An on-line procedure for the oxidation of Cr(III) to Cr(VI) was developed. The extent of oxidation was followed using the 1,5-diphenylcarbazide (DPC) spectrophotometric method accommodated in a flow injection analysis (FIA) system. Cerium(IV) sulphate and potassium peroxydisulphate were tested as oxidising agents. Despite the high oxidation power of the peroxydisulphate ion in acidic solution, the use of Ce(IV) is more convenient under dynamic conditions. The best experimental conditions were obtained at 45°C using a 0.025% m/\/Ce(IV) solution in 0.07 M H2SO4 with the Cr(III) solutions containing up to 3.0 × 10-2 M H 2S04.109564564

Baccan N.

De Andrade J.C.

Rocha J.C.

Repositorio da Producao Cientifica e Intelectual da Unicamp

ANALYST, MAY 1984, VOL. 109 645 
On-line Oxidation of Cr(lll) to Cr(VI) for Use with the Flow Injection 
Analysis Technique 
JoCo Carlos de Andrade," Julio Cesar Rocha and Nivaldo Baccan 
Universidade Estadual de Campinas, lnstituto de Quimica, CP 61 54, 13 100-Campinas-SP, Brazil 
An on-line procedure for the oxidation of Cr(lll) to Cr(VI) was developed. The extent of oxidation was followed 
using the 1,5-diphenyIcarbazide (DPC) spectrophotometric method accommodated in  a flow injection 
analysis (FIA) system. Cerium( tV) sulphate and potassium peroxydisulphate were tested as oxidising agents. 
Despite the high oxidation power of the peroxydisulphate ion in acidic solution, the use of Ce(lV) is more 
convenient under dynamic conditions. The best experimental conditions were obtained a t  45°C using a 
0.025% rnlVCe(lV) solution in 0.07 M H2S04 with the Cr(lll) solutions containing up to 3.0 x 10-2 M H2S04. 
Keywords: On-line oxidation; flow injection analysis; Cr(lll) oxidation 
The spectrophotometric determination of total chromium 
requires an oxidation step prior to the absorbance measure- 
ment. The procedure usually adopted involves the oxidation 
of Cr(II1) to Cr(V1) with permanganate1.2 or with Ag+ 
catalysed peroxydisulphate (persulphate).2-4 In the conven- 
tional analytical procedure an excess of permanganate is 
decomposed by reduction with azide or by precipitation as 
hydrous Mn02,2 while an excess of peroxydisulphate can be 
decomposed by boiling the solution4 or by reduction with 
azide.2 
Considering the great analytical interest in the speciation of 
chromium,5 the development of an inexpensive, on-line 
procedure for the oxidation of trace amounts of chromium 
should be both desirable and very useful. This would permit 
the use of continuous flow analysis systems in speciation 
studies. This paper shows that such an oxidation reaction can 
be followed using the flow injection analysis (FIA) approach, 
employing the highly sensitive and selective 1,5-diphenyl- 
carbazide (DPC) spectrophotometric method.6.7 
As permanganate absorbs near the A,. of the Cr - DPC 
complex, it cannot be used directly as an oxidising agent for 
chromium( 111) in the proposed FIA procedure. Therefore, 
Ce(1V) and peroxydisulphate ions were tested as possible 
on-line oxidants. We found that Ce(1V) was more efficient 
under the experimental conditions of FIA in spite of the high 
oxidising power of peroxydisulphate in aqueous acidic solu- 
tion .334 
Experimental 
Reagents 
Analytical-reagent grade chemicals and de-ionised water were 
used to prepare all solutions, which were stored in high- 
density polyethylene bottles. 
Stock standard Cr(ZZZ) solution, 1000 pg ml-I. Prepared by 
reduction of 2.829 g of K2Cr207 (G. Frederick Smith 
Chemical Co., 100% purity certified) with 3.5 g of sodium 
hydrogen sulphite (Fisher) in 2.6% V/V sulphuric acid 
(Merck). The mixture was heated to dryness and the residual 
solid was quantitatively transferred into a 1-1 calibrated flask 
to which sufficient sulphuric acid was added to obtain a final 
analytical concentration of 0.10 M. Except where stated 
otherwise, working solutions were prepared daily by dilution 
of aliquots taken from the stock solution. 
Stock Ce(1V) solution, 0.25% m/V. Prepared by dissolving 
1.25 g of cerium(1V) sulphate (Carlo Erba) in sufficient 
concentrated sulphuric acid to obtain a final concentration of 
0.20 M on completing the volume to 500 ml. This solution is 
stable for several weeks.* Except when previously stated, 
* To whom correspondence should be addressed. 
working solutions of Ce(1V) were prepared by dilution of 
aliquots taken from the stock solution, adding enough 
sulphuric acid to obtain a final analytical concentration of 
0.07 M. This procedure prevents the hydrolysis of Ce(1V). 
Potassium peroxydisulphate solution, 2.5% mlV. Prepared 
daily by dissolving 25.0 g of potassium peroxydisulphate 
(Merck) in 1 1 of 2.5 x 10-?40 mlV silver nitrate solution. 
Nitric acid, sulphuric acid and DPC solutions used for 
optimum colour development were prepared as previously 
described.7 
Experimental Conditions 
The FIA arrangement used to follow the on-line oxidation of 
Cr(II1) is shown in Fig. 1. An oxidation line has been added to 
the configuration previously reported,6,7 in which Cr(II1) is 
oxidised to Cr(V1) prior to injection of the sample into the 
main FIA reaction line. The oxidation coil is immersed in a 
thermostated bath (Ultra Thermostat MTA KUTESZ, Model 
606) in which the temperature of oxidation was controlled to 
within k O . 1  "C. The reagents and the sample were pumped at 
a flow-rate of 1.2 ml min-1 using an Ismatec Mini-S 840 
peristaltic pump and Tygon peristaltic pump tubing (Techni- 
con). Polyethylene tubing (i.d. 0.8 mm) was used for the 
oxidation, mixing and reaction coils. The absorbance 
measurements were made in a Zeiss PM 2A spectropho- 
tometer at 540 nm, using an 80-pl Zeiss flow cell (optical path 
length 10 mm). 
Results and Discussion 
Screening experiments on the on-line oxidation of Cr(II1) to 
Cr(V1) were carried out using the FIA arrangement shown in 
Fig. 1. The effect of the length of the oxidation coil and the 
1.00 rn 
Fig. 1. FIA system used for the on-line oxidation of Cr(lI1). A, Mixing 
coil, length 1.0 m; B,  proportional injector; C, reaction coil, length 
0.7 m; D, oxidation coil, length variable (see text); E and E , 
confluence points; F, thermostatic bath, temperature variable (see 
text); L, injection loop; W, waste; FC, spectrophotometric flow cell. 
I, 0.80 M HN03 or 0.80 M H,SO, (see text); 11, 0.05% rnlV DPC; 111. 
Cr(II1) or Cr(V1); and IV, 0.025% rnlV Ce SO4)? or 2.50% rnlV 
K&OR in 2.5 x 10-% rnlV AgN03 or Cr(vr() or water 
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View Article Online / Journal Homepage / Table of Contents for this issue
646 ANALYST, MAY 1984, VOL. 109 
0.20 
Q, 
C cu 
e s 
2 
0.10 
0 5 10 
Length of the oxidation coilim 
Fig. 2. Oxidation of Cr(II1) with Ce(1V): variation of the measured 
absorbance with length of oxidation coil and temperature of oxida- 
tion. Cr(III), 3.0 pg m1-l; Ce(SO&, 0.01% mlV in 0.10 M H,SO,; 
volume injected, 77 p1. A, 35 "C; B, 40°C; C, 45 "C; D, 50°C; E, 55 "C; 
and F, 60 "C 
0.20 
a 
C (0
n 
n 
6 
: 0.10 
v) 
0 
I I I I 
1.0 2.0 3.0 4.0 
Ce(S04)2, O/O m/V x lo2 
Fig. 3. Oxidation of Cr(I1I) with Ce(1V): effect of Ce(IV) concen- 
tration on the measured absorbance of the complex Cr - DPC. Cr(III), 
3.0 pg ml- 1 ;  Ce(IV) solution, in 1.0 M HZSO,; length of the oxidation 
coil, 2.0 m; temperature of oxidation. 45 "C; and volume injected. 
77 pl 
0.25 I I 
Fig. 4. Oxidation of Cr(II1) with Ce(1V): variation of the measured 
absorbance with analytical concentration of sulphuric acid in the 
Ce(1V) solution. Cr(II1). 3.0 pg ml- l ;  Ce(S0J2, 0.025% miV in 
HZS04; length of oxidation coil, 2.0 m; temperature of oxidation. 
45 "C; and volume injected, 77 pl 
influence of temperature on the extent of the oxidation 
reaction can be followed by the variation in absorbance shown 
in Fig. 2. Although the oxidation yield increases with 
increasing temperature, at temperatures higher than 45 "C 
bubbles are formed in the oxidation coil, particularly when 
longer coils are used. As the presence of bubbles interferes 
with the spectrophotometric detection, we fixed 45 "C as the 
limiting working temperature for the on-line oxidation of 
Cr(II1) by Ce(1V). 
As shown in Fig. 3, the concentration of the Ce(1V) solution 
used in confluence with the chromium solution also has an 
important influence on the over-all on-line oxidation process. 
This observation seems to be consistent with the rate law 
d[ Cr(VI)]/dt = k[Ce(IV>l2[Cr( III)]/[ Ce(III)] , reported by 
Tong and King9 for the oxidation of Cr(II1) by Ce(IV) in 
aqueous acidic sulphate media. Considering these data, 
0.025% miV was taken as the most appropriate concentration 
of Ce(1V) to be used in the on-line oxidation of Cr(II1). 
The dependence of the oxidation yield on the analytical 
concentration of sulphuric acid in the Ce(1V) solution is shown 
in Fig. 4. The variation in the absorbance values is not so 
pronounced, although the rate-law coefficient for the reaction 
between Cr(lI1) and Ce(1V) contains an inverse dependence 
on the concentration of hydrogen sulphate ion,9 indicating 
that the reaction proceeds more readily at lower concentration 
of this species. This effect of the sulphuric acid concentration 
on the yield may relate principally to the formation of the 
anionic species, Ce(S04)32-, which causes a slight variation in 
the formal potential of the Ce(1V) - Ce(II1) couple.lo Hence 
working solutions of Ce(IV) were prepared using the mini- 
mum amount of sulphuric acid necessary to dissolve the 
cerium(1V) salt and to prevent its hydrolysis. 
There is a marked influence of the sulphuric acid concentra- 
tion in the Cr(II1) solution on the oxidation yield, as indicated 
by Table 1. Possibly the chromium(II1) - sulphate complexes 
that may be formed when Cr(II1) is in sulphuric acid solution 
inhibit the oxidation reaction. In dilute solutions of sulphate 
and Cr(II1) ion, the important chromium species is the 
monosulphatochromium(II1) complex, present as an 
inert inner-sphere complex, [Cr(H,O),SO,] +, and as a 
labile outer-sphere association (ion-pair) complex ion , I1  
[CT(H*O)~~+  S042-]+. The species [Cr(SO,),]- appears when 
the sulphate ion concentration is increased. 11 The outer- 
sphere complex of sulphate and hexaaquochromium( 111) ion 
are in rapid equilibrium with their environment but the 
inner-phere chromium sulphates are not. As the rate of 
formation of these inner-sphere complexes is relatively slow ,9 
they do not interfere with the oxidation when the source of 
sulphate ion is the cerium(1V) solution but may interfere when 
the Cr(1II) is in a more concentrated sulphuric acid solution 
for some time. 
Having optimised the reagent concentrations, the variation 
of oxidation yield with the length of the oxidation coil was then 
re-investigated using the new experimental conditions. The 
results are given in Table 2. According to these data an 
oxidation coil of 1.0 m is sufficient to obtain the maximum 
oxidation yield. This result is very promising in terms of the 
application. of this on-line oxidation procedure in continuous- 
flow determinations of total chromium. 
As both the Cr(1II) and Ce(IV) are in sulphuric acid 
solution, we again tested this acid as a medium for the 
formation of the Cr - DPC complex, although previous work- 
had indicated better results with nitric acid for the formation 
of this complex. As shown in Table 2,  the absorbance values 
observed with DPC - sulphuric acid are lower than those with 
DPC - nitric acid, indicating that the formation of the Cr - 
DPC complex is more complete with nitric acid in confluence 
with DPC, although sulphuric acid can also be used. 
The relative oxidation yield was calculated, comparing the 
absorbance values observed for identical concentrations of 
Cr(V1) and oxidised Cr(III), both measured under the same 
experimental conditions. The flow-rate control necessary for 
this comparison was effected by measuring a ratio of flow- 
rates, &, defined as the ratio between the measured absorb- 
ance of a standard solution of Cr(V1) submitted to dilution 
with water at the confluence point E' [see Fig. 1, feedline 111, 
Cr(V1) standard solution; feedline IV, water] and the absorb- 
ance for the same Cr(V1) solution without on-line dilution 
[both feedlines I11 and IV pumping the Cr(V1) solution] under 
the same experimental conditions. This is a very convenient 
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View Article Online
ANALYST, MAY 1984, VOL. 109 647 
Table 1. Variation of the relative oxidation yield with analytical 
concentration of sulphuric acid in the Cr(II1) solution. Cr(III), 1.50 pg 
ml-1; Ce(SO&, 0.02S0/0 mlVin 0.07 M H2S04;  length of the oxidation 
coil, 1.0 m;  temperature of oxidation, 45 "C; volume injected, 164 pl; 
R f ,  0.433 (see text) 
HZS04/hj 
1.0 x 10-3 
1.0 x 10-2 
3.0 x 10-2 
1.0 x 10-1 
1.8 x 10-1 
3.0 X 10-1 
5.0 x 10-1 
1 .0 
1.5 
Relative oxidation 
yield, % 
95.6 
95.6 
95.6 
80.0 
63.7 
36.8 
18.7 
4.9 
0 
Table 2. Variation of the extent of oxidation with length of the 
oxidation coil. Cr(III), 3.0 pg ml-I; Ce(SO&, 0.025% miV in 0.07 M 
H,SO,; temperature of oxidation, 45 "C; volume injected, 77 p1 
Lengthim A(HN0,)" A(H,SO,)+ 
0.50 0.212 0.202 
1 .00 0.215 0.202 
2.00 0.215 0.204 
3.00 0.215 0.207 
4.00 0.217 0.213 
5.00 0.220 0.213 
7.00 0.226 0.213 
9.00 0.224 0.199 
* Absorbance measured using 0.80 M H N 0 3  in confluence with 
t Absorbance measured using 0.80 M H,S04 in confluence with 
0.05% m/V DPC.' 
0.05% mlV DPC. 
0.30 
0.20 
0 
m 
+? 
$ 
2 
0.10 
0 2  4 6  8 1 0  
Length of the oxidation coil/m 
Fig. 5 .  Oxidation of Cr(II1) with Ag+-catalysed peroxydisulphate: 
variation of the extent of oxidation with length of the oxidation coil 
and temperature o f  oxidation. Cr(III), 3.0 pg ml-l in 0.10 M H,SO,; 
K2S,0,, 2.50% m/V in 2.5 x 10 -3% mlV; volume injected, 77 pl. A ,  
40 "C; B, 50 "C; C, 60 'C; D ,  70 "C; and E ,  74 "C 
procedure for checking the extent of on-line dilution from 
time to time ( e . g . ,  before starting a run and then after each 2 h 
of continuous pumping) without stopping the run. The relative 
oxidation yield is constant for a given Rf value. 
Cr(II1) does not interfere in the reaction between Cr(V1) 
and DPC, which was used throughout this work to follow the 
extent of oxidation of Cr(II1) by Ce(1V). However, when 
Ce(1V) concentrations at or above 0.030% m/Vare used in the 
oxidation process, double peaks may be recorded, depending 
on the volume injected. These double peaks may indicate a 
lack of the DPC reagent in the centre of the sample zone as a 
result of a reaction between DPC and Ce(1V). Such double 
peaks are not observed when the same Cr(V1) solution is 
diluted with water [Fig. 1, feedline 111, Cr(V1); feedline IV, 
water] before it is injected into the main FIA line, using the 
same Rf value. 
The on-line oxidation can also be done using a potassium 
peroxydisulphate solution with Agf ions as a catalyst. Fig. 5 
shows the results obtained for the most suitable concentra- 
tions of S2082- and Ag+. However, peroxydisulphate pro- 
duces larger amounts of bubbles at lower temperatures, 
probably owing to S2082- decomposition in acidic solution4 
as, with higher temperatures and longer oxidation coils, more 
bubbles are observed. The poorer precision at higher temper- 
atures in Fig. 5 is a direct result of the interference of the 
bubbles in the absorbance measurements. Special techniques 
can be applied to eliminate the problem of bubbles, but this 
greatly increases the time required for a single analysis. Thus, 
the use of peroxydisulphate is not recommended for the 
on-line oxidation of Cr( 111) using the experimental conditions 
described in this paper. These observations indicate, once 
more, that conventional procedures used in static analysis do 
not necessarily translate into the best conditions for FIA.7 
Studies applying this on-line oxidation procedure for the 
sequential spectrophotometric determination of Cr(II1) and 
Cr(V1) are in progress. 
The authors are pleased to acknowledge helpful discussions 
with C. €1. Collins and C. Pasquini. They also thank the 
Conselho Nacional de Desenvolvimento Cientifico e Tecno- 
logic0 (CNPq) for a Fellowship to J. C. R. This work was 
partially supported by UNDP - UNESCO grant BRA/77/003. 
1. 
2. 
3. 
4. 
5 .  
6. 
7. 
8. 
9. 
10. 
11. 
References 
Sandell, E. B.,  "Colorimetric Determination of Traces of 
Metals," Second Edition, Interscience, New York, 1950. 
Marczenko, Z . ,  "Spectrophotometric Determination of Ele- 
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Yost, D .  M., J .  Am. Chem. SOC. ,  1926, 48, 152. 
House, D. A . ,  Chem. Rev., 1962, 62, 185. 
Florence, T. M.,  and Batley. G. E., CRC Crit. Rev. Anal. 
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Storgaard JGrgensen, S . ,  and Regitano, M. A .  B.. Analyst, 
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de Andrade, J. C.,  Rocha, J .  C., Pasquini, C.,  and Baccan. N., 
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Paper A31343 
Received October 3rd ,  1983 
Accepted November 28th, 1983 
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On-line Oxidation Of Cr(lll) To Cr(vi) For Use With The Flow Injection Analysis Technique

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On-line Oxidation Of Cr(lll) To Cr(vi) For Use With The Flow Injection Analysis Technique

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