland) and the thermoelement (DHN, Poland), both connected
to the pH controller (Cole-Parmer, USA) were submerged into the solution. The pH
controller automatically compensated the influence of the temperature on the pH value.
The solution was stirred vigorously by a mechanical stirrer with a glass propeller and
heated up to 60-70°C. Just before the start of the coprecipitation, the amount of water
lost due to evaporation was added by adding water to the level marked before heating
up. The pH was adjusted to 8.0 by addition of small amounts of diluted HNO3 (POCh,
Gliwice, Poland) a...
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