Experimentation precautions

In the process of passivation, metals usually are found only in one of the two extreme states, active or passive. The transition between these states occurs suddenly and discontinuously. The intermediate state in region BC can only be realized with special experimental precautions. It is in this sense that passivation differs from the inhibition of electrochemical reactions observed during adsorption of a number of surface-active substances, where the degree of inhibition varies smoothly with the concentration of added material. 

Finally, it must be said that coulometry, more especially the technique in non-aqueous media, requires strict adherence to a number of experimental precautions such as those described on pp. 222-223 and in Figs. 3.87 and 3.88162. 

Most biochemically relevant high-spin systems have such short 7j-relaxation times that their EPR is broadened beyond detection at ambient temperatures. An exception is the class of S = 5/2 Mn" systems with D hx. Also, S = 7/2 Gd"1-based MRI shift reagents exhibit readily detectable room-temperature EPR spectra. Otherwise, aqueous-solution transition ion bioEPR is limited to complexes of S = 1/2 metals, in particular Cu", and to a lesser extent VIV02+, NiIn, Ni1, Mov, and Wv. Cupric is the stable oxidation state of biological copper under aerobic conditions, however, the other metals are stable as Vv, Ni", MoVI, and WVI, and, therefore, the other oxidation states associated with S = 1/2 paramagnetism may exhibit oxidative or reductive reactivity and may thus require specific experimental precautions such as strict anaerobicity over the course of the EPR experiment. 

The most interesting liquids for low-temperature thermometry are 3He and 4He, especially for the calibration of resistance thermometers in the range from 0.5 to 4.2 K. Vapour pressure of H2 is also interesting to realize vapour pressure-fixed points included in ITS-90. The measure of He vapour pressure has been carried out with great accuracy [42,43] to establish the ITS-90 (see Section 8.3). There are several experimental precautions to be observed in order to obtain reliable measurements [2],... 

Step 2—acquisition of mass flow rate data some experimental precautions must be taken in order to obtain the proper data since the objective is to determine the two-phase critical flow rate by measuring the emptying time ATe, and... 

Experimental precautions are required for NMR measurements in this group because the adducts rapidly undergo ring opening. The NMR spectra of the selenophene adducts in solution are similar to those of the corresponding thiophene adducts. The H-NMR data for adducts 142-145, as obtained from dinitro and cyanonitroselenophene derivatives, are reported in Table XXI. It is of interest that, unlike 2-cyano-4-nitrothiophene, its selenophene analog is converted to the anionic a-adduct by nucleophilic attachment at the unsubstituted C-5 position without any competition from a reaction at the CN side chain.169... 

These metal analyses indicate a marked reduction of both titanium and iron in the dialytic extract relative to both the coal and the soxhlet extract. The question remaining is, how much of this metal is background It should be noted here that attainment of good trace element analyses in the low ppm range requires very careful experimental precautions and replicate analyses. This particular experiment is, by its nature, difficult to conduct in a scrupulous "trace element clean" manner. However, if it is assumed that contamination from any source (solvents, glassware, utensils, etc.) will usually add to the concentration of metal, we can use the metal content determined in the dialyzate as an upper limit for soluble metals content. The higher iron and titanium concentrations in the soxhlet extract indicate that these metals may be associated with material which is not truly soluble, such as microparticulate mineral matter. 

Cupric sulphide, CuS.—By adopting special experimental precautions, it is possible to prepare cupric sulphide free from cuprous sulphide and sulphur by precipitating a solution of cupric sulphate or chloride with hydrogen sulphide,8 although if no precautions are taken the precipitate is likely to be contaminated with these substances.9 At 180° C. concentrated sulphuric acid converts copper into a mixture of cupric sulphide and sulphur, from which the sulphur can be removed by heating at 160° C. in a rapid current of hydrogen.10... 

Reporting Experimental Pressure-Area Data with Film Balances, prepared for publication by L. Ter-Minassian-Saraga, Pure Appl. Chem. 57 (1985) 621. (Data representation, experimental precautions, checklist). 

Several methods allowing the determination of adsorption isotherms exist. The most economical one is the use of saturated salt solutions, generating known water partial pressures, followed by the measurement of the silica weight uptake. However, the time needed to obtain a suitable adsorption isotherm is excessively long (several weeks), and experimental precautions need to be taken to achieve meaningful results. Specialized equipments, quite expensive ones, have been developed for the water adsorption isotherm determination, based on highly sensitive microbalances. 

When the nature of the material s crystalhne phases is known, the volume of each of the phases present can be determined. We saw in Chapter 1 that the integrated intensity of each peak in a given phase is directly proportional to the volume of the phase. Therefore, quantitative phase analysis is performed from the very precise measurement of these integrated intensities. Quantitative phase analysis should not be confused, of course, with the quantitative chemical analysis which is used to determine the amount of each element present in a sample. The first quantitative phase analysis by X-ray diffraction was conducted in 1925 on a ceramic material, in order to determine the amount of mullite in burned clays [NAV 25]. Despite the fact that this type of quantitative measurement has existed for 80 years, these analyses remain very difficult to conduct [BIS 89, TOR 99a, TOR 99b] and require experimental precautions which we will now discuss. However, we should point out that X-ray diffraction is virtually the only method available for quantitative phase analysis. ... 

Experimental precautions necessary to achieve the imprinting satisfactorily are also presented [1]. 

A carbon-treated filtrate may have a different quality of color because of a change in pH. This may occur when a carbon that contains appreciable acid or alkali is used to decolorize a liquid containing indicator-type colored bodies. Such behavior requires experimental precautions. One method is to test the pH of each filtrate and adjust any deviation to the original pH. In the case of synthetic test solutions, these should be buffered to the natural pH of the dissolved dye. It is to be noted that the buffer can affect the adsorbability of the dye and the adsorption may not be the same from buffered as from unbuffered solutions. 

There is one further experimental precaution that must be observed in evaluating experiments—it is important to establish the nature of the entities desorbed in the high field. This is necessary for two reasons Although throughout we have written all relations for singly charged ions only, multiple ionization is likely in field desorption of materials... 

Any calorimeter with a suitable mixing device and designed for use with liquids can be applied to determine heats of solution, dilution, or mixing. To obtain good precision in the determination of heats of solution requires careful attention to detail in the construction of the calorimeter. The dissolution of a solid can sometimes be a relatively slow process and requires efficient and uniform stirring. Substantial experimental precautions are ordinarily made to ensure that heat input from the stirrer mechanism is minimized. 

If an acid is chosen with a concentration of 1 mol L then the result is a NHE (with N for normali.e., molar for a monoacid). However, it does not quite constitute a SHE because the real compounds are not close to their standard state. For example, the proton activity of an acid with a concentration equal to 1 mol is not equal to 1. The value of the NHE potential is about 6 mV/sHE at room temperature for hydrogen chloride. In practice, if one wishes to have a hydrogen electrode with a potential as close as possible to that of the SHE, then an acidic solution is implemented with a concentration slightly higher than 1 mol L Using this type of electrode is a difficult task and must be reserved to particular applications indeed, many particular experimental precautions are necessary for ensuring an equilibrium state in such a system. 

The internal solution Is in contact with the electrolyte of the system studied via a porous material. The latter Is essential because It ensures contact is made with a solution having a fixed, well known concentration during the experiment. Nevertheless, it introduces an ionic junction between the Internal solution and the electrolyte of the system being studied. This can be the cause of errors on the potentials measured, which are generally difficult to estimate. However, when certain experimental precautions are taken, these errors are considered insignificant most of the time (see section 3.4.2.2 and appendix A.hl). 

---