QUANTIFICATION OF TOTAL CHROMIUM AND HEXAVALENT CHROMIUM IN WATER BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY

The two primary oxidation states of chromium in natural waters, Cr(Ill) and Cr(VI), differsignificantly in biological, geochemical and toxicological properties. Whereas Cr(Ill) isconsidered essential for human in glucose, lipid and protein metabolism, Cr(VI) is toxicbecause of its ability to oxidize other species and its adverse effects on the lung, liver andkidney. Because of the different toxicities and bioavailability of Cr(Ill) and Cr(VI),determination of the total chromium content does not give full information about possiblehealth hazard. Hence monitoring of the concentration of the separate chromium species isof great importance. Many different techniques have been in use for Cr containing samplespreparation and metal ions speciation: ion chromatography, flow injection analysis, andatomic absorption spectrometry (AAS).Procedures for the quantification of total chromium and hexavalent chromium in watersamples arc presented. For the quantification of total chromium and hexavalent chromiumin water Chrornabond NH2 columns (arninopropyl phase with a 3ml volume and 500mg ofsorbent) obtained from Machery- Nagel (Duren, Germany) were used.The pH value of the water sample was adjusted to 5.5 using acetic acid or sodium acetateand sample was aspirated through the previously conditioned column. The column contentswere dried under vacuum and the hexavalent chromium selectively linked was eluted withnitric acid and quantification was performed by Electrothermal Atomic AbsorptionSpectrometry (ET AAS). For the detection of total chromium, Cr(Ill) was transformed intoCr(VI) by oxidizing the sample with 1% K2S20S solution and AgNOJ at 100°C for 15 min.Oxidized solution was eluted through a Chromabond column and total Cr level wasquantified by ET AAS using the same instrumental conditions as for hexavalent chromium.Total chromium was also quantified directly in the water samples using ET AAS. Thetemperature programme of the graphite furnace, the use of chemical modifiers, the atomictechnique employed and the effectiveness of deuterium background correction wereinvestigated. Chromium was reliably determined by without chemical modifiers orbackground correction.The detection limits were 0.4 and 0.5~gll for total chromium and hexavalent chromiumrespectively. The linearity changed under the optimized conditions was 0.4 - 50j.lgll and0.5-50j.lgll and the relative standard deviation was less than 3.5%. The validation of bothprocedures was performed by the standard addition method and the recoveries were higherthan 96% in all cases. It is proved that the method can be successfully employed as analternative to the commonly used preconcentration and speciation analytical techniques.The direct procedure was adopted for the estimation of total chromium in water samples becauseboth procedures applied for total chromium gave similar results. The methods were applied to thedetermination of total chromium and hexavalent chromium in 40 water samples.

An investigation into the effect of rate of stirring of bath electrolyte on the properties of electrodeposited CdTe thin film semiconductors

Electrodeposition (ED) has been recognized as a low cost and scalable technique available for fabrication of CdS/CdTe solar cells. Photovoltaic activity of these electrodeposited semiconductor materials drastically depends on the ED growth parameters namely; electrodeposition potential, concentrations and ratios of concentrations of precursors used to prepare the bath electrolyte, pH of the electrolyte, deposition temperature and rate of stirring of the electrolyte. In order to grow thin films with good photovoltaic properties, it is essential to maintain these variables at their optimum ranges of values during electrodepositions. Hence, this study was conducted to investigate the dependence of the properties of electrodeposited CdTe thin film material on the rate of stirring of the bath electrolyte. The CdTe material was grown on glass/FTO (23 cm2) and glass/FTO/CdS (23 cm2) surfaces in bath electrolytes containing 1.0 mol/L CdSO4 and 1.0 mmol/L TeO2 solutions at different rates of stirring within the range of 0-350 rpm while keeping the values of pH of the electrolyte, deposition temperature and cathodic deposition potential with respect to the saturated calomel electrode at 2.3, 65 °C and 650 mV respectively. After the heat treatment at 400 °C in air atmosphere, the deposited samples with a good visual appearance were selected and evaluated based on their morphological, elemental, structural, optical and electrical properties in order to identify the optimum range of rate of stirring for electrodeposition of CdTe thin film semiconductors. Results revealed that, rates of stirring in the range of 60-85 rpm in a 100 mL volume of electrolyte containing the substrate and the counter electrodes in the center of the bath with a separation of 2.0 cm between them can electrodeposit CdTe layers exhibiting required levels of morphological, structural, optical and electrical properties on both glass/FTO and glass/FTO/CdS surfaces

Electrodeposition and characterization of as-deposited and annealed CdTe thin films

Thin films of CdTe semiconductor materials were grown on fluorine doped tin oxide (FTO) conducting glass substrates using the technique of electrodeposition. CdSO4 at high concentrations and CdCl2, TeO2 at low concentrations were used as precursor salts for electrodeposition. The range of deposition potentials was estimated using cyclic voltammetric measurements. The electrical, optical, structural and morphological characteristics of as-deposited and annealed CdTe thin films were characterized using photo-electrochemical (PEC) cell studies, UV-Vis spectrophotometry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). These particular samples were converted from n-type into p-type after heat treatment. UV-Vis spectrometric measurements for CdTe layers indicated that, the energy band gaps of 1.45±0.02 eV for both as-deposited and annealed samples which exhibited the required optical property for fabricating CdS/CdTe solar cells. Little increase in (220) and (311) peaks of XRD spectra were observed for annealed layers compared to the as-deposited material. However, annealing exhibited a small reduction of cubic phase preferential orientation (111). The optical transmission for both as-deposited and annealed CdTe samples were about 60% for wavelengths longer than about 850 nm

Effect of stirring rate of electrolyte on properties of electrodeposited CdS layers

CdS is the most matching window material available for the CdTe absorber layer of CdS/CdTe solar cells and electrodeposition is a promising technique adaptable for fabrication of thin films of CdS owing to its simplicity, low cost, scalability and manufacturability. The quality of electrodeposited thin film semiconductor layers depends significantly on the electrodeposition potential, concentrations of precursor salts, pH, temperature and the rate of stirring of the electrolyte. In this study, the attention was focused on the effect of “stirring rate of electrolyte” since it has not been studied in detail in the past, despite of its strong relation to the rate of mass transport towards the working electrode where the thin film semiconductors are electrodeposited. This study was carried out via electrodepositing of CdS thin layers on fluorine doped tin oxide conducting glass working electrodes at different rates of stirring of the electrolyte while keeping the rest of the electrodeposition parameters unchanged at a previously identified set of values. The morphological, electrical and optical properties of the CdS layers grown at different stirring rates were used to determine the effect of stirring rate on the quality of CdS layers. The study revealed that, a stirring rate in the range of 60–125 rpm which produced orderly flows in the electrolyte around the working electrode (1 × 3 cm2) placed at the center of a 100 ml electrolytic bath with a distance of 2 cm apart between the graphite counter electrode and the conducting glass electrode could produce good quality CdS layers when electrodeposition was carried out at a cathodic deposition potential of 660 mV with respect to the saturated calomel electrode. The concentrations of CdCl2 and Na2S2O3 in the bath used were 0.10 and 0.01 M respectively. The temperature and pH of it were 60 °C and 1.80 respectively

Necessity and relevance of precipitate free clear electrolytes for electrodeposition of CdS semiconductor materials with enhanced photovoltaic properties

Cadmium sulfide (CdS) is a well-known window material used for fabrication of second generation thin film solar cells including CdS/CdTe and CdS/CuInGaSe2. Among the CdS fabrication techniques, electrodeposition is a simple, cost effective and scalable method that has been stepped towards large scale commercialization. However, the presence of precipitates in baths used for electrodeposition of CdS has been found to be a persistent problem which had produced CdS thin films with poor photovoltaic properties. Hence, an investigation was carried out to identify a set of optimum physiochemical conditions that can produce clear stable electrolyte for electrodeposition of CdS thin film semiconductors using CdCl2 and Na2S2O3 precursor salts. The study revealed that, electrolytes containing 0.10 mol/L CdCl2 and 0.01 mol/L Na2S2O3, within the pH range of 1.50–2.00 and the temperature range of 55–65 °C can provide clear and stable electrolytes for electrodeposition of CdS thin films. Further, the results showed that, the electrical, optical, morphological and structural properties of CdS layers electrodeposited from electrolytes within above physiochemical conditions were remarkably better to those electrodeposited from the turbid electrolytic baths formed beyond the ranges of predetermined optimum physiochemical conditions