Materials cylinder was then rotated at _ RPM in

Materials and methods

 The chemicals used in this experiment were benzoic acid,
NaOH, oxalic acid and HCl sourced from the SD Fine Chemicals Limited. NaOH and
HCL was standardized using oxalic acid(0.2N). Benzoic acid cylinder was
supported on central steel shaft which can be rotated at varying RPM controlled
by digital RPM meter . A D.C Motor is connected to it can be seen in Figure 1.  Benzoic Acid solid is considered  pure and clay-free. The shaft and D.C Motor  have been obtained from Balaji Scientific
Group, Dombivali. Conical Flasks, beakers, burette, measuring cylinders and
pipettes  used in  the experiment were brought from a local
vendor in contract with the institution. Burette and pipette have an
uncertainty of ± 0.1 ml and measuring cylinder has an uncertainty of ±1 ml.  Schematic of the experimental setup is shown
in Figure 1.

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Figure 1.schematic diagram of experimental
setup

To find the
dissolution in the presence of reaction, we measure the  dimensions of benzoic acid cylinder  which was then fixed in a vertical position
with the D.C. motor . The cylinder was then rotated at _ RPM in aqueous NaOH
solution (300 ml) in a beaker and it was made sure that the cylinder was dipped
completely inside the NaOH solution. After 10 minutes the benzoic acid cylinder
was removed from the solution and the NaOH sample was titrated against HCl to
determine the unreacted NaOH concentration. The cylinder dimensions were
measured again.  For 5 different NaOH
concentrations ranging from 0.1 N to 0.5N and four different speeds
between  _RPM this procedure was
repeated.

Physical
dissolution

To determine
the physical dissolution rate, the benzoic acid cylinder was rotated at
same  RPM as above in 200 mL of water for
30 minutes. At every 5 min intervals 5 mL samples  were analyzed for benzoic acid content by
titration against 0.02N NaOH solution using phenolphthalein indicator. Every
time sample is withdrawn  5 mL of makeup
water is added .