The Buret

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

FIGURE 4.15 The correct way for a right-handed person to position his or her hands when performing a titration. 

Volumetric glassware used to deliver variable, but known volumes of solution. 

Volumetric buret showing a portion of its graduated scale. 

A titration in which the reaction between the analyte and titrant is an acid—base reaction. 

The utility of acid-base titrimetry improved when NaOH was first introduced as a strong base titrant in 1846. In addition, progress in synthesizing organic dyes led to the development of many new indicators. Phenolphthalein was first synthesized by Bayer in 1871 and used as a visual indicator for acid-base titrations in 1877. Other indicators, such as methyl orange, soon followed. Despite the increasing availability of indicators, the absence of a theory of acid-base reactivity made selecting a proper indicator difficult. 

Developments in equilibrium theory in the late nineteenth century led to significant improvements in the theoretical understanding of acid-base chemistry and. 

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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. 

In practice, however, any improvement in the sensitivity of an acid-base titration due to an increase in k is offset by a decrease in the precision of the equivalence point volume when the buret needs to be refilled. Consequently, standard analytical procedures for acid-base titrimetry are usually written to ensure that titrations require 60-100% of the buret s volume. 

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 buret contains a sodium hydroxide solution of known concentration. 

Consider the following titration curves. The solution in the buret is 0.1 M. The solution in the beaker has a volume of 50.0 mL. Answer the following questions. 

G.8 A student investigating the properties of solutions containing carbonate ions prepared a solution containing 8.124 g of Na2C03 in a flask of volume 250.0 mL. Some of the solution was transferred to a buret. What volume of solution should be dispensed from the buret to provide... 

In a titration, the volume of one solution is known, and we measure the volume of the other solution required for complete reaction. The solution being analyzed is called the analyte, and a known volume is transferred into a flask, usually with a pipet. Then a solution containing a known concentration of reactant is measured into the flask from a buret until all the analyte has reacted. The solution in the buret is called the titrant, and the difference between the initial and the final volume readings of the buret tells us the volume of titrant that has drained into the flask. The determination of concentration or amount by measuring volume is called volumetric analysis. 

C04-0141. A student prepared 1.00 L of a solution of NaOH for use in titrations. The solution was standardized by titrating a sample of potassium hydrogen phthalate whose mass was 0.7996 g. Before titration, the buret reading was 0.15 mL. When the indicator changed color, the buret reading was 43.75 mL. Calculate the molarity of the NaOH solution. 

The pipet is designed to deliver an exact volume of liquid the volumetric flask is designed to hold an exact volume of liquid and the buret is designed to deliver precisely measurable volumes of liquid. (Not drawn to scale.)... 

A student began a titration with an air bubble in the buret tip. The bubble came out of the tip after 5 mL of Na2S203 solution had been released. 

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. 

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. 

The addition of titrant from the buret must be stopped at precisely the correct moment—the moment at which the last trace of substance titrated is consumed by a fraction of a drop of titrant added, so that the correct volume can be read on the buret. That exact moment is called the equivalence point of the titration. In order to detect the equivalence point, an indicator is often used. An indicator is a substance added to the reaction flask ahead of time in order to cause a color change at or near the equivalence point, i.e., to provide a visual indication of the equivalence point. For example, the use of a chemical named phenolphthalein as an indicator for a titration in which a strong base is used as the titrant and an acid as the substance titrated would give a color change of colorless to pink in the reaction flask near the equivalence point. The color change occurring near, not exactly at, the equivalence point is usually not a concern. The reason will become clear in a later discussion. The point of a titration at which an indicator changes color, the visual indication of the equivalence point, is called the end point of the titration. As we will see, equivalence points can be determined in other ways too. 

The three volumetric glassware products we will discuss are the volumetric flask, the pipet, and the buret. Let us study the characteristics of each type individually. 

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... 

Assemble the apparatus for a titration. The buret should be a 50-mL buret and should be washed thoroughly with a buret brush and soapy water. Clamp the buret to a ring stand with either a buret clamp or an ordinary ring stand clamp. The receiving flask should be a 250-mL Erlenmeyer flask. You should clean and prepare three such flasks. Place a piece of white paper (a page from your notebook will do) on the base of the ring stand. This will help you see the end point better. 

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. 

We should emphasize that the concentrations of the acid and base are important to consider. These concentrations in the discussions thus far have been 0.10 M 0.10 M NaOH in the buret and 0.10 M HCI in the reaction flask The lower the concentration of the acid, the fewer H+ ions there are present, the higher the initial pH, and the higher the pH level of the initial steady increase. The lower the concentration of the base, the lower the level of the pH after the inflection point. See Figure 5.1 (b). 

This reaction is complete at the first inflection point. On the way to the second inflection point, the bicarbonate from the first reaction reacts with more hydrogen ions from the buret to form carbonic acid ... 

Oxidation-reduction reactions represent yet another type of reaction that titrimetric analysis can utilize. In other words, a solution of an oxidizing agent can be in the buret, and a solution of a reducing agent can be in the reaction flask (and vice versa). In this section, we review the fundamentals of oxidization-reduction chemistry and discuss the titrimetric analysis applications. 

Record the buret readings at the end points for all three beakers. Calculate the percent KHP in the sample for all three titrations and include at least a sample calculation in your notebook, along with all results and the average. Calculate the ppt relative standard deviation. If your three results do not agree to within 10.0 ppt relative standard deviation, repeat until you have three that do or until you have a precision satisfactory to you instructor. 

When the solution is red, there is an excess of HC1. At this point, titrate the solution with the standardized NaOH to a green end point. Record the readings of the buret. 

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