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[Espectrofotometria] Análise de AAS por método FIA

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safety
reasons, these PTFE tubes were passed through the walls of the
microwave oven using the external oven air vents. The
hydrolysed sample passed through the PTFE coil B2 (100 cm,
0.8 mm id), immersed in a water bath. To avoid the presence of
bubbles in the detector, produced during the passage of the
sample in the coiled reactor inserted in the microwave oven, a
T-shaped glass debubbler (aspiration rate, 0.2 ml min21) was
incorporated between the water bath and the confluence point y.
Then, the sample zone is mixed with the R2 reagent [0.50%
(w/v) Fe(NO3)3 in 0.4 mol l21 nitric acid, flowing at 1.3
ml min21] in the reactor coil B3 (100 cm, 0.8 mm id). The
absorbance resulting from the FeIII–salicylate complex,
measured at 525 nm, is proportional to the ASA concentration
of the sample. To avoid damage in the magnetron a beaker
containing 500 ml of water was placed inside the microwave
cavity. The volume of water that evaporated during the
hydrolysis step was continuously replaced by pumping water in
at a flow rate of 4.8 ml min21 (C3 channel).
Results and discussion
ASA can be hydrolysed rapidly in an alkaline medium to its
constituent salicylic and acetic acids.1 In the conventional
analytical procedure,2 complete hydrolysis is attained after 10
min heating at reflux, followed by back-titration with standard-
ized acid solution.
After hydrolysis the SA content is proportional to the ASA
concentration present in the sample. SA can be quantitatively
determined by complexation with FeIII ions and the resulting
colour read at 525 nm. In this work, a domestic microwave oven
was exploited as the energy source to accelerate the hydrolysis
of ASA (about 20 s) in order to minimize the time of analysis
and increase the sensitivity of the detection.
Study of chemical parameters
Initially, the FI manifold was used to investigate the conditions
of ASA hydrolysis. In all preliminary experiments, the coil
lengths of B1, B2 and B3 were 300, 200 and 100 cm,
respectively. The L1 and L2 loop volumes were 250 ml and fixed
flow rates of 3.0 ml min21 were used for C1 and C2 and 3.9
ml min21 for R2 reagent. The maximum power (600 W) of the
microwave oven was used.
The acidity of the R2 reagent is important because the nitric
acid concentration should be sufficient to neutralize the sample
zone and to give an acid pH in which the reaction between FeIII
ions and SA occurs. Increment of NaOH (R1 reagent)
concentration requires an increase in the R2 reagent acid
concentration. 
The effect of the alkalinity on the baseline stability was
studied replacing the sample (L1) by water. Within the
concentration range 0.05–0.25 mol l21 NaOH studied no
baseline drift was observed maintaining a concentration ratio of
2 : 1 (HNO3, R2 : NaOH, R1). A ratio lower than 2 : 1 could not
be used because it caused baseline noise, due to incomplete
neutralization of the injected alkaline zone.
The effect of NaOH concentration (R1 reagent) on the ASA
hydrolysis was evaluated over the concentration range from
0.05 to 0.25 mol l21. In this experiment, the nitric acid
concentration of R2 reagent [1% (w/v) Fe(NO3)3] was twice that
of NaOH (R1). ASA standards in the concentration range from
50 to 400 mg ml21 were used. Fig. 2 shows that the absorbance
intensity increased with an increase in NaOH concentration up
to 0.20 mol l21, above which it increased only slightly. In this
Fig. 1 Schematic diagram of the FI system for spectrophotometric determination of ASA. The three rectangular pieces represent a scheme of the sliding
bar injector. S, sample or standard solutions; L1, sample loop (50 cm, 250 ml); L2, R1 reagent (0.2 mol l21 NaOH) loop (50 cm, 250 ml); C1, C2 water carriers
streams, flowing at 3.4 ml min21; C3, water auxiliary channel pumped at 4.8 ml min21; B1, B2 and B3, reactor coils (200, 100 and 100 cm, respectively);
R2 reagent solution, 0.5% (w/v) Fe(NO3)3 in 0.4 mol l21 nitric acid, flowing at 1.3 ml min21; Db, glass T-debubbler, aspiration at the top 0.2 ml min21; D,
spectrophotometer at 525 nm; x and y, confluence points; W, waste. The numbers in parentheses are the flow rates in ml min21.
1012 Analyst, May 1998, Vol. 123
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work, a 0.2 mol l21 NaOH solution and Fe(NO3)3 in 0.4 mol l21
nitric acid solution were chosen for the R1 and R2 reagents,
respectively.
Study of microwave oven conditions
The efficiency of hydrolysis of the ASA is influenced by the
microwave oven power level (Fig. 3). The microwave oven has
five power levels corresponding to 60–600 W output. Increas-
ing the power level (60–600 W) gave continuous increase in the
sensitivity followed by irreproducible results (probably due to
the pulsed way that the energy is supplied by the magnetron)
was observed up to 420 W. The best hydrolysis efficiency and
more reproducible results were obtained using maximum power
(600 W). However, when maximum power was used bubble
formation occurred and caused problems in the measurements.
Efficient removal of the bubbles generated during the micro-
wave-assisted hydrolysis was obtained when a T-debubbler was
incorporated in the FI system maintaining a higher sensitivity.
Therefore, a maximum power level was selected for this work.
To obtain reproducible results, it is recommended that sample
injections are started 5 min after turning on the microwave oven.
Study of FI parameters
The effect of the reactor coil length B1 placed inside the
microwave was studied between 100–300 cm using PTFE
tubing (0.8 mm id) and maintaining constant all other
parameters as described above. Increasing the length of B1 coil,
a continuous increase of the sensitivity was observed (Fig. 4).
There was an increase of only 15% in sensitivity when B1 coil
length was increased from 200 to 300 cm. Therefore, a length of
200 cm was chosen as a compromise between sensitivity and
sample throughput.
The effect of the sample and R1 reagent volumes (L1 and L2)
was studied by changing the lengths of L1 and L2 loops in the
manual injector. The volumes were varied equally between 62.5
and 312.5 ml. The absorbance was found to increase with the
sample volume up to 250 ml, without meaningful increase to
312.5 ml. Thus, 250 ml volumes were chosen for the sample and
R1 reagent. 
The influence of the reactor coil lengths B2 and B3 on the
sensitivity of the FI method was studied in the range 100–300
and 50–200 cm, respectively. The results showed that the
absorbance decreased with increasing length due to the
dispersion of the sample zone. A length of 100 cm was chosen
for B2 in order to minimize the dispersion and maintain an
efficient cooling time. A length of 100 cm was chosen for B3 in
order to obtain ideal mixing of the hydrolysed sample with
Fe(NO3)3 solution in 0.4 mol l21 nitric acid and attain an
appropriate acid medium for the solution flowing through the
detector.
The C1 and C2 water carrier flow rates were optimized by
varying simultaneously these streams while keeping constant
the flow rate of R2 reagent stream at 3.9 ml min21. When flow
rates were increased from 1.5 ml min21 to 4.0 ml min21 it was
found that the sensitivity increased up to 3.4 ml min21, above
which it showed a slight decrease. Therefore, flow rates of 3.4
ml min21 were selected for both C1 and C2 water carrier
streams.
The effect of flow rate of ferric nitrate reagent solution on
sensitivity was studied using 1.0% (w/v) Fe(NO3)3 in 0.4
mol l21 nitric acid and varying the flow rate from 1.0 to 4.0
ml min21. A decrease was observed in the sensitivity with an
increase of the flow rate probably due to a decrease in the final
pH. The intensity of absorption of the FeIII–salicylate complex
depends on the pH value and it has been reported that maximum
intensity occurs at pH 2.5–3.0 and is five times higher than that
at pH 1.5.15 In all further experiments, a flow rate of 1.3 ml
min21 was selected for the R2 reagent. No baseline drift was
observed for blank