Brazilian Journal of Analytical Chemistry - BRJAC - Brazilian Journal ...

containing the same proportion of n-propanol as in
the sample solution.
3.2 sTabiliTy of na an d k in Th e so l v e n T
The final glycerin solution is an aqueous solution
containing 30% v/v n-propanol, 1% v/v HCl, 0.2%
Cs w/v and the glycerin aliquot. After preparing the
raw glycerin solution in a polypropylene flask, the Na
and K signal intensities were measured every 10 min
up to 120 min. For comparison purpose, the signal
intensities were measured for an aqueous solution
containing 0.5 mg L -1 of Na and K. As shown in
Figure 2, the analytes were stable in the medium of
30% v/v n-propanol during the investigated period
of time.
fig u r e 2. si g n a l i n T e n s i T y sTabiliTy f o r na a n d k: (-n-) r a w
g l y C e r i n d i l u T e d w iT h 30% v/v n-pr o p a n o l in w a T e r a n d (-Δ-)
a q u e o u s sT a n d a r d s Co n T a i n i n g 0.5 m g l -1 of na an d k.
3.3 fi g u r e s of me r iT
The figures of merit are shown in Table I for both
analytes. For comparison purposes, the slopes of the
calibration curves using aqueous standards are also
presented. It is shown that the signal intensities increase
in the presence of 30% v/v n-propanol, probably
due to the lower surface tension of the measuring
solution, as already discussed. The proposed
Br J Anal Chem
sim p l e an d fa s T me T h o d fo r Th e de T e r m i n a T i o n of na an d k in r a w gl y C e r i n fr o m biodiesel
method requires calibration in 30% v/v n-propanol,
also containing 1% v/v HCl and 0.2% w/v Cs, as in
the samples solutions.
Ta b l e i. fi g u r e s of me r iT fo r Th e de T e r m i n a T i o n of
na an d k in r a w gl y C e r i n by faes, a f T e r diluTion wiT h 30%
v/v n-pr o p a n o l in wa T e r, n=3.
Cal i b r a T i o n
(0.1 – 1.0 m g l -1 )
slo p e (l m g -1 )
r
loq* (m g g -1 )
rds (n=3) %
p r o d u C T i o n by fl a m e aT o m iC emission speCTromeTry
in wa T e r
0.399
0.9992
0.07
5
na k
30% v/v
n-pr o p a n o l
0.505
0.9998
0.02
in wa T e r
0.417
0.9990
0.1
* LOQ values for 500 mg of sample diluted 1000 times (in two steps).
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30% v/v
n-pr o p a n o l
0.600
0.9984
0.08
Good linear correlation coefficients, R>0.998, were
obtained, especially considering the flame emission
technique used is usually of reduced linearity. The
precision, estimated by the relative standard deviation
(RSD), was below 6%, being appropriate for the
analysis of raw glycerin. The limit of detection (LOD)
was defined as 3 times the standard deviation of
10 measurements of the blank divided by the curve
slope. The limit of quantification (LOQ) was taken as
3.3 times the LOD. For the sample, the LOQ depends
on the dilution, the values for the adopted procedure
being: 0.02 mg g -1 for Na and 0.08 mg g -1 for K, considering
500 mg of the sample diluted 1000 times
(in two steps), which was adequate for raw glycerin
samples produced by alkaline catalysis.
3.4 aC C u ra C y va l i d a T i o n
As no certified reference material for glycerin is
available, the accuracy was checked by comparing
the results obtained after total acid digestion in a
microwave oven with those obtained after dilution
with 30% v/v n-propanol. Calibration in the same
medium, according to the preparation procedure,
was carried out. The analysis of three raw glycerin
samples, produced by using alkaline catalysis, lead to
the results shown in Table II, which are in agreement,
according to the student-t test for a 95% confidence
level, except for one result. In sample A, the concentration
of K was below the quantification limit and
the comparison is not conclusive for this case. This
experiment demonstrates that the proposed method
leads to accurate results.
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