Chemistry Reference
In-Depth Information
130µL, V
2
475µL, L
1
650cm, L
2
360cm, L
3
60cm, θ
1
=θ
2
=θ
3
=0.5mm, Me
=glass-calomel electrode
Source: Reproduced with permission fromTaylor and Francis, UK
[221]
injection volumes are optimised in such a manner that the confluence of the injected
plugs at point A is asymmetric, the plug travelling along channel 1 and merging with the
tail of the plug circulating through channel 2. A plug with two reaction zones is formed in
reactor L
3
. The first of such zones corresponds to the reaction between 1,5-
diphenylcarbazide and Cr(VI) from the sample, while the second one is due to the overall
chromium content (Cr[III]) being previously oxidised to Cr[VI] by injected Ce[IV]). The
detection of this plug provides two peaks per simultaneous injection, which are related to
the Cr(VI) concentration and the total chromium present in the samples, respectively. The
presence of two passive interfaces between the two detectors and the microprocessor
means that the pH and absorbance data can be acquired and processed. A printer provides
the results, which are expressed as the concentration of each species.
Three calibration curves were run:
(1) Curve for Cr(VI) from the absorbance of the first peak, A
1
, which allows the direct
determination of this species in the sample.
(2) Curve for the contribution, A
2
, of Cr(VI) from the sample to the absorbance of the
second peak.
(3) Curve for the contribution, A
2
′
, of Cr(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:
The calculation program allows, from a series of Cr(III) and Cr(VI) standards, the above-
mentioned calibration curves to be established; and, hence, the concentrations of both
oxidation states of chromium can be calculated. These data, together with the pH for each
sample, permits the determination of the concentration of the different species on the
basis of the following equations:
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