Calibration of Volumetric Glassware

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  

An empty weighing bottle had a mass of 10.283 g. After water was added from a 25-mL pipet, the mass was 35.225 g. The temperature was 23 C. Find the volume of water delivered by the pipet. 

Test Yourself if the tenqierature had been 29 C, and the mass of water was 24.942 g, what volume was delivered (Answer 25.069 ntL) 

A 10-mL pipet delivered 10.(XX) 0 g of water at 15°C to a weighing bottle. What is the true volume of the pipet  

Each instrument that we use has a scale of some sort to measure a quantity such as mass, volume, force, or electric current. Manufacturers usually certify that the indicated quantity lies within a certain tolerance from the true quantity. For example, a Class A transfer pipet is certified to deliver 10.00 0.02 mL when you use it properly. Your individual pipet might always deliver 10.016 0.004 mL in a series of trials. That is, your pipet delivers an average of 0.016 mL more than the indicated volume in repeated trials. Calibration is the process of measuring the actual quantity of mass, volume, force, electric current, and so on, that corresponds to an indicated quantity on the scale of an instrument. 

Temperature (°C) Density (g/mL) Volume of 1 g of water (mL) At temperature Corrected shown to 20 C  

Page 38 gives a detailed procedure for calibrating a buret. 

The concentration of the solution decreases when the temperature increases. 

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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 ... 

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. 

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... 

British Standards Institution. 1987. Schedule for Tables for Use in the Calibration of Volumetric Glassware [BS1797], BSI, London. (This is a good example of a BSI monograph, and despite its date is still the current version.)... 

The volumes and concentrations can be measured with high accuracy. Calibration of volumetric glassware by discharge into weighed containers... 

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. 

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. 

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). 

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. 

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. 

See Testing of Glass Volumetric Apparatus, NBS Circ. 602, April 1, 1959. Apparatus meeting the specifications of NB SIR 74-461 ( The Calibration of Small Volumetric Laboratory Glassware ), as well as of ANSI/ASTM E 694-79 ( Specifications for Volumetric Ware ), is also acceptable. 

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. 

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