Sample metallization

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

The condition of the test metal is important. Clean metal samples with uniform finishes are preferred. The accelerating effects of surface defects lead to deceptive results in samples. The ratio of the area of a defect to the total surface area of the metal is much higlier in a sample than in any metal in service. This is an indication of the inaccuracy of tests made on metals with improper finishes. The sample metal should have the same type of heat treatment as the metal to be used in service. Different heat treatments have different effects on corrosion. Heat treatment may improve or reduce the corrosion resistance of a metal in an unpredictable manner. For the purpose of selectivity, a metal stress corrosion test may be performed. General trends of the performance of a material can be obtained from such tests however, it is difficult to reproduce the stress that actually will occur during service. 

Samples Metallic Content % Co Pt Nominal unit cell coripo on... 

Sample Metal loading (wt.%) Na exchanged (%) Metal exchange level (%)... 

Figure 1.4 shows a typical curve demonstrating the dependence of concentrations of copper, lead, and cadmium in the filtrate on the volume of seawater sampled. Metal levels become constant after 1-1.51 of sample have been filtered, and it can be concluded that at this point, contamination of the sample by the filtration equipment is negligible. 

Fig. 4. Separated metal fractions from different grain size classes during sample reduction (by operators metal >5 cm from mixed sample metal > 1 cm from split < 1 cm metal from 1 cm to 2 mm from BA split <2 mm metal <2 mm).

Thus the value of the dielectric constant at the sample/metal interface determines the shift of the resonance. When adsorption of molecules at the metal surface results in the change of the refractive index or of the local value of the dielectric constant, the change of reflectivity is observed. This phenomenon has been used as the mechanism for detection of gases (Fig. 9.18a) and of adsorbed biomolecules (Fig. 9.18b). The depth of penetration of the surface plasmon is comparable to that of the evanescent field, that is, 100-500 nm for the visible-near infrared range. 

When exposed to oxygen in air, dissolved metals precipitate in unpreserved water as oxides or hydroxides. This process is prevented by the addition of 1 1 nitric acid solution to water samples. Metals in water with the pH of less than 2 will be permanently dissolved in a form of highly soluble nitrates. 

Approaches to minimising sampling errors (Guder and Wahlefeld, 1983) and preventing the introduction of chemical contamination (Versieck et al., 1982 Heydorn, 1984 Aitio and Jarvisalo, 1994) have been published. To reduce the risks of introducing errors during sampling, metal-free and sterile devices, and standardised procedures should be used. 

These methods must all be tested carefully, as the presence of a chelating agent, solid or dissolved, can shift the equilibrium between sorbed, complexed, and free ions in the sample. Metals in water samples can exist in several different forms. They can be sorbed on filterable particles, complexed... 

To avoid any kind of metal contamination, no hand jewellery or medical dressings were worn during sampling. If medical dressings were required, heavy-duty rubber gloves were recommended to be worn at all times to avoid contamination of samples. Metal-free polyethylene or unpainted wooden spades or scoops, metal-free nylon sieve-mesh housed in inert wooden or plastic frames, and metal-free funnels and sample collection containers were used at all stages of sample collection. 

Fig. 14 a. Schematic arrangement for Flow Injection Analysis for a simple spectrophotometric determination, b A typical experimental output from such a system, showing lack of carry over even with large samples. (Metal ion Bismuth Reagent Pyrocatechol violet, 5 x 10 3 M pH 2-4 Sample size 200 pi Analysis rate 80 hr-1)... 

In soils and other natural samples, metal ions may partition to more than one reactive site, with each sorbent-sorbate complex providing a unique spectroscopic signal. In addition, the x-ray beam hitting the sample will inevitably bombard the... 

Sample [metal content / mmol g ] Pre treatment Adsorbed amount / mmol g V. V Vs per metal S, / m g- ... 

The creep test can be also used for solid foods and as in the dynamic tests, a compressive or elongational stress can be employed (Rao et al., 1995). However, when a compressive stress is imposed, the sample-metal platen interfece should be either lubricated or bonded to minimize frictional effects. 

Back titrations based on the addition of excess EDTA followed by back titration of the excess reagent are useful when reactions are slow or a suitable direct indicator is not available. The excess generally is determined by titration with standard solutions of magnesium or zinc ion. These titrants are chosen because they form EDTA complexes of relatively low stability, thereby preventing the possible titration of EDTA bound by the sample metal ion. Examples of the indirect method are the following ... 

Because protein oxidation may occur at any stage of the manufacture of the product, it is necessary to assess the susceptibility of the protein to oxidation through several mechanisms. Hydrogen peroxide, tert-butyl hydroperoxide (TBHP), and light are often used to promote oxidation in protein samples. Metal ions such as Cu and Fe have been added to protein formulations to purposefully assess whether a protein is susceptible to metal-catalyzed oxidation. 

Sample Metal content (%) Crystallinity (%) Ethane conversion ... 

Fig.3 Arrhenius plot of the rates of isotopic exchange between 02 and the of the support on fresh and on coked RhA catalyst samples. Metal carbon is eliminated from the coked sample before exchange measurements.

The uncertainty associated with the sample metal content is estimated from the weighing, dilution and interpolation sources (Fig. lb). The model used for the calculation of the contribution from the interpolation source assumes negligible standards preparation uncertainty when compared with the instrumental random oscillation [5, 10],... 

The weighing operations are present in the dry-base correction factor (three) and in the sample metal content (one). Two contributions for the associated uncertainty, weighing, were studied ... 

The sample metal content, M, is obtained from the interpolated concentration in the calibration curve, Cmter, the mass of the diluted digested sample, a, and the dilution factor, /dU., (digested sample volume times dilution ratio). 

Sample Metal(s) detected Added metal salt at 20 ppm each Solvent Preparation method ... 

---