Simultaneous and Sequential Determination of Chromium(VI) and Chromium(III) by Unsegmented Flow Methods Juan Ruz, Angel Rios, M. D. Luque de Castro, and Miguel Valcfircel Department of Analytical Chemistry, Faculty of Sciences, University of C6rdoba, C6rdoba, Spain

Simultane und sequentielle Bestimmung von Chrom(VI) und Chrom(III) ~durch unsegmentierte DurchfluBmethoden

Zusammenfassung. Verschiedene Artender simultanen und sequentiellen photometrischen Cr(III)- und Cr(VI)-Bestim- mung nach der Technik der reversed-flow Injektionsanalyse werden beschrieben. Die relative Standardabweichung ffir die Bestimmung dieser Chromspezies im i~g/ml-Bereich bei einer Probenfrequenz von 30 bis 100 je Stunde betrfigt weni- ger als + 1% fiir die Simultan- und weniger als + 3% ffir die sequentiellen Methoden. Eine simulierte kontinuerliehe Oberwachung von Chrom(VI) sowie eine periodische von Chrom(III) in natiirlichem Wasser und Abwasser wird be- schrieben. Die h/iufigsten St6rungen werden diskutiert.

Summary. Several different configurations for simultaneous and sequential photometric speciation of Cr(VI) and Cr(III) based on the reversed flow injection analysis and completely continuous modes are proposed in this paper. The determi- nation of these species at the gg �9 ml- ~ level is achieved with sampling frequencies between 30 and 100 h-1 and an r.s.d. of less than _+ 1% for simultaneous methods and of less than _+ 3% for the sequential method. The proposed methods are suitable for chromium speciation in waters. A simulation of the continuous monitoring of Cr(VI) and periodical of Cr(III) in natural and waste waters has been performed. The most frequent interferents in these types of samples have been investigated.

previous paper [7] we have tackled the - strictly speaking - simultaneous determination of these species using a single photometric detector. A sequential method also using a single detector which allows the fast and easy determination of both oxidation states of chromium is proposed as well.

Experimental

Reagents. Aqueous stock solutions of: Ce(IV) (0.500 g �9 1-1), Cr(III) (100 gg �9 ml-1), Cr(VI) (100 ~tg. m1-1) and HzSO4. (0.25 M). 1,5-diphenylcarbazide (I,5-DPC): 0.425 g were dissolved into 100 ml of ethanol and diluted up 250 ml with water.

Apparatus. A Pye-Unicam SP-500 single-beam spectro- photometer equipped with Hellma 178.12QS flow-cell (inner volume 18 gl) was used. Gilson Minipuls-2 and Ismatec S- 840 peristaltic pumps, Rheodyne 5041 and dual home made injection valves with variable injection volumes and Tecator TM III chemifold were also used.

Results and Discussion

A distinction has been made between configurations for simultaneous and sequential determination of Cr(VI) and Cr(III). All configurations are based on the reversed FIA mode.

Introduction

The joint determination of chromium(VI) and chro- mium(III) in samples, especially in natural or industrial waste waters, is of great importance in speciation studies concerning this element [1]. Some papers on chromium speciation involving the use of Flow Injection Analysis, FIA, and based on the indicator reaction Cr(VI)-l,5-diphenyl- carbazide have been recently published [ 2 - 6].

In this paper, in which the above-mentioned indicator reaction is also used, simultaneous and sequential determinations of Cr(VI) and Cr(III) are suggested using the reversed FIA (rFIA) mode, which is very suitable for studies of speciation in waters in which the samples are generally abundant. For the first time, as a continuation of a

Offprint requests to: M. Valcfircel

A. Simultaneous Determination of Cr(VI) and Cr(III)

A. 1. With Completely Continuous Monitoring of Cr(VI)

This method corresponds to the configuration shown in Fig. I a. The sample of Cr(VI) and Cr(III) is continuously aspirated by the pump and carried through the channel into which the injection of the oxidant - Ce(IV) - is performed. The oxidation of Cr(III) to Cr(VI) takes place along reactor L1, the sample merging later with the reagent (I,5-DPC) stream and yielding the indicator reaction (reactor L2), which is monitored at 540 nm. In this manner Cr(VI) is continously monitored (baseline) and every time that the oxidant is injected, a transient signal is generated in the detector (FIA peak) which is directly related to the Cr(III) concentration in the sample.

The optimum reagent and oxidant concentrations are 0.17% (w/v) with 40% of ethanol and 0.500 g �9 1-1 (in 0.1 M HNO3), respectively. High temperatures favour the oxida- tion of Cr(III) by Ce(IV); therefore, reactor L1 is thermo-

Fresenius Z Anal Chem (1985) 322:499-502 �9 Springer-Verlag 1985

o)

ce4§

SAMPLE

1.5-DPC

qlmL.min -1] w

A I

b)

ce4-, r - " - t 2 2 [ 1 LI~500. cm

/ V~=130-NL ~ ~ V W k t ~ 540nm• SAMPLE / " I ~)-wvwv~, L3:B0cmf- - - ,~ . r --k 1 2 . 2 / . . . . . . . . i I

w q (rnL,min -1 }

c)

1.5-D P C

SAMPLE< H§

Ce4§

1.5-DPC

V2:90,,,L / ,w

q(mL. rnin -1 ]

A |

Fig. la - -c Manifolds designed for speciation analysis of Cr(VI) and Cr(III) by reversed FIA. a Completely continuous determination of Cr(VI) and periodical of Cr(III); b Asynchronous merging zones mode for simultaneous determination of both species; e Sequential method for this speciation by use of a selecting valve for determination of Cr(VI) (channel 1) or overall chromium (channel 2). q flow- rate; V~; Vt, V2 injection valves; S selecting valve; Wwaste

stated at 45 ~ C. The optimum value of each variable is in- dicated in the configuration in Fig. I a. The inner diameter of the reactors used is 0.5 mm. Under these working conditions, the absorbance-concentration equations obtained-for each of these species separately are:

Acr(vi) = 0.548[Cr(VI)] + 0.008 for [Cr(VI)] = 0.10 - 1.25 pg - ml-1 (r = 0.999) Act(m) = 0.093[Cr(III)] + 0.033 for [Cr(III)] = 0.50 - 5.00 g g ' ml-1 (r = 0.994).

The slopes of these straight lines show the determination of Cr(VI) to be five times as sensitive as that of Cr(III). The sampling frequency for Cr(III) is 30 h - 1.

Mixture Resolution. It has been shown that the increase in Cr(VI) concentration in the mixture affects the Cr(III) signal negatively. This fact is due to the singularity of this configu- ration. The oxidant is injected into the stream which contains the sample, and the extent of development of the reaction:

Cr(III) + 3 Ce(IV) ~ Cr(VI) + 3 Ce(III)

decreases with increasing Cr(VI) concentration in the me- dium, which tends to displace the equilibrium towards the left.

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The Cr(III) concentration does not affect the Cr(VI) signal; thus, its concentration is directly obtained by applying the above equation:

[Cr(VI)] = (Acr(i i i ) - 0 . 0 0 8 ) / 0 . 5 4 8 ( l a g - ml- 1). (I)

The determination of Cr(III) in the mixture requires running several calibration curves in the presence of different Cr(VI) concentrations. The slopes and intercept of these curves change linearly with the Cr(VI) concentration according to:

slope: -0.030[Cr(VI)] + 0.131 intercept: -0.183[Cr(VI)] + 0.081

from these, each calibration curve for Cr(III) is defined as a function of the Cr(VI) concentration in the samples. The equation finally obtained is:

[Cr(III)] = (0.183Acr(vi) + 0.556Acr0n) - 0.046)/ (II) (-0.030Act(v1) + 0.073 (gg" ml-1).

The solution of Eqs. (I) and (II) allows the simultaneous determination of Cr(VI) and Cr(III) in the samples. The relative standard deviation, r.s.d., obtained by analyzing 11 samples (P = 0.05) containing 0.25 and 2.00 gg �9 ml-1 of

Table 1. Resolution of some synthetic mixtures of Cr(VI) and Cr(III) with the proposed configurations

Concentrations added Concentrations found Relative errors (gg- ml- t) (~tg. ml- ~) (%)

Cr(VI) Cr(III) Cr(Vl) Cr(III) Cr(VI) Cr(III)

(a) Completely continuous 0.10 5.00 0.10 4.81 0.0 - 3.8 configuration 0.25 4.00 0.25 4.08 0.0 2.2

0.25 3.00 0.25 3.12 0.0 4.0 0.50 1.00 0.51 0.92 2.0 - 8.0 1.30 5.00 1.10 5.00 0.0 0.0

(b) Merging zones 0.40 3.50 0.39 3.58 - 2.5 2.2 configuration 0.60 3.50 0.58 3.58 - 3.3 2.2

0.40 1.50 0.39 3.50 - 2.5 0.0 1.00 2.50 1.00 2.57 0.0 2.8 1.00 1.50 1.00 1.48 0.0 - 1.3

(c) Sequential 0.50 2.00 0.46 2.04 - 8.0 2.0 configuration 0.50 1.50 0.47 1.56 - 6.0 4.0

1.00 1.50 0.90 L60 - 10.0 6.6 0.50 1.50 0.47 1.58 - 6.0 5.3 1.00 1 .00 1.04 t .05 4.0 5.0

Cr(VI) and Cr(III) were + 0 . 3 8 % and +0 .46%, re- spectively.

The method has been applied to the resolution o f A synthetic samples of Cr(VI) and Cr(III) in waters and the results obtained are shown in Table I (a). The values found for Cr(IV) are quite acceptable in spite of the low concen- 0900- trations determined. The errors are larger for Cr(III), es- pecially at low concentrations.

Simulation of the Monitoring of Cr(VI) and Cr(IlI). Since this type of configuration is suitable for the control of Cr(VI) and Cr(III) in waste waters [continuous monitoring of Cr(VI) and periodical of Cr(III)], a simulation of their deter- mination in a hypothetical waste water has been performed. For this purpose, a 2-1 reservoir (furnished with a magnetic stirrer) initially containing 200 ml of water, has been used. F rom this, the water is continuously aspirated and analyzed. There are also three burettes containing solutions of Cr(VI), Cr(III) and water, respectively, which pour their content randomly into the reservoir. Figure 2 shows the recording obtained.

A. 2. With the Merging Zones Mode

In this configuration (Fig. 1 b) the sample is previously divided into two channels, each featuring an injection valve. The oxidant is injected into channel 1, whilst 1,5-DPC is injected into channel 2. Reactor lengths and injection volumes are optimized in such a manner that the confluence o f the injected plugs at point A is asymmetric, the plug travelling along channel 1 merging with the tail of the plug

0,600-

0.300-

j- (

10 I

2.0 310 4.0 t (h)

Fig. 2. Simulation of the speciation analysis of Cr(VI) and Cr(HI) over a working morning. Recording obtained

whilst the second is due to the overall chromium content [Cr(III) being previously oxidized to Cr(VI) by Ce(IV) injected]. The detection o f this plug provides two peaks for each simultaneous injection (Fig. 1 b) which are related to the Cr(VI) concentration and that of overall chromium in the samples, respectively. The opt imum values of the F IA variables are shown in Fig. 1 b, the reagent concentrations being the same as for the above configuration.

The absorbance-concentration equations for each of these individual species are:

Acr(vi) = 0.588[Cr(VI)] -- 0.002 (1st peak)( r = 0.999) 7 Acr(v~) 0.600[Cr(VI)] 0.002 (2nd peak) (r 0.999) Ac,(m) = 0.109[Cr(III)] + 0.036 (r = 998)

[Cr(VI)] = 0.10 - 1.60 jag. m l - t

[Cr(III)] = 0.50 - 5.00 gg- ml-1

circulating through channel 2. A plug is formed in reactor L3 with two reaction zones. The first corresponds to the reaction between 1,5-DPC and Cr(VI) from the sample,

Mixture Resolution. The above equations can be directly applied to the resolution of C r ( V I ) - Cr(III) mixtures. In Table 1 (b) are shown some of the mixtures resolved by this

501

method, whose results are acceptable, especially for Cr(III), for which smaller errors than those found with the above configuration are encountered. The accuracy of this method for the resolution of mixtures containing 1.00 gg �9 ml - 1 of Cr(VI) and Cr(III) shows in the r.s.d. (+0 .31% and _+ 0.71%, respectively). The sampling frequency achieved is 70h -1.

B. Sequential Determination of Cr (VI) and Cr (III)

The configuration used in this case is illustrated in Fig. 1 c. The sample containing the Cr(VI) - Cr(III) mixture is split into two channels feeding two sub-configurations. In sub- configuration 1 Cr(VI) is determined by injecting the reagent (1,5-DPC) after merging with the H2SO4 stream, which ensures an acidic pH for the sample. In subconfiguration 2 the overall chromium content is determined, thanks to the confluence of the sample channel with a stream of Ce(IV) in acidic medium prior to the injection of 1,5-DPC. Valve S located in front of the detector is a key piece in this configuration. It is an ordinary 4-way injection valve trans- formed into a selecting one for channels 1 and 2. This valve has been used by the authors in other configurations [7, 8]. In one of its positions, it allows channel 1 to attain the detector and waste the stream flowing along channel 2. In the other position the situation is just the opposite. In this manner Cr(VI) and overall chromium in the samples can be determined depending on the position of valve S. The sampling frequency is higher with this configuration. The optimization of each sub-configuration has been performed separately. . \

The individual determination of each species is carried out on the basis of the following equations:

Acr(vi) = 0.315[Cr(VI)] - 0.002 (channel 1)(r = 0.998)] Act(v1) 0.268[Cr(VI)] 0.002 (channel 2) (r 0.998)f Acr0ii) = 0.123[Cr(III)] + 0.133 (r = 999)

where the equation corresponding to Cr(VI) in channel 2 indicates the contribution of this species to the absorbance of the peak corresponding to overall chromium, the absorbance due to Cr(III) being obtained by difference.

Mixture Resolution. The above equations have been directly applied to the sequential determination of Cr(VI) and Cr(III) in samples containing a mixture of both. In Table 1 (c) the synthetic samples resolved by this procedure are shown. The errors in these determinations are larger than for the preceding configurations. Such errors are usually negative for Cr(VI) and positive for Cr(III). The r.s.d, values obtained for 11 different samples containing 1.00 ~tg �9 ml - ~ of Cr(VI) and Cr(III) are + 1.29% and _+ 3.03%, re- spectively. The sampling frequency in this case is 100 h-1 (substantially higher that of the above configurations).

Study of Interferents

This study has been performed using the configuration depicted in Fig. 1 b since this provides good accuracy and smaller errors in mixture resolution. Since rFIA is suitable for parameter control in abundant samples as natural and waste waters, and taking into account the importance of the chromium speciation studies in these types of samples, the

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Table 2. Interferents in chromium speciation

Tolerated ratio Added species of the foreign species/chromium Cr(VI) Cr(III)

100 CI-, NOff, CO~-, C1-, NO;, CO3 z-, PO~-, CN-, Br-, PO4 a-, CN-, Br-, F - , CH3COO , F - , C H 3 C O O - , SO 2 - , Ca 2 +, SO 2 , Ca 2 +, Mg 2+, pb 2+, Mg 2+, pb 2+, Mn 2+ ' Cd 2+ ' Cd 2+ ' Co 2+ ' Na +, Co 2+ ' Fe 3+, Na +, K +, NH + K +, NH2

50 EDTA EDTA 10 Mn 2+, Fe 3+ l NO2, Hg 2+ NO2, Hg 2+

< 1 Fe 2+, S 2- , I - Fe 2+, S 2-, I- , Cu 2+

interferents tested have been those species frequently present in this type of waters. In Table 2 the results found are listed. The most serious interferences are caused by reductants such as S 2-, I - , Fe 2+ and NO2. In general, an improvement in the number and level of interferents is achieved with respect to the conventional manual method.

Conclusions

Three configurations are suggested for the simultaneous and sequential determination of Cr(VI) and Cr(III), using

[Cr(VI) = 0.20 - 2.20 gg. ml-1

[Cr(III)] = 0.50 - 3.50 g g - m l - ~

reversed FIA and a single detector. They are useful for speciation studies of chromium by FIA featuring the novelty of the use of rFIA and completely continuous mode, not only in sequential determinations (the most usual in the papers reported so far), but also for the simultaneous determinations of Cr(VI) and Cr(III) with good accuracy and sensitivity and small errors. The rFIA mode is the most suitable technique for determining these species in water since it uses the reagent sparingly and offers increased sensitivity with respect to the normal FIA. The simplicity of these configurations make them suitable for automation.

References

1. Florence TM, Barley GE (1980) CRC Crit Rev Anal Chem 2; 219

2. Bubnis BP, Straka MR, Pacey GE (1983) Talanta 30:841 3. Lynch TP, Kernoghan NJ, Wilson JN (1984) Analyst 109:839 4. Jorgensen SS, Regitano MAB (1980) Analyst 105:292 5. Andrade JC, Rocha JC, Baccan N (1984) Analyst 109:645 6. Andrade JC, Rocha JC, Baccan N (1983) Analyst 110:197 7. Ruz J, Rios A, Luque de Castro MD, Valcfircel M, Water Res

(in press) 8. Rios A, Luque de Castro MD, Valcfircel M (1985) Analyst

110:277

Received April t0, 1985; revised June 18, 1985