Calibration buret

The spreadsheet header table is shown in Figure 22-8. Create Names was used to assign the names in cells D3 D9 to cells C3 C9. Figure 22-9 shows a small portion of the data (entered in columns A and C) and the corrected volume and pH values. Vcorr is obtained by multiplying the nominal volume by the buret calibration factor, and pHcorr by adding the correction factor C to the measured pH. Columns of intermediate calculations are illustrated in Figure 22-10. 

Table 9.1), with the choice of buret determined by the demands of the analysis. The accuracy obtainable with a buret can be improved by calibrating it over several intermediate ranges of volumes using the same method described in Chapter 5 for calibrating pipets. In this manner, the volume of titrant delivered can be corrected for any variations in the buret s internal diameter. 

Good measurement practices (GMPs) describe operations specific to a technique. In general, GMPs provide instructions for maintaining, calibrating, and using the equipment and instrumentation that form the basis for a specific technique. For example, a GMP for a titration describes how to calibrate a buret (if nec-... 

It is convenient to calibrate the burets so that the liquid is divided into 20 equal portions. Then, in the addition of the reagents, these calibrations aid in synchronizing the rates. 

Calibrate the detector tube pump for proper volume measurement at least quarterly. Simply connect the pump directly to the bubble meter with a detector mbe in-line. Use a detector mbe and pump from the same manufacturer. Wet the inside of the 100 cc bubble meter with soap solution. For volume calibration, experiment to get the soap bubble even with the zero ml mark of the buret. For piston-type pumps, pull the pump handle all the way out (full pump stroke) and note where the soap bubble stops for bellows-type pumps, compress the bellows fully for automatic pumps, program the pump to take a full pump stroke. 

The device most commonly used to measure volume in general chemistry is the graduated cylinder. A pipet or buret (Figure 1.8) is used when greater accuracy is required. A pipet is calibrated to deliver a fixed volume of liquid—for example, 25.00 mL—when filled to the mark and allowed to drain. Variable volumes can be delivered accurately by a buret, perhaps to 0.01 mL. 

FIGURE G.9 A buret is calibrated so that the volume of liquid delivered can be measured. 

Graduated cylinders are not as precisely calibrated as burets or volumetric pipets. Briefly explain why it is acceptable to measure the K1 and HC1 solutions used in the titration with graduated cylinders rather than with pipets or burets. 

Given the care with which laboratory equipment (balances, burets, instruments, etc.) is calibrated at the factory, why should the chemical analyst worry about errors ... 

In order to effectively utilize the stoichiometry of the reaction involved in a titration, both the titrant and the substance titrated need to be measured exactly. The reason is that one is the known quantity, and the other is the unknown quantity in the stoichiometry calculation. The buret is an accurate (if carefully calibrated) and relatively high-precision device because it is long and narrow. If a meniscus is read in a narrow graduated tube, it can be read with higher precision (more significant figures) than in a wider tube. Thus a buret provides the required precise measurement of the titrant. 

Electronic devices such as automatic titrators and digital burets may be used in place of the traditional glass buret and manual titration. Such devices provide electronic control over the addition of titrant and thus, with proper calibration, are accurate, high-precision devices. These will be discussed in Section 4.9. 

Using either a pipet or a buret, quantities of standard solutions are measured. (If a buret is used, separate measurements of the initial and final volumes are needed.) Solvent may be added to dilute the samples if needed. These are the known solutions from which a calibration curve will be constructed. 

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. 

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. 

Bromine (80 mmole 12.8 g) is introduced by syringe into a 1-L reaction bulb (Fig. 1) containing a Teflon-coated magnetic stirring bar. The bulb is then placed on the vacuum line, cooled to -196°, and evacuated. Pentaborane(9) (78 mmole, 7.8 mL liquid volume at 0°) is distilled from a calibrated buret on the vacuum line and condensed into the reaction vessel, which is then allowed to warm to room temperature while the contents are vigorously stirred. A -196° bath is kept close at hand in the event the reaction proceeds too vigorously. After several hours, only a slight color remains. 

Calibration of Glass Tube. Determine the volume in mis of the 15.5cm heating tube by running in mercury from a buret until the tube is filled to the level at which the ground glass... 

In reactions with aliphatic azides gas evolution did not usually begin until approximately 5 min had elapsed from the time of initial addition. In contrast, immediate gas evolution was observed after the initial addition of azido nitriles and phenoxy azides to the nitrosonium salt. Total gas evolution was measured on the closed system by water displacement from a calibrated gas buret. Total gas evolution reflected the total amount of reacted azide and the different pathways for the production of gaseous products (nitrosative decomposition and Curtius rearrangement). The rate of production of gaseous products slowed markedly after the evolution of 40-60 mL (1-2 mmol of reacted azide) with the exception of nitrosative reactions wi th 4-azidobutanonitrilc and 5-azidopentanonitrile, gas evolution terminated when approximately 50 % of the azide had reacted. Gas evolution in the nitrosative reactions of aliphatic azides continued to completion as a result of protonic decomposition. Reactions were usually complete within 2 h. 

Hydrogen Analysis. The thermal decomposition technique was used to determine the hydrogen elemental composition. The sample was heated in vacuo to 925°C and was maintained at that temperature until all evolved gas was transferred by way of a Toepler pump to a calibrated gas buret. 

A volumetric flask is calibrated to contain a particular volume of solution at 20°C when the bottom of the meniscus is adjusted to the center of the mark on the neck of the flask (Figure 2-9, Table 2-3). Most flasks bear the label TC 20°C, which means to contain at 20°C. (Pipets and burets are calibrated to deliver, TD, their indicated volume.) The temperature of the container is relevant because both liquid and glass expand when heated. 

Pipets deliver known volumes of liquid. The transfer pipet in Figure 2-1 la is calibrated to deliver one fixed volume. The last drop does not drain out of the pipet and should not be blown out. The measuring pipet in Figure 2-1 lb is calibrated like a buret. It is used to deliver a variable volume, such as 5.6 mL, by starting delivery at the 1.0-mL mark and terminating at the 6.6-mL mark. The transfer pipet is more accurate, with tolerances listed in Table 2-4. 

Page 38 gives a detailed procedure for calibrating a buret. 

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