Precision volume measurement

As indicated in Section 4.1 (and as should be apparent from the discussion thus far in this chapter), titrimetric analysis methods heavily utilize solution chemistry, and therefore volumes of solutions are prepared, measured, transferred, and analyzed with some degree of frequency in this type of analysis. It should not be surprising that analytical laboratory workers need to be well versed in the selection and proper use of the glassware and devices used for precise volume measurement. 

Precise volume measurements are needed when you perform a titration. Chemists use special glass apparatus to collect these measurements. (See Figure 10.15.) As well, an acid-base indicator is needed to monitor changes in pH during the titration. 

Cost. Usually, the gravimetric technique is costlier than the volumetric technique. The volumetric technique requires only high precision pressure transducers and high precision volume measurements. The gravimetric, however, requires a high precision vacuum balance and, perhaps, considerable set-up effort. 

If the mobile phase is a liquid, and can be considered incompressible, then the volume of the mobile phase eluted from the column, between the injection and the peak maximum, can be easily obtained from the product of the flow rate and the retention time. For more precise measurements, the volume of eluent can be directly measured volumetrically by means of a burette or other suitable volume measuring vessel that is placed at the end of the column. If the mobile phase is compressible, however, the volume of mobile phase that passes through the column, measured at the exit, will no longer represent the true retention volume, as the volume flow will increase continuously along the column as the pressure falls. This problem was solved by James and Martin [3], who derived a correction factor that allowed the actual retention volume to be calculated from the retention volume measured at the column outlet at atmospheric pressure, and a function of the inlet/outlet pressure ratio. This correction factor can be derived as follows. 

Equation (3) merely sums the two peaks to produce a single envelope. Providing retention times can be measured precisely, the data can be used to determine the composition of a mixture of two substances that, although having finite retention differences, are eluted as a single peak. This can be achieved, providing the standard deviation of the measured retention time is small compared with the difference in retention times of the two solutes. Now, there is a direct relationship between retention volume measured in plate volumes and the equivalent times, which is depicted in Figure 6. 

Anyone making a measurement has a responsibility to indicate the uncertainty associated with it. Such information is vital to someone who wants to repeat the experiment or judge its precision. The three volume measurements referred to earlier could be reported as... 

There are two major factors that influehce retention volume measurement and they are temperature and solvent composition. In order to measure retention volume with adequate precision it is necessary to know the relationship between retention time and temperature so that the control limits of the column temperature can be specified. 

It is seen that in order to measure retention volumes with a precision of 0.1%, the temperature control must be +/- 0.04°C. This level of temperature control on a thermostat bath is not difficult to achieve but it is extremely difficult, if not impossible, to return to a specific temperature to within +/- 0.04°C after prior change. To achieve a precision of retention volume measurement of 1%, the temperature control must be +/- 0.4°C. This is far more practical as most column oven temperature can be set to a given temperature to within +/-0.25°C. Although the data was obtained for three specific solutes, the results can be taken as reasonably representative for all solutes and phase systems. In most practical analyses, the precision limits of retention volume measurement will be about 1% but this will not include the reproducibility of the flow rate given by the pump. As... 

Loops are not calibrated accurately and a loop of nominally 20 p.l is unlikely to have this exact volume. This will not affect either the precision of measurement and, as long as the same loop is used for obtaining the quantitative calibration and for determining the unknowns , the accuracy of measurement. 

Gas pycnometry (gas displacement) Rapid Nondestructive Easy to use Expensive instrumentation Large sample size Accuracy limited by precision of volume measurements... 

The estimation of the overall precision of a methodfrom its unit operations A frequent problem in analysis is the estimation of the overall precision of a method before it has been used or when insufficient data are available to carry out a statistical analysis. In these circumstances the known precision limits for the unit operations of the method (volume measurement, weighing, etc.) may be used to indicate its precision. Table 2.6 gives the normal precision limits for Grade A volumetric equipment. 

Similarly, the overall precision of the volume measurement is obtained. An allowance for the uncertainty in the colour change observation at the end point must be included (e.g. 0.03 cm3... 

Previously, we discussed the need to know when a weight measurement does and does not need to be precise so that the appropriate balance is chosen for the measurement. We now repeat many of our previous comments, but for volume measurements rather than weight. 

Third, if a volume measurement is only incidental to the overall result of an accurate quantitative analysis, then it need not be precise. This means that if the volume measurement to be performed has no bearing whatsoever on the quantity of the analyte tested, but is only needed to support the chemistry or other factor of the experiment, then it need not be precise. 

These last two points require that the analyst carefully consider the purpose of the volume measurement and whether it will directly affect the quantity of analyte tested, the numerical value to be reported. It is often true that volume measurements taken during such a procedure need not be precise even though the analytical results are to be precisely reported. 

The volume of viscous materials, such as grease or ointments, can be measured with a special pycnometer made of aluminum. The chamber is milled to ensure precise volume. A cap with a capillary opening is screwed into the body such that any air is expelled. See Figure 15.16 and Workplace Scene 15.3. 

Ann McNichol is a Research Specialist at the National Ocean Sciences Accelerator Mass Spectrometry Facility at the Woods Hole Oceanographic Institution, which produces high-precision 14C measurements from small-volume samples. Dr. McNichol s research interests include the study and use of carbon, nitrogen, and oxygen isotope techniques to quantify bio-geochemical processes, the study of the fate of organic matter (both natu-... 

A pipette is used to measure and transfer a precise volume of liquid. You rinse a pipette with the solution whose volume you are measuring. This ensures that the solution will not be diluted or contaminated. 

The instruments for polymer HPLC except for the columns (Section 16.8.1) and for some detectors are in principle the same as for the HPLC of small molecules. Due to sensitivity of particular detectors to the pressure variations (Section 16.9.1) the pumping systems should be equipped with the efficient dampeners to suppress the rest pulsation of pressure and flow rate of mobile phase. In most methods of polymer HPLC, and especially in SEC, the retention volume of sample (fraction) is the parameter of the same importance as the sample concentration. The conventional volumeters— siphons, drop counters, heat pulse counters—do not exhibit necessary robustness and precision [270]. Therefore the timescale is utilized and the eluent flow rate has to be very constant even when rather viscous samples are introduced into column. The problems with the constant eluent flow rate may be caused by the poor resettability of some pumping systems. Therefore, it is advisable to carefully check the actual flow rate after each restarting of instrument and in the course of the long-time experiments. A continuous operation— 24h a day and 7 days a week—is advisable for the high-precision SEC measurements. THE or other eluent is continuously distilled and recycled. 

Contact methods of measuring surface roughness (see Chapter 7) are not likely to be successful with uncured rubber because of its softness. It is unlikely that roughness needs to be known very precisely and a simple method has been given by Orlovskii et al139. The volume of a disc is calculated using the overall thickness measured on top of any irregularities and compared to the true volume measured by a liquid displacement method. 

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