Zirconium(IV) oxide coated on a silica gel surface was found to be very effective for extracting CrVI from aqueous solutions at a pH of about 3. A microcolumn packed with this material was inserted into a flow injection system for the separation and preconcentration of CrVI. By this process, a concentration level of 2 ppb was detected with a relative standard deviation of 2.4%. Because of the high selectivity of the immobilized substrate, it was possible to determine CrVI in the presence of larger amounts of other ions. The analysis was also carried out on natural water spiked with CrVI, and compared with a standard method.11761029103

Baccan N.

Gushikem Y.

Peixoto C.R.M.

Repositorio da Producao Cientifica e Intelectual da Unicamp

ANALYST, JUNE 1992, VOL. 117 1029 
Selective Spectrophotometric Determination of Trace Amounts of 
Chromium(v1) Using a Flow Injection System With a Microcolumn of 
Zirconium(iv) Oxide Modified Silica Gel 
Carlos R. M. Peixoto, Yoshitaka Gushikem" and Nivaldo Baccan 
lnstituto de Quirnica, Unicamp, CP 6154, 13081 Campinas, Siio Paulo, Brazil 
Zirconium(iv) oxide coated on a silica gel surface was found to  be very effective for extracting Crvl from 
aqueous solutions at a pH of about 3. A microcolumn packed with this material was inserted into a f low 
injection system for the separation and preconcentration of  Crv'. By this process, a concentration level of 2 ppb 
was detected with a relative standard deviation of 2.4%. Because of the high selectivity of the immobilized 
substrate, it was possible t o  determine CrV' in the presence of larger amounts of  other ions. The analysis was 
also carried out on natural water spiked with Crvl, and compared with a standard method. 
Keywords: Zirconium(iv) oxide modified silica; silica gel surface coated with zirconium(1v) oxide; 
chromium(v1) separation; flow injection determination of chromium(v1) 
Extraction of CrvL from solution by using an organic ion- 
exchange resin is a very well-known method. However, a 
worsening of the sorption characteristics is observed under 
multi-cyclic operating conditions owing to the partial oxida- 
tion of the matrix by the sorbate. Consequently, it is more 
appropriate in this instance to find a sorbent based on an 
inorganic matrix having high stability towards the action of 
oxidants and exhibiting anion-exchange properties.' 
In recent work, the use of zirconium(1v) oxide coated on a 
silica gel surface as a selective sorbent for Crvl was described.2 
In this instance, the substrate and the matrix are very resistant 
towards oxidation; moreover, the metal oxide was not 
detached from the surface under multi-cyclic operation. 
In general, hydrated metal oxides, obtained by a precipita- 
tion process, are normally fine powders with low mechanical 
resistance and, in most instances, are not useful as packing 
materials for column chromatography.3-5 The utilization of a 
matrix coated with an ion-exchanger substrate could consti- 
tute an advantage, because, in the chemically modified solid, 
the characteristics of the original silica matrix such as rigidity, 
specific surface area and the average diameter of the pores are 
not affected.6 
In the present study, a selective method is reported, 
involving on-line separation and preconcentration of Crv* in a 
microcolumn packed with zirconium(1v) oxide coated on a 
silica gel surface inserted into a flow injection (FI) system. The 
determination of Crvl at very low concentration levels in water 
is described. 
Experimental 
Preparation of Chemically Modified Silica 
Silica gel having an average pore diameter of 0.60 nm and a 
specific surface area, SBET, of 500 m* g-1 was previously dried 
at 423 K under vacuum (0.13 Pa) before use. About 50 g of the 
activated silica were added to a solution containing 11.6 g 
(0.05 mol) of pure ZrC14 dissolved in 300 cm3 of anhydrous 
ethanol. The mixture was heated under reflux for 8 h under a 
dry nitrogen atmosphere, and the resulting solid was washed 
by decantation and then heated at 400K to eliminate the 
solvent. This dry material was hydrolysed by immersing it in 
de-mineralized water. The solid was washed with water to  
remove all the C1- and then dried in an oven at 393 K for 5 h. 
The amount of Zr attached on the surface was determined by 
X-ray fluorescence analysis and yielded Z r 0 2  = 3.25%. 
* To whom correspondence should be addressed. 
Preparation of Solutions 
Solution of 1,5-diphenylcarbazide (DPC) .  Prepared by 
dissolving 0.25 g of DPC (Merck) in 20 cm3 of acetone and 
diluting the solution to  500 cm3 with de-mineralized water. 
Stock solution of Crvl (1000 pg cm-3). Prepared by dissol- 
ving 2.829 g of previously dried K2Cr207 (Merck) in lo00 cm3 
of water. 
Solution of tris(hydroxymethy1)methylamine ( T H A M )  
(0.1 mol dm-3). Prepared by dissolving 12.1 g of the sub- 
stance in 1000 cm3 of de-mineralized water. 
Adsorption Isotherm 
The adsorption isotherm of Crvl obtained from an aqueous 
solution by using the chemically modified silica was measured 
at 298 K. The batch technique was used. 
Solutions (50 cm3) containing 4.00 x 10-4 mol dm-3 Crvl, 
the initial pH values of which were adjusted to between 1.0 
and 8.0 with HCl or  NaOH, were shaken in a flask with 0.2 g 
of the sorbent material for 30 min. The pH of the supernatant 
solution in equilibrium with the solid phase in each flask was 
measured. 
The amount of Crvl adsorbed by the solid was calculated by 
applying the equation: 
Nf = (N,  - N,)/m 
where N, is the number of moles of Crvl added, N ,  is the 
number of moles of Crvl in the solution phase in equilibrium 
with the solid phase, and m is the mass of the solid phase. 
The distribution coefficient, D, was calculated from the 
equation: 
D = Nf/c 
where c is the concentration of Crvl in the solution phase in 
equilibrium with the solid phase. 
Flow Injection 
The FI configuration used is shown in Fig. 1. Two injectors 
were used, one for addition of CrV1 and another for addition of 
eluent (THAM). Time-controlled injections of the samples 
were used. 
The microcolumn C (length, 2 cm; i.d., 0.3 cm) was packed 
with the sorbent material (about 50 mg). The operation of the 
FI system was carried out in four steps. 
1. Activation: 0.1 mol dm-3 H N 0 3  was passed through the 
column at a flow rate of 1.7 cm3 min-1 for 20 s. 
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View Article Online / Journal Homepage / Table of Contents for this issue
1030 
o.-------o 
W W - 0  6 0  
w--0 
crv' ' 
ANALYST, JUNE 1992, VOL. 117 
o--------o 
O 0 - w  
0 - 4  
0.1 rnol dm-3 - 
w-0 
w--0 
R THAM 
*W 
-NO3 b . 8  rnol dm-3 HN03 
1 x 10-3 mo5 > 0.05% DPC 
-W 
W-- 0.1 rnol d m - 5  
THAM 
w--0 
W-0 
W--0 
RC 
MC P *W 
0.05% DPC 
dm-3 HN03 
(d) 
\ 
A 0  
CrVl < 
0.1 rnol dm-3 
HN03 
-W 
0.1 rnol d m - s  -
THAM R 
I I I I -  m 
mW 
0.05% DPC 
Fig. 1 f low injection configurations: (a) activation; (b) injection; (c) washing; and (d) elution. W, Waste; MC, mixing coil; 
RC, reaction coil; C, microcolumn; P. spectrophotometer; and R. recorder 
2 .  Injection of CP': a Crvl solution (pH 3) was passed 
through the column at a flow rate of 3.4cm3min-1 for a 
pre-determined time (1-6 min). 
3. Washing: the column was washed with 1 x 
10-3 rnol dm-3 HCI at a flow rate of 2 cm3 min-1 for 10 s .  
4. Elution: Crvl was eluted from column C in a reversed flow 
with 0.1 rnol dm-3 THAM at a flow rate of 2 cm3 min-1. A 
solution of DPC (0.05% m/v) was previously mixed in MC 
(Fig. 1) with 0.8 rnol dm-3 HN03. The resulting solution was 
allowed to  react in R C  with Crvl carried by the THAM 
solution. The absorbance of the complex formed was 
measured at 540nm in the spectrophotometer P, and the 
signal was recorded at R. 
Interference Study 
Stock solutions (1000 pg cm-3) of Cr3+, Mg2+, Mn2+, Cu2+, 
Fe3+, Ce3+, Ce4+, Mo6+, V5+, S042-, Mn04- and Po43- 
were prepared for interference studies. An appropriate 
aliquot of each of these stock solutions was added to the Cr"' 
solution (0.1 pg cm-3), and the resulting mixtures were 
analysed in the FI system as described above. 
Chromium(v1) in Natural Water 
Samples of natural water were collected from a lake on the 
university campus, and to each sample was added a few drops 
of concentrated H N 0 3  to  bring the p H  of the solution to about 
3. Each solution was allowed to stand overnight and then 
passed through a fine pore filter of 0.45 pm. The determina- 
tion of Crvl was carried out as described above. 
Results and Discussion 
Characteristics of the Material 
Hydrated zirconium(1v) oxide adsorbs either cations or  
anions, depending on the solution pH. At low pH, anion 
exchange can occur by a mechanism in which OH- is displaced 
from the surface of the solid phase.1.7 For zirconium(rv) oxide 
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ANALYST, JUNE 1992, VOL. 117 1031 
120 I 1 Table 3 Interference study on the adsorption of Crvl by zirconium(1v) 
oxide coated on a silica gel surface 100 t 
Concentration 
5 
5 
5 
5 
1 
2 
3 
5 
0.1 
0.5 
0.1 
1 
1 
3 
5 
0.1 
0.5 
1 
1 
5 
0.01 
0.05 
0.1 
1 
(PPm) 
Interference 
factor 
1 .oo 
0.98 
0.98 
1.02 
0.98 
0.96 
0.93 
1.01 
0.93 
1 .oo 
0.89 
0.97 
1.01 
1.04 
0.94 
0.92 
0.83 
0.99 
0.89 
0.98 
0.94 
0.83 
0.84 
- 
I 
2o t Ce"' 
CelV 0 2 4 6 8 
PH 
MoV1 Na2Mo04 Fig. 2 
solution. [Crvl] = 4.00 x 
Plot of distribution coefficient (D) of Crvl versus pH of the 
mol dm-3 
VV 
Table 1 Calibration graph for Crvl with use of zirconium(1v) oxide 
coated on a silica gel surface as sorbent 
soj2- 
Injection Volume Mass of 
time/s injectedkrn' CrV'/pg Absorbance 
10 0.666 0.0333 0.028 
40 2.67 0.133 0.119 
70 4.67 0.233 0.201 
100 6.67 0.333 0.275 
Mn04- KMn04 
Pods- Na2HP04 
Table 4 Comparison of the results obtained for the analysis of natural 
water spiked with Crvl 
Table 2. Relative standard deviation in the determination of CrV'. 
Number of injections = 10 in each instance 
Crvi con- 
Time/s* centration (ppb) RSD (%) 
500 2 2.4 
200 10 1.8 
60 30 1.6 
40 50 1.2 
Sample No. DPC method FI method 
1 0.20 0.18 
2 0.13 0.12 
3 0.055 0.050 
* Flow rate: 3.2 cm3 min-1. 
was plotted against the mass of Crv' (data in Table 1). A linear 
relationship [ A  = 0.0049 + 0.823 (Crvl); (CrV1) mass in 
micrograms], with a correlation coefficient ( r )  of 0.999, was 
obtained. 
The precision of the method for different Crvl concentra- 
tions was obtained by carrying out ten replicate measurements 
for each solution. Solution concentrations ranging from 2 to 
50ppb were injected into the column at a flow rate of 
3.2 cm3 s-1. The results are presented in Table 2. The values 
of the relative standard deviation (RSD) are reasonable in 
comparison with the results obtained with other methods for 
determining Cr"' at approximately the same concentration 
level, e.g. ,  by using electrothermal atomic absorption spec- 
trometry. 879 
coated on the silica surface, the exchange property of the 
immobilized substrate was preserved. 
In Fig. 2, the distribution coefficient ( D )  for Crvl is plotted 
against the solution pH. The maximum exchange capacity 
achieved by the material was 80 X 10-6 mol g-* at pH 3. The 
change of D with solution pH can be explained by the 
following reaction: 1.7 
EZrOH + H +  + Crz072- - ~ Z r + C r ~ 0 ~ ~ -  + H20 
where EZrOH denotes the metal oxide attached to the silica 
surface. At higher pH, the reaction is shifted towards the 
left-hand side whereas at lower pH (<3), the competition 
reaction between Cr2O72- and CI- (introduced as hydro- 
chloric acid to adjust the solution pH) becomes significant. At 
pH 2, the concentration ratio [CI-] : [Cr207*-] = 160. 
Interference Study 
The interference study was carried out by passing a binary 
mixture of 0.1 ppm Crvl and the interferent ion, in variable 
concentrations, through the column. The results are presented 
in Table 3. The interference factor, listed in the last column, 
was calculated from the ratio of the absorbance of the Crvl and 
interferent binary solution to  that of the pure solution of Crvl. 
The interference of cations was studied for Cr3+, Hg*+ , Mn2+, 
Cu2+ and Ce3+ until a ratio of [interferent] : [CrV1] = 50 : 1 was 
reached. For Ce4+ and Fe3+, for which the tendency to 
hydrolyse is greater,") the interference factor was consider- 
ably higher, mainly for the former ion. 
The interference of anions was more significant because, at 
a solution pH of 3, the solid surface was positively charged and 
hence acted as an anion exchanger. Greater interference was 
observed for SO4*- and P043-, confirming the previous 
FI Experiments 
In the FI experiments the microcolumn C (Fig. 1) packed with 
the sorbent was, in each complete cycle of operation, loaded 
with Crvl at a solution pH of 3, and elution was effected with 
THAM solution at pH8-9. Under such conditions, the 
sorption and desorption operations could be repeated many 
times without any change in the exchange properties of the 
material. A 0.1 mol dm-3 NaOH solution was also used as an 
eluent, but, in this instance, after a few operations, the 
packing material showed changes in its sorption capacity. An 
attack of the surface by the strong alkali used was observed. 
In order to test the possibility of using the material for the 
determination of Crvl, a 50 ppb solution of the ion was injected 
into the column at different times (from 10 to 100s). The 
absorbance of the CrVL-DPC complex eluted from the column 
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1032 ANALYST, JUNE 1992, VOL. 117 
observations reported by Boichinova and Tsaikov.11 A high 
affinity for these two anions by the hydrated zirconium(1v) 
oxide was described. 
Chromium(v1) in Natural Water 
In order to determine the amount of Crvl, the solution was 
adjusted to pH 3 and then subjected to analysis in the FI 
system as described under Experimental. However, in the 
water sample collected, no Crvl was detected (less than 2 ppb). 
In order to determine the reliability of the process, the 
determination of Crvl was also carried out directly in the 
sample, without prior separation and preconcentration, with 
use of DPC as the reagent. However, as the limit of detection 
with this reagent was 50ppb, the samples of natural water 
were spiked with Crvl. After allowing the solutions to stand 
overnight, the amounts of Crvl in the solutions were deter- 
mined by spectrophotometry. The same sample was also 
analysed by the FI technique as described previously. The 
results are summarized in Table 4. 
The agreement between the results obtained by the two 
procedures is very reasonable and indicates that the material 
can be used for the determination of Crvl in natural water. 
Conclusions 
The high selectivity shown by zirconium(1v) oxide coated on a 
silica gel surface towards Crv' represents a considerable 
improvement in the determination of this species in compari- 
son with the previously described method.12-14 With this 
methodology for the separation of Crvl, most of the common 
ions usually present in water samples did not interfere. 
However, higher concentrations of S042- and PO$ inter- 
acted with the column material, resulting in an incomplete 
separation of Crvl. 
C. R. M. P. is indebted to Conselho Nacional de Pesquisa 
(CNPq) for a fellowship, and Y. G. to Financiadora de 
Estudos e Projetos (FINEP) for financial support. 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
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Paper 1 /0432OJ 
Received August 19, 1991 
Accepted January 7, 1992 
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Selective Spectrophotometric Determination Of Trace Amounts Of Chromium(vi) Using A Flow Injection System With A Microcolumn Of Zirconium(iv) Oxide Modified Silica Gel

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Selective Spectrophotometric Determination Of Trace Amounts Of Chromium(vi) Using A Flow Injection System With A Microcolumn Of Zirconium(iv) Oxide Modified Silica Gel

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