Imprecision volume measurement

The measurement uncertainty of weighing will usually lead to a deviation due to inaccuracy and imprecision of not more than 1 % [10]. Volume measurements however, may show a deviation of 1.5 % under the most favourable conditions. Under less favourable conditions this can easily rise to 3 % or more (see Sect. 29.1.7). 

The more diluted solutions are, the less satisfactory the titrations will be. With concentrations about 10 mol/L, titrations are still satisfactory since the random error resulting from the imprecision due to the pH measurements remains weaker than that inherent to the volumes measured with graduated glass flasks (error about 0.1%). Until concentrations of 10 mol/L, titrations are still reasonably good. For 10 mol/L solutions, the precision is about 1%, provided care is taken to eliminate carbonic gas. 

Student 3 has obtained a set of results which are widely scattered and hence imprecise, and which give a mean which is less than the correct answer. Thus the analysis contains random errors or possibly, looking at the spread of the results, three defined errors which have been produced randomly. The analysis was thus poorly controlled and it would require more work than that required in the case of student 2 to eliminate the errors. In such a simple analysis the random results might simply be produced by, for instance, a poor pipetting technique where volumes both higher and lower than that required were measured. 

HPLC has more or less supplanted GC as a method for quantifying drugs in pharmaceutical preparations. Many of the literature references to quantitative GC assays are thus old and the precision which is reported in these papers is difficult to evaluate based on the measurement of peak heights or manual integration. It is more difficult to achieve good precision in GC analysis than in HPLC analysis and the main sources of imprecision are the mode of sample introduction, which is best controlled by an autosampler, and the small volume of sample injected. However, it is possible to achieve levels of precision similar to those achieved using HPLC methods. For certain compounds that lack chromophores, which are required for detection in commonly used HPLC methods, quantitative GC may be the method of choice, for analysis of many amino acids, fatty acids, and sugars. There are a number... 

Dilute Samples and Manual Method. As already discussed (Figure 3), a usable but nonlinear response that is obtained can allow measurements down to 1 X 10 M chloride. If 50-pL volumes are coupled with measurements down to 10 M, then the amount of chloride needed can be reduced further, from 0.09 to 0.002 pg. Imprecision will increase below 5 X 10-5 M. 

Section 12.2 defines specific heat capacity as the amount of heat required to increase the temperature of 1 g of material by 1 K. That definition is somewhat imprecise, because, in fact, the amount of heat required depends on whether the process is conducted at constant volume or at constant pressure. This section describes precise methods for measuring the amonnt of energy transferred as heat during a process and for relating this amonnt to the thermodynamic properties of the system under investigation. 

For trace analysis it is preferable to use a method that relies on the relative response factor of each compound to be measured against a marker introduced as a reference. This means that any imprecision concerning the injected volumes, the principal constraint of the previous method, is compensated. As above, this... 

There are several ways to approach such problems. When you are uncomfortable with calculus, it may initially be the simplest to use algebraic expressions or series expansion. For example, the volume V of a sphere, expressed in terms of its diameter d, is 4l3)w(d/2)3 = ird3l6. When the measurement produces a diameter d Ad, where Ad is an estimate of the experimental imprecision in d, then the volume follows as V AV ir(d Ad )3 / 6 = ttI 6) X (d 3 3d2 Ad + 3d( Ad )2 Ad)3) ttI 6) X d3 3d2 Ad) = ird3/6) X (1 3Adid) when we make the usual assumption that Ad d, so that all higher-order terms in Ad can be neglected. In other words, the relative standard deviation of the volume, A V/V, is three times the relative standard deviation A did of the diameter, a result we could also have obtained from the above-quoted rules for multiplication, because r3 = rX rX r. 

Any device or system that has one or more physical properties (e.g., electrical resistance, electrical potential, length, pressure at constant volume, or volume at constant pressure) that vary monotonically and repro-ducibly with temperature may be used to measure temperature. The science of the measurement of temperature is called thermometry. In the past, the measurement of high temperature was known as pyrometry but now that term usually refers to radiation thermometry at any temperature. Although the accuracy of a measurement refers to the difference between the measured value and the true value of the quantity being measured, and the precision of measurement refers to the degree of agreement among repeated measurements of the same quantity, it follows that a set of measurements of the same quantity, it follows that a set of measurements may be very precise but terribly inaccurate. Since in many instances the word accuracy is used when inaccuracy is meant and the word precision is used when imprecision is meant, perhaps it would be better always to refer to uncertainties of measurement, statistical and systematic, rather than to accuracy and precision. 

Thus, explosives like TNT are casually referred to as more powerful, although that is an imprecise term. In truth, explosives cannot be measured by a single factor. Detonation velocity is the speed of the reaction. Brisance is the ability to fragment (i.e., the shell casing) it is a combination of density, heat, detonation rate, and gas volume released. 

The internal standard method is based on the use of the relative response factor of each component to be measured with respect to a marker introduced as reference. This avoids the imprecision related to the injected volumes, which is a disadvantage of the previous method. However, it requires the addition of a component to a sample dilution. In general, a calibration curve is built by ap>plying different solutions of increased concentrations of the standard analyte with a constant quantity of internal standard. When injecting such samples, we obtain the relation between the areas of the analyte and the internal standard then, it is marked in a graph according to the concentration of analyte in each solution. By means of interpolation in the graphic, we get the relation of the areas of an unknown sample, which has to contain the same quantity of internal standard. 

The imprecision of the read-out can be made as small as possible by choosing a graduated pipette, syringe or measuring cylinder with a nominal capacity as near to the volume to be measured as possible. For example a volume of 0.5 mL should be measured by a 0.5 mL automatic pipette or with a 1.0 mL traditional graduated pipette. A volume of 21 mL should preferably be measured with a 25 mL graduated pipette or a measuring cylinder of a nominal volume of 25 mL. 

The sources that need to be taken into consideration in the evaluation of the measurement uncertainty of the photometric method include instrument-related uncertainties, for example imprecision and drift in absorbance readings reagent-related uncertainties, for example uncertainties in the volume and tenqterature of the reagents, incomplete mixing, pH dependence, and evaporation and system-related uncertainties, for example system nonlinearity. 

As the name implies, cup viscosity tests employ a cup-shaped gravity device that permits the timed flow of a known volume of liquid through an orifice located at the bottom of the cup. Under ideal conditions, this rate of flow would be proportional to the kinematic viscosity that is dependent upon the specific gravity of the draining liquid. However, the conditions in a simple flow cup cannot be considered ideal for true measurements of viscosity. Cup viscosity tests, however imprecise, are practical, easy-to-use instruments for making flow comparisons under strictly comparable conditions (2,3). 

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