Volumetric glassware calibration

Volumetric procedures incorporate several important sources of systematic error. Chief amongst these are the drainage errors in the use of volumetric glassware, calibration errors in the glassware, and indicator errors. Perhaps the commonest error in routine volumetric analysis is to fail to allow enough time for a pipette to drain properly, or a meniscus level in a burette to stabilize. Pipette drainage errors have a systematic as well as a random effect the volume delivered is invariably less than it should be. The temperature at which an experiment is performed has two effects. Volumetric equipment is conventionally calibrated at 20°C, but the temperature in... 

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. 

Instrument/Equipment Effects Examples include the calibration and precision of an analytical balance, the specified tolerance for volumetric glassware and a temperature controller that maintains a mean temperature which is different (within specification) from its indicated value. 

Some volumetric glassware products have a large A imprint on the label. This designates the item as a class A item, meaning that more stringent calibration procedures were undertaken when it was manufactured. Class A glassware is thus more expensive, but it is most appropriate when highly precise work is important. This imprint may be found on both flasks and pipets. 

Standardization of a Titrant For wet chemistry analytical methods, a titration is often used and the titrant, or the solution to which an unknown sample is compared, must be standardized. This can be done by comparing it with another standard. The important thing here is that the standard with which it is compared is ultimately traced to a SRM. The procedure utilizes volumetric glassware heavily, and thus the analyst must be assured that these are properly calibrated, as discussed above. Auto-titrators can be used (Figure 5.4). In this case, the automated equipment can be calibrated against manual equipment, i.e., volume readings obtained with the auto-titrator must match the volume readings obtained with a calibrated buret for the same sample. If they do not match (within accepted limits), the auto-titrator must be taken out of service and repaired, just like the defective balance. 

The pipet delivers less volume at 20°C than at 27 C because glass contracts slightly as the temperature is lowered. Volumetric glassware is usually calibrated at 20°C. 

The accurate calibration of volumetric glassware must also take buoyancy into account. For example, in the previous problem, if the observed weight of water is that needed to fill a 100 ml volumetric flask exactly to the mark, we could easily calculate the true volume of the volumetric flask just as we did on p 86 ... 

General principles of calibration of course apply to speciation analysis (Griepink, 1993 Quevauviller et al., 1996a). All efforts made to obtain a good sample and perform the extraction under the proper conditions are spoiled if the calibration is wrong. Basic principles of QA apply here, including calibration of balance and volumetric glassware, verification of the calibrant purity and stoichiometry, verification of the stability of stock solutions, etc. 

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. 

A calibration schedule details the calibration of balances, volumetric glassware, automatic pipettes, thermometers, pH and conductivity meters, wavelength and photometric scales etc. The schedule consists of periodic external checks, employing a suitably accredited calibration service, supported by more regular in-house performance checks. 

There is at present no generally accepted criterion for the accuracy of instruments, although tolerances for the calibration of volumetric glassware and thermometers have been published. Manufacturers claims of accuracy within 1% are diflScult to assess without knowledge of the samples tested or method used to obtain the value. It should be possible for manufacturers to specify the accuracy of calibration of many instruments. As a general rule, it seems desirable that any inaccuracy in an instrument should not contribute significantly to the total inaccuracy of the result. 

Volumetric glassware is marked with one or more graduation marks to indicate liquid volumes. Calibration of glassware is usually at 20 °C but in the USA sometimes calibration is at 25 °C. 

The laboratory has to be able to check that the calibrated equipment is still within the specifications required each time it is used. It also has to check other similar items of equipment in the laboratory against the officially calibrated ones. So, for balances, it would keep some special check weights and record their weights each time the balance is used. Volumetric glassware must be of a recognised specification (such as BS 1583) and be checked regularly, in the manner of the practical exercise in Section 5. 

Certain variables— water quality, calibration of analytical balances, calibration of volumetric glassware and pipettes, stability of electrical power, and the temperature of heating baths, refrigerators, freezers, and centrifuges—should be monitored on a laboratorywide basis because they wiU affect many of the methods in the laboratory (see Chapter 1). In addition, there wiU be certain variables that relate more directly to individual analytical methods, and these require... 

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. 

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