Potential sources of difficulty

Another potential source of peaks in the NIR is called Fermi resonance. This is where an overtone or combination band interacts strongly with a fundamental band. The math is covered in any good theoretical spectroscopy text, but, in short, the two different-sized, closely located peaks tend to normalize in size and move away from one another. This leads to difficulties in first principle identification of peaks within complex spectra. 

W1/W2 theory and their variants would appear to represent a valuable addition to the computational chemist s toolbox, both for applications that require high-accuracy energetics for small molecules and as a potential source of parameterization data for more approximate methods. The extra cost of W2 theory (compared to W1 theory) does appear to translate into better results for heats of formation and electron affinities, but does not appear to be justified for ionization potentials and proton affinities, for which the W1 approach yields basically converged results. Explicit calculation of anharmonic zero-point energies (as opposed to scaling of harmonic ones) does lead to a further improvement in the quality of W2 heats of formation at the W1 level, the improvement is not sufficiently noticeable to justify the extra expense and difficulty. 

In practice, the application of x-ray measurement techniques to thin films involves some special problems. Typical films are much thinner than the penetration depth of commonly used x-rays, so the diffracted intensity is much lower than that from bulk materials. Thin films are often strongly textured this, on the other hand, results in improved intensity for suitable experimental conditions but complicates the measurement problem. Measurements at other than ambient temperature, not usually attempted with bulk materials, constitutes additional complexity. Since typical strains are on the order of 1 X 10 , measurements of interplanar spacing with a precision of the order of 1 X 10 are needed for reasonably accurate results hence, potential sources of error must be kept to a low level. In particular, the sample displacement error can be a major source of difficulty with a heated sample. The sample surface must remain accurately on the axis of the instrument during heating. 

Silver-silver chloride reference electrodes are available from all the principal manufacturers of ion-specific and pH electrodes. They usually are furnished with a saturated KC1 (saturated with AgCl) filling solution, but some manufacturers will supply the electrodes in a dry condition so that the user may add the electrolyte. This can be an advantage because a saturated KC1 filling electrolyte is a source of difficulty in laboratories where the temperature fluctuates or goes much below 25 °C. For the latter condition potassium chloride will precipitate in the junction and will have to be dissolved with dilute KC1 or water and flushed. For this reason most authorities recommend the use of 3.5 M KC1 (saturated with AgCl) for use in the silver chloride reference electrode. Junction potentials are not seriously altered, so that pH measurements are unaffected. 

Although a standard deviation of about 10% is now quite usual for rate constant measurements from fast-flow discharge and pulsed photolysis studies, it is still found that the same reaction studied in different laboratories by the same technique may give results of similar precision, but which differ by far more than would be expected on the basis of that precision. Potential sources of such discrepancies are many but, with experience, likely errors can sometimes be identified in particular cases. For example, reactions between short lived radical intermediates are common in combustion processes. The measurement of the rate constants of such reactions, where the reaction is second order with respect to the transient species, pose particular but well recognized difficulties stemming from the need to determine the absolute concentration of the reacting radical (Chapter 1). This is difficult to achieve and has been a common source of error in this type of determination as exemplified in the series of studies on the rate of the reaction CHO -I- CHO — CH2O -H CO. 

The complexities involved in silicate weathering have caused difficulties for interpretation of laboratory dissolution data and have been the basis of heated controversies concerning the mechanisms of dissolution. This difficulty has been exacerbated by attempts to make direct comparisons of the results of dissolution experiments conducted in different pH regions. Because the extrapolation of rate and mechanistic data obtained from low or high pH weathering studies to more neutral conditions is a potential source of error, and because the majority of soils have pH values between 4 and 9, the interpretations made in this paper will be restricted to the pH range of 4 to 9, unless otherwise stated. 

Polynuclear aggregates of octahedral V(III) (cf, S = 1) are also a potential source of SMMs. The fact that the number of known V(III) clusters is small is due to its air-sensitivity, which adds some difficulty to their preparation and handUng. Among the relatively few paramagnetic vanadium clusters reported [188-190], only one structural type has proved to be a SMM. 

Membrane material for covering electrodes may be a source of difficulty since many synthetics contain plasticizers which are highly toxic and diffuse out from the film very slowly, causing cellular embarrassment. This can clearly be shown by using tissue culture situations to test potentially useful materials. Rapid failure of cell respiration, followed by cell death, can often be observed around so called inert plastics. Even different batches of a normally safe plastic may be biologically dangerous. 

Coulometric titrations are subject to live potential sources of error (1) variation in the current during electrolysis, (2) departure of the process from lfX)% current efliciency, (3) error in the current measurement, (4) error in the measurement of lime, and (5) titration error due to the difference between the equivalence point and the end point. The last of these difficulties is common to volumetric methods as well. I-or situations in which the indicator error is ihe limiting factor, the two methods are likely to have comparable reliability. 

A risk event is a discrete random occurrence that (if occurring) affects the project. Risk events are identified based on the difficulty to achieve the required project outcome (the characteristics of the product or service), constraints on schedules and budgets, and the availability of resources. The environment in which the project is performed is also a potential source of risk. Historical information... 

One potential source of confusion concerns the manner in which dates and times are formatted. Different companies and nations use different formats. For example, 03/5/06 could represent March 5, 2010 (United States) or 3 May 2010 (United Kingdom). In order to circumvent these difficulties, the international convention ISO 8601 (ISO 2004) can be used. It calls for dates to be formatted as follows YYYY-MM-DD. So, May 3, 2010, would be 2010-05-03. 

There are many potential sources of error as people process information. Someone may have difficulty selecting and understanding competing information. One may have to integrate information from several sources to recognize a pattern for an event that is starting to occur. 

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