Chromium sample pretreatment

Table 1 gives composition, structural and textural characteristics of the catalysts. The evolution of the BET surface areas with the chromium content as well as the results of X ray analysis of the samples pretreated in air at 410°C. 

The problems of contamination obviously extend to any sample pretreatment procedures, which, therefore, should be reduced to the absolute minimum for ultratrace determinations. If sample pretreatment is necessary or considered appropriate, great care must be taken to ensure that chromium from the reagents or containers used, or indeed from the environment, does not contaminate the sample and invalidate the results. 

The determination of chromium can be performed by a great number of physicochemical methods, with different detection powers, working range, and application field. Here, only those techniques that have found more general applications in different fields can be briefly discussed. The selection of a method for a special analytical task is not only dependent on the detection power and other performance characteristics of the technique, but is highly dependent on the sample constitution, the required sample pretreatment and means of sample introduction, and, last but not least, on its availability (Table 3). 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

XPS Spectrum of Surface. Figure 2 shows XPS spectra of CDC nickel-plated sheets with various pretreatment. Carbon, oxygen, chromium and nickel are observed in the surface of CDC nickel-plated sheets oiled with DOS and ATBC. The carbon spectra show ester carbonyl carbon and hydrocarbon species on both samples. The ester carbonyl carbon reflects the ester bond of DOS (C8Hi7C00C8Hi60C0C8Hi7) and ATBC ((C3H7COO)3C(CH2)OCOCH3). 

The methodologies used for chromium speciation in liquid samples are very diverse preconcentration on different types of columns, chemical reactions such as complexation or oxidation-reduction, and other separation procedures have been used and will be reviewed briefly below. These pretreatments with subsequent atomic spectrometric determination allow good limits of detection and recoveries for samples with low concentrations of chromium (Comelis 1996). Polarography is also suitable for the determination of chro-... 

Water solutions of mercury in the fig/L concentration range have been found to be unstable due to absorption to the storage containers and/or evaporation after spontaneous reduction to Hg(0). Pretreatment of glass vessels with an acid-dichromate solution has been reported to considerably reduce loss of mercury from dilute (0.3 fig/L) standards, probably due to saturation of the available adsorption sites with chromium (Litman et al., 1975). Numerous other container treatment procedures and preserving additives have been suggested to avoid loss of mercury from aqueous samples (see section aqueous samples ). 

The catalysts varied in concentration from 1.9 per cent to 34.5 per cent chromium after reduction. All catalysts with a chromium concentration of more than 2 per cent were diluted down to 2 per cent with -y-alumlha before pelleting. This procedure had the advantage that the several samples could be compared in activity without changing the space velocity, or any other condition. The samples might be said to differ not in total chromia or total alumina but in the ratio of alumina used as support to that used merely as a mechanical diluent. The pellets were 4 mm. long and 5 mm. in diameter. The pelleted catalysts were heated at 500° for 20 hours as a pretreatment, and to bum off the stearic acid used as an aid in pelleting. 

The above AES results can be correlated to bond performance. Hsu et found that the DQSK adherends, containing the higher concentration of silicon, showed interfacial failure, while only cohesive failure was observed for CRS adherends. The chromium-to-iron ratios, determined from the depth-profile study of pretreated steel surfaces,correlated with the wedge test performance as shown in Figure 24, with minimal crack extension in samples with high Cr/Fe ratios. 

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