Chemicals strength errors

The feed equipment is not the only source of error. The strength of the chemical may vary, thus introducing an error. When measuring the chemical strength, a percent error should be included, depending on the method used. Other parameters, such as temperature, altitude, pressure, and vacuum variability, may cause errors as well. All of these errors can combine in unpredictable ways to result in an amount delivered that is different from the intended amount. 

Because they are weak acids or bases, the iadicators may affect the pH of the sample, especially ia the case of a poorly buffered solution. Variations in the ionic strength or solvent composition, or both, also can produce large uncertainties in pH measurements, presumably caused by changes in the equihbria of the indicator species. Specific chemical reactions also may occur between solutes in the sample and the indicator species to produce appreciable pH errors. Examples of such interferences include binding of the indicator forms by proteins and colloidal substances and direct reaction with sample components, eg, oxidising agents and heavy-metal ions. 

Over the last several years, the number of studies on application of artificial neural network for solving modeling problems in analytical chemistry and especially in optical fibre chemical sensor technology, has increase substantially69. The constructed sensors (e.g. the optical fibre pH sensor based on bromophenol blue immobilized in silica sol-gel film) are evaluated with respect to prediction of error of the artificial neural network, reproducibility, repeatability, photostability, response time and effect of ionic strength of the buffer solution on the sensor response. 

D espite many safety precautions within chemical plants, equipment failures or operator errors can cause increases in process pressures beyond safe levels. If pressures rise too high, they may exceed the maximum strength of pipelines and vessels. This can result in rupturing of process equipment, causing major releases of toxic or flammable chemicals. 

Figure 24.9a shows a plot of measured total carbon (CO plus CO2, mole percent) versus equivalence ratio. The solid line was calculated assuming chemical equilibrium at the measured temperatures. The data points represent the measured CO and CO2 mole fractions (dry basis) using the fast extractive-sampling system. Horizontal bars represent the uncertainty in (f> due to reading and calibration errors vertical bars represent the uncertainty in the CO and CO2 mole-fraction sum due to line strength and absorption measurement uncertainty. The data are consistent to within 4% of the equilibrium predictions at all values of (p, indicating reliable operation of the system. 

Fig. 3 Evolution of orientational order parameter Pi at different electrode distances plotted versus the electric field strength E. The letters next to the marked values correspond to the SFM images in Fig. 2a-h. The error of Pi due to inhomogeneities in the film and the phase contrast can be estimated to be about 0.02. Reprinted with permission from Macromolecules [20]. Copyright 2008 American Chemical Society...

The errors involved in the absorbance measurements are usually smaller than those associated with chemical operations. In some methods the colour reaction is not reproducible. In other methods, the colour varies with time, and the absorbance should be measured after a strictly determined lapse of time. In some systems even small variations of temperature (e.g., 3-5 °C) result in changes in colour. Some reactions are sensitive to changes of pH. A small change in pH, e.g., through 0.1 unit, may cause a 5% error. Other possible errors are caused by competitive reactions occurring in the system, or by changes in the ionic strength. 

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