Stopcock, buret

The titration of small volumes requires special biurets. Modified stopcock burets have been described for use in the microliter range, but the accuracy is never greater than 1 (il (P4). Manipulation is very diflBcult and not suited for the routine laboratory. 

The titrant in a conventional titration is replaced in a coulometric titration by a constant-current source whose current is analogous to the titrant s molarity. The time needed for an exhaustive electrolysis takes the place of the volume of titrant, and the switch for starting and stopping the electrolysis serves the same function as a buret s stopcock. 

The ethylmagnesium bromide is prepared in dry tetrahydrofuran and stored no longer than 1 week in a 250-ml. tube fitted with a 3-way vacuum stopcock and a dropping buret. The solution is decanted into the buret, and the correct volume is transferred to the reaction flask with positive nitrogen pressure. The tetrahydrofuran is purified by distillation from lithium aluminium hydride. See Org. Syn., 46, 105 (1966), for warning regarding the purification of tetrahydrofuran. 

The correct way for a right-handed person to position his or her hands to turn the buret s stopcock during a titration is shown in Figure 4.15 The natural tendency with this positioning is to pull the stopcock in as it is turned. This will prevent the stopcock from being pulled out, causing the titrant to bypass the... 

FIGURE 4.14 The three styles of buret stopcocks. Left, all Teflon center, Teflon stopcock and glass barrel right, all glass. 

Give both your acid and base solutions one final shake at this point to ensure their homogeneity. Rinse the buret with 5 to 10 mL of NaOH twice, and then fill it to the top. Open the stopcock wide open to force trapped air bubbles from the stopcock and tip. Allow this excess solution to drain into a waste flask. Bring the bottom of the meniscus to the 0.00-mL line. Using a clean 25-mL pipet (volumetric), carefully place 25.00 mL of the acid solution into each of the three flasks. Add three drops of phenolphthalein indicator to each of the three flasks. 

Define standard solution, volumetric flask, volumetric pipet, standardization, buret, stopcock, titration, titrant, substance titrated, equivalence point, indicator, end point, and 9automatic titrator. 

Give your EDTA solution one final shake to ensure its homogeneity. Then, clean a 50-mL buret, rinse it several times with your EDTA solution (the titrant), and fill, as usual, with your titrant. Be sure to eliminate the air bubbles from the stopcock and tip. Titrate the solutions in your flask, one at a time, until the last trace of red disappears in each with a fraction of a drop. This final color change will be from a violet color to a deep sky blue, but should be a sharp change. All three titrations should agree to within 0.05 mL of each other. If they do not, repeat until you have three that do. Record all readings in your notebook. 

His paper on the reactions that take place in the Gay-Lussac towers of sulfuric acid plants resulted from his successful experiments on the absorption of obnoxious sulfur dioxide fumes from an ultramarine plant In order to analyze the gases, he invented the Winkler gas buret with a three-way stopcock, and perfected his own methods. In the meantime he made his living by producing nickel and cobalt on a commercial scale. 

Close the stopcock of the buret and allow the reaction to proceed, while occasionally shaking the flask until the evolution of gas ceases. Consider the reaction as complete if the volume of the generated gas in the buret remains within 1 ml for a period of 20 mins. When the reaction is complete, adjust the level on the gas buret to that of the water in leveling bulb. If the volume of gas is not sufficient to fill the 450-ml bulb of the gas butet, add to the 600-ml flask a known amt of w from 25-ml buret until a readable volume is reached on the gas buret... 

Burets with ground-glass stopcocks should not be used, as leaking is caused by polymethylene formed preferentially on the ground surfaces. A buret such as Ultramax F and P, having a stopcock of plastic material, is satisfactory. The buret should be filled immediately before commencement of the reaction to keep the diazomethane solution cool and thus to minimize polymerization. The technique used is very similar to that of a titration, and a number of methylations of prepared batches can be quickly performed with one filling of the buret. 

Figuro 2-8 An air bubble trapped beneath the stopcock should be expelled before you use the buret...

For precise location of the end of a titration, we deliver less than one drop at a time from the buret near the end point. (A drop from a 50-mL buret is about 0.05 mL.) To deliver a fraction of a drop, carefiilly open the stopcock until part of a drop is hanging from the buret tip. (Some people prefer to rotate the stopcock rapidly through the open position to expel part of a drop.) Then touch the inside glass wall of the receiving flask to the buret tip to transfer the droplet to the wall of the flask. Carefully tip the flask so that the main body of liquid washes over the newly added droplet. Swirl the flask to mix the contents. Near the end of a titration, tip and rotate the flask often to ensure that droplets on the wall containing unreacted analyte contact the bulk solution. 

Error can be caused by failure to expel the bubble of air often found directly beneath the stopcock (Figure 2-8). If the bubble becomes filled with liquid during the titration, then some volume that drained from the graduated portion of the buret did not reach the titration vessel. The bubble can be dislodged by draining the buret for a second or two with the stopcock wide open. You can expel a tenacious bubble by abruptly shaking the buret while draining it into a sink. 

The electrolysis current (not to be confused with the detector current) for the Br2-generating electrodes can be controlled by a hand-operated switch. As the detector current approaches 20.0 p,A. you close the switch for shorter and shorter intervals. This practice is analogous to adding titrant dropwise from a buret near the end of a titration. The switch in the coulometer circuit serves as a stopcock for addition of Br2 to the reaction. 

Hg leveling bulb to be lowered asthe generator produces gas after the stopcock opens the buret to the generating system. 

Magnetic stir plate 50-ml buret Buret clamp Clamp support Stopcock Small glass funnel... 

Fig. 5.9. Toepler pump and gas buret. In this installation a constant-volume" manometer is used for measuring the pressure. Stopcock B allows the attachment of a sampling bulb or a large expansion bulb for use with large volumes of gas. Stopcock A is used for initial evacuation of the calibrated volume and sample bulbs. It is closed during the operation of the pump. When a gas is to be measured, the mercury levels in the right arm of the Toepler pump and in the left arm of the manometer are adjusted to levels (C) for which the gas volume has been calibrated. An excellent version of this pump is manufactured by the Rodder Instrument Co., Los Altos, Calif.

Fig. 1.19. A gas buret For the measurement or delivery of gases. When the elbow (on left) is attached to the top of the buret, gas is collected and its volume is measured after equalizing the mercury levels. For example, a hydrolysis flask similar to the reaction flask in Fig. 1.18 may be attached to this elbow. When the stopcock assembly is used (at the right), the buret can be filled with gas and a measured volume of this gas can be injected into a stream of flowing inert gas. 

The buret s stopcock is opened and titrant is slowly added until the solution permanently changes color or the pH rapidly changes. This is the end point of titration, and a final buret reading is made. 

Place the buret above the Erlenmeyer flask so that its tip reaches 1 cm below the rim. Open the stopcock of the buret slightly and allow the dropwise addition of the... 

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