Calibration of volumetric apparatus

The Lunge-Rey pipette is shown in Fig. 3.3(b). There is a small central bulb (5-10mL capacity) closed by two stopcocks 1 and 2 the pipette 3 below the stopcock has a capacity of about 2 mL, and is fitted with a ground-on test-tube 4. This pipette is of particular value for the weighing out of corrosive and fuming liquids. 

In piston burettes, the delivery of the liquid is controlled by movement of a tightly fitting plunger within a graduated tube of uniform bore. They are particularly useful when the piston is coupled to a motor drive, and in this form serve as the basis of automatic titrators. These instruments can provide automatic plotting of titration curves, and provision is made for a variable rate of delivery as the end point is approached so that there is no danger of overshooting the end point. 

In all calibration operations, the apparatus to be calibrated must be carefully cleaned and allowed to stand adjacent to the balance which is to be employed, together with a supply of distilled or de-ionised water, so that they assume the temperature of the room. Flasks will also need to be dried, and this can be accomplished by rinsing twice with a little acetone and then blowing a current of air through the flask to remove the acetone. 

To test the delivery time, again separate the components of the stopcock, dry, grease and reassemble, then fill the burette to the zero mark with distilled water, and place in the holder. Adjust the position of the burette so that the jet comes inside the neck of a conical flask standing on the base of the burette stand, but does not touch the side of the flask. Open the stopcock fully, and note the time taken for the meniscus to reach the lowest graduation mark of the burette this should agree closely with the time marked on the burette, and in any case, must fall within the limits laid down by BS 846 (1985). 

Grade 2 water to reverse osmosis or de-ionisation, followed by filtration through a membrane filter of pore size 0.2 jum to remove particulate matter. Alternatively, Grade 2 water may be redistilled in an apparatus constructed from fused silica. 

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The following table, which is designed for gravimetric calibration of volumetric apparatus, gives the specific volume of water at standard atmospheric pressure as a function of temperature. 

With regard to the width of the neck, this should be of such a size that the error in reading the volmne is not greater than the allowable error in the calibration of volumetric apparatus, which may be taken as o o5 per cent, of the volume measured. In the case of smaller flasks, for 50 c.c. or less, the Regnault flask (Regnault pyknometer) is very useful. This has a very narrow neck, so as to diminish the error in reading and in order to obtain sufficient airspace to ensure ready mixing, the neck is widened at the top (Fig. i). 

The capacity of a glass vessel varies with the temperature, and it is therefore necessary to define the temperature at which its capacity is intended to be correct in the UK a temperature of 20 °C has been adopted. A subsidiary standard temperature of 27 °C is accepted by the British Standards Institution, for use in tropical climates where the ambient temperature is consistently above 20 °C. The US Bureau of Standards, Washington, in compliance with the view held by some chemists that 25 °C more nearly approximates to the average laboratory temperature in the United States, will calibrate glass volumetric apparatus marked either 20 °C or 25 °C. 

In the volumetric apparatus, the amount of water distilled from a calibrated capillary tube was determined by measurement of the change in level of the water in the capillary with a slide micrometer. The estimated error is about 1% of the amount adsorbed at low coverages, and less than 0.2% at high coverages. 

Most of the volumetric apparatus available in the United States is calibrated at 20°, although the temperatures generally prevailing in laboratories more nearly approach 25°, which... 

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. 

Volumes of liquids or gases are measured by using glass or plastic apparatus that have been calibrated to read in units of volume as shown in FIGURE B.l. Certain apparatus such as graduated cylinders, volumetric flasks, and some pipets are designed to contain a specific volume of liquid when filled to the appropriate calibration mark. Other apparatus such as burets and some pipets are designed to deliver a specific volume of liquid into another container. 

See, for example I. M. Kolthoff and P. J. Elving, "Treatise on Analytical Chemistry." Part 1, "Theory and Practice of Analytical Chemistry." Interscience, New York, NY, 1959 - 75 or J. C. Hughes, "Testing of Glass Volumetric Apparatus," in "Apparatus in Precision Measurement and Calibration," Handbook 77, Vol. 3, Natl. Bur. of Standards, Washington, DC, 1961. 

Recently an automatic apparatus for surface areas and pore size determination has been described [125]. The Isothermegraphe is a volumetric apparatus, with a calibrated tube, which draws complete adsorption-desorption isotherms using a piston of mercury which modifies the pressure slowly at a programmed speed. 

Most of the instruments, commonly used in an analytical laboratory, such as UV-Spectrophoto-meter, IR-Spectrophotometer, single—pan electric balance, pH-meter, turbidimeter and nephelometer, polarimeter, refractometer and the like must be calibrated duly, before use so as to eliminate any possible errors. In the same manner all apparatus, namely pipettes, burettes, volumetric flasks, thermometers, weights etc., must be calibrated duly, and the necessary corrections incorporated to the original measurements. 

Significant time is saved in this calibration procedure by minimizing sample preparation and the number of calibrated test apparatus wherein only a balance, a 5-mL volumetric flask, a stopwatch, a thermal probe, two standards, one HPLC column, and one mobile phase are used. The use of MS Word template forms (see Figure 10) saves considerable time by eliminating any notebook entries and also improves the consistency of calibration records. Two important system parameters, dwell volume, and... 

The volumetric method is mainly used for the purpose of determining specific surface areas of solids from gas (particularly nitrogen) adsorption measurements (see page 134). The gas is contained in a gas burette, and its pressure is measured with a manometer (see Figure 5.4). All of the volumes in the apparatus are calibrated so that when the gas is admitted to the adsorbent sample the amount adsorbed can be calculated from the equilibrium pressure reading. The adsorption isotherm is obtained from a series of measurements at different pressures. 

The single most important application of statistical methods in science is the determination and propagation of experimental uncertainties. Quantitative experimental results are never perfectly reproducible. Common sources of error include apparatus imperfections, judgments involved in laboratory technique, and innumerable small fluctuations in the environment. Does the slight breeze in the lab affect a balance When a motor starts in the next building, does the slight power surge affect a voltmeter Was the calibrated volumetric flask perfectly clean ... 

Another concern for accuracy is based on how accurately the user can read calibrations on the volumetric ware. The reproducibility of an individual user will be more consistent than the reading made by a variety of users. Therefore, if there will be a variety of users on any given apparatus, all who are likely to use it should make a series of measurements. This way, the individual errors can be calculated. The ASTM has analyzed the range of errors made by trained personnel, and the reproducibility of these results are shown in Table 2.9. 

Procedure Transfer 1.00 g of sample into a 150-mL glass beaker, add 10 mL of water, and, while stirring continuously, slowly add 20 mL of 1 A hydrochloric acid to dissolve the sample. Boil rapidly for 1 min, then transfer into a 250-mL plastic beaker, and cool rapidly in ice water. Add 15 mL of 1 M sodium citrate and 10 mL of 0.2 M disodium EDTA, and mix. Adjust the pH to 5.5 0.1 with 1 A hydrochloric acid or 1 A sodium hydroxide, if necessary transfer into a 100-mL volumetric flask dilute to volume with water and mix. Transfer a 50-mL portion of this solution into a 125-mL plastic beaker, and measure the potential of the solution with the apparatus described under Calibration Curve. Determine the fluoride content, in micrograms, of the sample from the Calibration Curve. 

A volumetric/manometric sorption measuring apparatus consists of a calibrated and thermostated sample bulb, a burette for controlled admission of the sorptive gas and some manometers. The sample is dried mostly in a separate degassing station in vacuum at elevated temperature. Preferably optimum degassing conditions are found out thermogravimetrically. 

The quality of apparatus is crucial. The quality of the pipets directly affects the reliability of the analytical information. It is necessary to use measuring pipets of high quality for reliable measures of volume, e.g., clinical, serological pipets, micropipets, syringe pipets. Syringe pipets must be used for measurement of microliter volumes. In volumetric analysis more reliable burets are necessary. For small quantities of sample, micro- and ultramicro-micrometer burets are recommended. Only well-calibrated pipets and burets will assure the accuracy of volume determination. 

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