Mohr pipet

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. 

Which is more accurate, a Mohr pipet or a volumetric pipet Why ... 

With a Mohr pipet, the meniscus must be read twice, but with a serological pipet, the meniscus may be read only once, if desired. Explain this. 

FIGURE 12.10 A homemade soap bubble flow meter constructed from an old Mohr pipet, a piece of glass tubing, and a pipet bulb. 

The serological pipet is calibrated through the tip and may be used by adjusting the meniscus to the line that would deliver the desired volume by letting it drain completely (and blowing it out). The Mohr pipet is calibrated only to the lowest calibration line and cannot be drained below this line. 

If I chose a Mohr pipet, it would not be calibrated for blowout. If I chose a serological pipet, it would be calibrated for blowout. A blowout pipet would have one or two frosted rings completely circumscribing the top of the pipet. See Figure 4.12 for details on how to make the delivery. 

Figure 2-11 (a) Transfer pipet and (b) measuring (Mohr) pipet. [Courtesy A. H. Thomas Co., Philadelphia, PA.]... 

F Volumetric pipet courtesy of VWR Scientific, Division of Univar. G Mohr pipet courtesy of VWR Scientific, Division of Univar. 

Fractional volumes of liquid are transferred with graduated pipets, which are available in two types—Mohr and serological. Mohr pipets (Figure 1.4G) are available in long- or short-tip styles. Long-tip pipets are especially attractive for transfer to and from vessels with small openings. Virtually all Mohr pipets are TD and are available in many sizes (0.1 to 10.0 mL). The marked subdivisions are usually 0.01 or 0.1 mL, and the markings end a few centimeters from the tip. Selection of the proper size of pipet is especially important. For instance, do not try to transfer 0.2 mL with a 5 or 10 mL pipet. Use the smallest pipet that is practical. 

The use of a Mohr pipet is similar to that of a volumetric pipet. Draw the liquid into the pipet with a pipet filler to a level about 2 cm above the 0 mark. Lower the liquid level to the 0 mark. Remove the last drop from the tip by touching it to the inside of the glass container. Transfer the pipet to the receiving container and release the desired amount of solution. The solution should not be allowed to move below the last graduated mark on the pipet. Touch off the last drop. 

Serological pipets (Figure 1.4H) are similar to Mohr pipets, except that they are graduated downward to the very tip and are designed for blow-out. Their use is identical to that of a Mohr pipet except that the last bit of solution remaining in the tip must be forced out into the receiving container with a rubber bulb. This final blow-out should be done after 15 to 20 seconds of draining. 

Pipettor pump courtesy of Cole-Parmer Instrument Co E Pasteur pipet courtesy of VWR Scientific, Division of Univar F Volumetric pipet courtesy of VWR Scientific, Division of Univar 6 Mohr pipet courtesy of VWR Scientific, Division of Univar H Serological pipet courtesy of VWR Scientific, Division of Univar... 

A section of a Mohr pipet or graduated centrifuge tube sealed to a 14/20 standard taper outer joint have proven to be quite satisfactory. 

Pipet 0.5 mL of the ascorbic acid standard (1.0 mL Mohr pipet) into a 100 mL volumetric flask and dilute to the mark with 20 mM H2SO4 (now 0.5 ppm). 

Fill a 10 mL Mohr pipet with the hot buffer and spread a thin layer of it on each acetate strip. Be careful. Try not to let it run over the side, but if it does, let it cool before you try to remove it. 

Add 1.5 mL of ammonium hydroxide (a buret is preferred, but a Mohr pipet is acceptable.). CarefuV. Concentrated NH4OH is hazardous. Do in a hood if possible, or else very quickly. 

A Mohr pipet has graduations down the length of the tube, whereas a delivery pipet has only one graduation at the top. 

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