Measurement volumetric glassware

To ensure that S eas is determined accurately, we calibrate the equipment or instrument used to obtain the signal. Balances are calibrated using standard weights. When necessary, we can also correct for the buoyancy of air. Volumetric glassware can be calibrated by measuring the mass of water contained or delivered and using the density of water to calculate the true volume. Most instruments have calibration standards suggested by the manufacturer. 

Densities are perhaps the most straightforwardly determined and unambiguous physical property of ionic liquids. Given a quality analytical balance and good volumetric glassware the density of an ionic liquid can be measured gravimetrically (i.e., the sample can be weighed). 

It is important that any method chosen is scientifically sound under the conditions it will be applied. It is also necessary to demonstrate that the equipment, which will be used, is suitable and its use will not influence the results adversely. This includes all types of equipment, e.g. does the volumetric glassware have a suitable tolerance and do the instruments have sufficient sensitivity over the entire range of measurement The process for demonstrating equipment capability is called equipment qualification and is dealt with in Chapter 5. The staff carrying out validation need to be both qualified and competent in the tasks that they need to carry out. 

First, volumetric glassware should not be used for any volume measurement when the overall objective is strictly qualitative or when quantitative results are to be reported to two significant figures or less. 

Second, if the results of a quantitative analysis are to be reported to three or more significant figures, then volume measurements that enter directly into the calculation of the results should be made with volumetric glassware so that the accuracy of the analysis is not diminished when the calculation is performed. 

Fourth, if a volume measurement does directly affect the numerical result of an accurate quantitative analysis (in a way other than entering directly into the calculation of the results), then volumetric glassware must be used. 

Concentrations in the region of 0.1 mol 1 1 are often convenient but it obviously depends upon such factors as the amount of substance available, the cost, the solubility, etc. From this stock solution, a series of accurate dilutions are prepared using volumetric glassware and the absorbance of each dilution measured in a 1-cm cuvette at the wavelength of maximum absorbance for the compound. A plot of absorbance against concentration will give an indication of the validity of the Beer-Lambert relationship for the compound and a value for the molar absorption coefficient may be calculated from these individual measurements or from the slope of the linear portion of the graph ... 

Systematic error arises from imperfections in an experimental procedure, leading to a bias in the data, i.e., the errors all lie in the same direction for all measurements (the values are all too high or all too low). These errors can arise due to a poorly calibrated instrument or by the incorrect use of volumetric glassware. The errors that are generated in this way can be either constant or proportional. When the data are plotted and viewed, this type of error can usually be discovered, i.e., the intercept on the y-axis for a calibration is much greater than zero. 

Typical volumetric glassware (a) volumetric flask, (h) transfer pipette, (c) measuring pipette,... 

This chapter will deal with volumetric analysis, that is analysis carried out by the accurate measurement of volumes. To measure volumes accurately, use must be made of volumetric glassware. There are three pieces of volumetric glassware that are fundamental to successful volumetric analysis. These are the volumetric flask, the pipette and the burette, and each will be described below (see Figure 6.1). It should be stated, however, that no amount of reading about these pieces of apparatus (no matter how eloquently written ) is sufficient to educate a student. Analytical pharmaceutical chemistry is first and foremost a practical subject, and the laboratory is the best place to get to grips with the techniques required for consistent, reproducible analysis. 

Volumetric glassware is calibrated by measuring the mass of a liquid (usually distilled or deionized water) of known density and temperature that is contained in (or delivered by) the volumetric ware. In carrying out a calibration, a buoyancy correction must be made (Section 2D-4), since the density of water is quite different from that of the masses. 

Besides the general rules given above, particular and severe requirements underpin the BCR certification studies. In all cases, detailed protocols and reporting forms are prepared, requesting each participant to demonstrate the quality of the measurements performed, in particular the validity of calibration (including calibration of balances, volumetric glassware and other tools of relevance, use of calibrants of adequate purity... 

Erom this method calibration, simple extrapolation of a measurement unknown will produce a concentration estimate. However, from a QA perspective, day-to-day comparisons of these simple calibration measurements, and the associated statistics, can often highlight aspects of reagent quality, apparatus (balance, volumetric glassware, etc.), and operator competence when compared. These data can be invaluable in assisting regulatory compliance, and ensuring that the complete measurement system is under control. 

Calibration is the process of relating the actual quantity (such as mass, volume, or electric current) to the quantity indicated on the scale of an instrument. Volumetric glassware can be calibrated to measure the volume that is actually contained in or delivered by a particular piece of equipment. Calibration is done by measuring the mass of water contained or delivered and using Table 2-5 to convert mass to volume ... 

Figure A-2 Measurement of the volume of water using two types of volumetric glassware.

We commonly measure liquid volumes with precision volumetric glassware such as burets, pipets, and volumetric flasks. The National Institute of Standards and Technology in the United States has established specifications for Class A glassware two examples are listed in Table 2.2 on the next page. We may accurately determine the volume of a vessel at one temperature from the mass of a liquid of known density, such as water, that fills the vessel at this temperature. 

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