Pipet measuring

Common types of pipets and syringes (a) transfer pipet (b) measuring pipet (c) digital pipet (d) syringe. 

FIGURE4.il The tips of two styles of measuring pipets. The Mohr pipet is shown on the left, and the serological pipet on the right. The graduation lines on the Mohr pipet stop short of the tip, but on the serological pipet, pass through the tip. 

Compare a volumetric pipet with a serological measuring pipet in terms of ... 

It is obvious that the flow rate must be precisely controlled. The pressure from the compressed gas cylinder of carrier gas, while sufficient to force the gas through a packed column, does not provide the needed flow control. Thus a flow controller valve is built into the system. The flow rate of the carrier gas, as well as other gases used by some detectors, must be able to be carefully measured so that one can know what these flow rates are and be able to optimize them. Flow meters are commercially available. However, a simple soap bubble flow meter is often used and can be constructed easily from an old measuring pipet, a piece of glass tubing, and a pipet bulb. See Figure 12.10. With this apparatus, a stopwatch is used to measure the time it takes a soap bubble squeezed from the bulb to move between two graduation lines, such as the 0- and 10-mL lines. The commercial version uses an electronic sensor to measure the flow rate based on the bubble movement. See Workplace Scene 12.3. 

Prepare calibration standards that are 0.05, 0.10, 0.15, and 0.20 mg/mL in caffeine from the stock standard. Use 25-mL volumetric flasks and measuring pipets or pipetters suggested by your instructor. Calculate the concentration of sodium benzoate in each of these solutions. 

By dilution measure out 6.22 mL of the 4.021 M solution as precisely as possible (measuring pipet), dilute it to 100.0 mL (volumetric flask), and shake. By weight of the pure solid weigh out 2.650 g on an analytical balance, place in a 100-mL volumetric flask, dissolve in water, dilute to 100.0 mL, and shake. 

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. 

Which is more accurate, a transfer pipet or a measuring pipet ... 

A dilution procedure typically involves two types of glassware a pipet and a volumetric flask. A pipet is a device for accurately measuring and transferring a given volume of solution. There are two common types of pipets measuring pipets and volumetric pipets, as shown in Fig. 4.10. Measuring pipets are used to measure out volumes for which a volumetric pipet is not available. For example, we would use a measuring pipet as shown in Fig. 4.11 to deliver 28.6 milliliters of 17.5 M acetic acid into a 500-milliliter volumetric flask and then add water to the mark to perform the dilution described above. 

Volumetric and measuring pipets are filled to a calibration mark at the outset the manner in which the transfer is completed depends on the particular type. Because an attraction exists between most liquids and glass, a small amount of liquid tends to remain in the tip after the pipet is emptied. This residual liquid is never blown out of a volumetric pipet or from some measuring pipets it is blown out of other types of pipets (Table 2-2). 

Burets, like measuring pipets, make it possible to deliver any volume up to the maximum capacity of the device. The precision attainable with a buret is substantially greater than the precision with a pipet. 

Measuring pipet A pipet calibrated to deliver any desired volume up to its maximum capacity compare with volumetric pipet. Mechanical entrapment The incorporation of impurities... 

Volumetric pipet A device that will deliver a precise volume from one container to another also called a measuring pipet. 

Glassware for measuring volumes of liquids. Left to right, graduated cylinder, buret, and measuring pipet. 

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. 

As in weighing operations, there will be situations where you need to accurately know volumes of reagents or samples measured or transferred (accurate measurements), and others in which only approximate measurements are required (rough measurements). If you wish to prepare a standard solution of 0.1 M hydrochloric acid, it can t be done by measuring an accurate volume of concentrated acid and diluting to a known volume because the concentration of the commercial acid is not known adequately. Hence, an approximate solution is prepared that is then standardized. We see in the table on the inside back cover that the commercial acid is about 12.4 M. To prepare 1 L of a 0.1 M solution, about 8.1 mL needs to be taken and diluted. It would be a waste of time to measure this (or the water used for dilution) accurately. A 10-mL graduated cylinder or 10-mL measuring pipet will... 

Note The values are theoretical standard deviations based on random event considerations and measured pipetting variation 8). PFU = plaque forming units CPM = counts per minute. 

A transfer pipet is more accurate than a measuring pipet. Tolerances for Class A (the most accurate grade) transfer pipets in Table 2-3 are the allowed error in the volume that is actually delivered. 

D. Which is more accurate, a transfer pipet or a measuring pipet What is the uncertainty in microliters when you deliver either 10 jxL or 100 piL from a 100-jxL adjustable micropipet ... 

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