Automation of a flow-injection system for multispeciation

Differentiating between the two most frequent oxidation states ofchromium is fundamental in speciation studies of this element on account of the toxicity of hexavalent chromium. In fact, most of the works on this element published so far deal solely with this aspect [1]. Flowinjection analysis (FIA) has been used several times for determining this element in one [2 and 3] or in its two most common oxidation states [4-10].


Introduction
Differentiating between the two most frequent oxidation states of chromium is fundamental in speciation studies of this element on account of the toxicity of hexavalent chromium. In fact, most of the works on this element published so far deal solely with this aspect [1]. Flowinjection analysis (FIA) has been used several times for determining this element in one [2 and 3] or in its two most common oxidation states [4][5][6][7][8][9][10].
Nevertheless, it is known that there is a large variety of chemical forms of Cr(III) and Cr(VI), which show different properties depending on the groups or elements to which they are bound. Their relative abundance is a function of the type of medium. Since the pH is a major parameter in any type of solution (especially in aqueous ones) and taking into account that its value dictates the predominance of a particular species of Cr(III) and Cr(VI) (hydroxylated, dimer etc.), the following species of chromium must be considered to be present in waters. + Chromium(III)" Cr(H20)6 + Cr(OH) 2+, Cr(OH), Or(OH)3, Or(OH)i-. Chromium(VI)" H2CrO4, HCrOi-, Cr207 -, CrO4 -. Among these species, Cr(OH)3 is very insoluble, its concentration in water being negligible, and HCrO4 is a relatively strong acid (pKa 1"0), which is unlikely to occur in water. Consequently the following equilibria, with their respective constants, must be considered:  [9], and asymmetric merging zones [9] modes have also been utilized, together with the indicator reaction reported by several authors [3][4][5][6][7][8][9][10] involving Cr(VI) and 1,5diphenylcarbazide.
There is only one previous report on this type of work: a recent theoretical study on the effect of the pH on the Cr(VI) species present in solution [12]. This was carried out by the conventional technique and Cr(III) species were not considered.

Experimental
Reagents: aqueous stock solution of." Ce(IV) (0.500 g 1-1), Cr(III) (100 g m1-1), Cr(VI) (100 g m1-1) and H2SO4 (1 M Manifold: the pH of the water sample, which travels through the system, is continuously monitored (glasscalomel microelectrode-ME-in figure 1), prior to its confluence with a sulphuric acid stream, whereupon the s'tream is split into two channels with an injection valve each. The oxidant (Ce[IV]) is injected into channel 1, while 1,5-DPC is injected into channel 2. The reactor lengths and injection volumes are optimized in such a manner that the confluence of the injected plugs at point A is asymmetric, the plug travelling along channel and merging with the tail of the plug circulating through channel 2. A plug with two reaction zones is formed in reactor L3. interfaces between the two detectors and the microprocessor means that pH and absorbance data can be acquired and processed. A printer provides the results, which are expressed as the concentration of each species.
The optimization of physico-chemical and FIA variables affecting the system provides the results shown in figure l's caption. The calibration of the microelectrode with suitable buffers was performed at the working flow-rate, owing to fact that the response of this sensor is affected by the flow velocity.
Principle behind the calculation The suggested configuration (figure 1), allows the pH value for each sample to be directly obtained and the Cr(III) and Cr (VI) content from the double peak to be obtained on each simultaneous injection of reagents.
Three calibration curves must be previously run: (1) Curve for Cr(VI) from the absorbance of the first peak, A, which allows the direct determination of this species in the sample.
(2) Curve for the contribution, A2, of Cr(VI) from the sample to the absorbance of the second peak.
(3) Curve for the contribution, A,ofCr(III) from the sample to the absorbance of the second peak.
The equations of these straight lines, regression coefficients and determination ranges found are as follows.

Table
shows the values corresponding to several synthetic water samples containing different chromium Table 1. Chromium speciation with the suggested configuration and using the MECROM program.  In a second series of experiments the influence of the pH on the distribution of these species was studied. With this purpose, synthetic samples were prepared with a constant concentration of both oxidation states of chromium and different pHs. Table 2 shows the relative abundance of the different Cr(III) and Cr(VI) species for pH values between 3 and 10.

Discussion
This report demonstrates that it is possible to automate studies with a non-segmented flow system and a microprocessor. The system described provides a concentration profile ofthe different chemical forms in which an element can occur in natural or artificial samples. all aspects of automation and mechanization in analytical, clinical and industrial environments. The Journal publishes original research papers; short communications on innovations, techniques and instrumentation, or current research in progress; reports on recent commercial developments; and meeting reports, book reviews and information on forthcoming events. All research papers are refereed.

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