Calibration of glassware

In a similar manner, the calibration of glassware, such as volumetric flasks, pipettes, burettes, measuring cylindres are duly carried out by specific methods recommended by Indian Standards Institution (ISI), British Standards Institution (BSI), National Physical Laboratory (NPL), United States Pharmacopoeia (USP) at specified temperatures (See Chapter 2). 

Is it all worth it Calibration of glassware and careful temperature measurements during an experiment cause significant reduction in the uncertainty of the volumes of glassware. However, if the uncertainty in the glassware is only a minor contributor to the whole, you must judge each case on its merits and act accordingly. 

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. 

Calibration of glassware and tedious cleaning to ensure proper drainage are completely eliminated. 

Example 2.1 illustrated how Equation 2.1 may be used in the calibration of glassware, to correct for buoyancy of the water used for calibration, that is, to correct to weight in vacuum. Dividing the weight of the water in vacuum by its density at the given temperature will convert it to volume. 

Calibration of glassware. If your instructor wishes youip use the data from these experiments to calibrate the volumes of the glassware, use Table 2.4 to convert the weight in air to the volume. The temperature of the water delivered must be determined the Table 2.4 spreadsheet is on your CD, and you can enter data in that by copying it to your computer desktop. 

The term calibration is often used when in fact what is meant is verification . Calibration of an instrument or a piece of equipment (e.g. glassware) involves making a comparison of a measured quantity against a reference value. For example, to calibrate a spectrophotometer response, the appropriate reference material is selected and the spectrophotometer response to it, under the specified conditions, is measured. Then, the measured value is compared to the value quoted in the literature. Either a correction is made to the results from subsequent measurements or an adjustment is made to the instrument. 

What is essential in establishing traceability is that the measurand is specified unambiguously. This may be, e.g. in terms of extractable cadmium from soil by using a named acid mix or the concentration of a metal in a particular oxidation state, e.g. Fe(n) or Fe(m). The units used to report the result should also be known and acceptable SI units are preferred. The method used will be validated and if used in accordance with the written procedures should produce results that are fit for purpose . The class of glassware to be used will be specified in the method procedure, e.g. Class A pipettes and volumetric flasks, as these are manufactured to a specified tolerance. Instruments will be regularly calibrated and their performance verified daily. In terms of the chemicals used, these will... 

As indicated previously, most pipets are pieces of glassware that are designed to deliver (TD) the indicated volume. Pipets come in a variety of sizes and shapes. The most common is probably the volumetric pipet, or transfer pipet, shown in Figure 4.8. This pipet, like the volumetric flask, has a single calibration... 

A volumetric flask is a precision piece of glassware with a single calibration line in a narrow neck An Erlenmeyer is not a precision piece of glassware and has graduation lines on it only as a rough indication of volume. 

It is possible to check the calibration of a pipet, flask, or buret. The process involves weighing with a calibrated analytical balance. The volume of water (temperature noted) delivered or contained by the glassware is weighed. Then the analyst converts this weight to volume (using the density of water at the temperature noted), corrects the result to 20°C (the usual temperature of the factory calibration), and compares it to the factory calibration. If the difference is not tolerable, the piece of glassware is either not used for accurate work or a correction factor is applied. It should be pointed out that the thermometers used must be properly calibrated and that the timer used to measure the delivery time for the burets and pipets must also be calibrated. 

Thus it can be seen that the compound error from the glassware differs little from the largest error in the process. Of course the glassware errors can be eliminated by calibration of the glassware prior to use but in general analysts will accept manufacturers tolerances. The tolerated random error from glassware could be readily eliminated other random errors such as variation in the extraction efficiency are more difficult to control. 

Cleaning of Glassware Used in Calibration of TNT Thermometers" is described in Vol 2, pp C7-R C8-L, under "Calibration of TNT Thermometers"... 

Accuracy in the determination depends on the care with which the glassware is cleaned, how solvents are purified, how solutions are prepared and clarified, the calibration of the photometer, the determination of the specific refraction increment (dn/dc), etc. Consequently, the accuracy is often not better than 10%. 

Suppose a very large number of measurements could be made under conditions of measurement that allow all possible variation that could occur, including systematic effects from calibrations of balances, glassware, and so on. Also suppose that the material being measured was identical for all of these measurements, which were correctly applied with any identified systematic effects corrected for. The reality of measurement uncertainty is that these measurements would not be identical but would scatter around the value of the measurand. In the absence of any other information, this dispersion is assumed to be normally distributed, which can be described by two parameters, the mean (p) and the standard deviation (cr). It is not... 

The standard recognizes (section 5.6.1) that the traceability requirements should apply to aspects of the method that have a significant influence on the result of the measurement. For an analytical chemistry laboratory, as well as for the reference materials used for calibrating the response of an instrument, balances will need to be calibrated from time to time, and appropriate certification of the traceability of glassware and thermometers must be available. 

Calibration of chemical glassware at plants manufacturing expls, etc 2 C6... 

For greatest accuracy, we calibrate volumetric glassware to measure the volume actually contained in or delivered by a particular piece of equipment. We do this by measuring the mass of water contained or delivered by the vessel and using the density of water to convert mass into volume. 

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 ... 

Burette Calibrations See under Calibration of Laboratory Glassware... 

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